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Should Pluto be promoted to a planet again?

Pluto as seen by New Horizons during a flyby at a distance of 450,000 kilometers. The irridiscent colors are the result of scientists combining four images taken by the spacecraft. Credit: NASA.

In 2006, Pluto was relegated from being the ninth planet in the solar system after the International Astronomical Union (IAU) demoted it to a “dwarf planet”. Officially, Pluto is now known as  “minor planet 134340 Pluto.” 

Many astronomy buffs were disappointed by the new assignment, seeing the criteria for Pluto’s demotion as rather nitpicky or even arbitrary.

But don’t scratch Pluto off as a planet just yet. There are still many authoritative voices in science who argue that the frozen world on the outer rim of the solar system should truly be classed as a planet in its own right.

Why was Pluto demoted in the first place?

The existence of Pluto was first proposed in the early 20th-century by Percival Lowell, whose calculations showed that wobbles in the orbits of Uranus and Neptune must be caused by the gravitational pull of an unknown ninth planet. Pluto was confirmed a decade later, on March 13, 1930, at the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W. Tombaugh.

Since then Pluto’s status as the solar system’s ninth planet remained unchallenged until astronomers discovered 2003 UB313, also known as Eris, which is nearly as large as Pluto, but with a lot more mass.

The discovery of Eris triggered a fiery debate among astronomers. If Pluto is a planet, then so should Eris. Eventually, both were classed as dwarf planets after the IAU issued new criteria for what constitutes a planet. Essentially, IAU says that a planet must:

  • Orbit around the sun;
  • Have sufficient mass to assume hydrostatic equilibrium (gets squished into a sphere-like object due to gravity);
  • clears the neighborhood around its orbit.

In August 2006, the IAU ruled that Pluto would no longer be considered a planet because it doesn’t “clear the neighborhood around its orbit.” Pluto’s oblong orbit overlaps that of Neptune, so it was disqualified.

Instead, the IAU classed Pluto as a dwarf planet, which is a celestial body that meets the first two criteria. Besides Pluto and Eris, there are three other known dwarf planets in the solar system: Ceres in the asteroid belt between Mars and Jupiter; Haumea, which is located beyond Neptune’s orbit, and Makemake, the second-largest Kuiper Belt object.

Why should Pluto be considered a planet again?

Bearing this in mind, it’s clear that had the IAU not stuck to its new rules, we would have had at least 13 planets in the solar system now. But why should that be a problem?

Many have criticized IAU’s definition of a planet, claiming the criteria are arbitrary. Among these critics is NASA administrator Jim Bridenstine, who during a keynote at the 2019 International Astronautical Congress said:

“I am here to tell you, as the NASA administrator, I believe Pluto is a planet,” before adding that “Some people have argued that in order to be a planet you have to clear your orbit around the sun. If that’s the definition we’re going to use then you could undercut all the planets—they’re all dwarf planets—because there isn’t a planet that clears its entire orbit around the sun.”

The NASA chief is referring to asteroids, which regularly whizz past all the planets in the solar system.

“I think it’s a sloppy definition,” said Bridenstine. “I think the way you should define a planet is based on its intrinsic value, not values that constantly change like orbital dynamics.” 

Naming planets: a bureaucratic decision?

Bridenstine’s stance on Pluto is supported by the authors of a study published in a 2019 edition of the journal Icarus by a team of researchers led by Philip Metzger, a planetary physicist with the Planetary Science faculty at the University of Central Florida. Although the paper doesn’t focus on Pluto specifically, the authors explain how even asteroids were recognized as planets until the 1950s.

“In the 1950s, developments in planet formation theory found it no longer useful to maintain taxonomic identification between asteroids and planets, Ceres being the primary exception. At approximately the same time, there was a flood of publications on the geophysical nature of asteroids showing them to be geophysically different than the large planets. This is when the terminology in asteroid publications calling them planets abruptly plunged from a high level of usage where it had hovered during the period 1801–1957 to a low level that held constant thereafter,” the scientists wrote.

In effect, asteroids were reclassified as non-planets based on their geophysical characteristics. By extension, all cosmic bodies should also be classified by their geophysical characteristics and not arbitrarily through voting by a panel.

“I believe the IAU made several deep mistakes and so the definition is not valid and not scientifically useful and should be rejected. First, definitions should never be voted on for taxonomical concepts like “planet” because taxonomy is supposed to evolve and develop as an integral part of the science. By voting on a particular taxonomical choice they shut down that portion of the scientific process. So the entire vote was anti-science to begin with,” Metzger told ZME Science.

Metzger adds that the IAU actually violated their principles, “including their explicit statutes and bylaws, which require the actual language of a proposed vote to be reviewed by all the members for four months before the assembly.”

Since the IAU was in a hurry to force a decision, the organization violated its by-laws by not sharing the proposal text until the 2006 assembly.

“That was the first violation, the first mistake. Then the proposal was rejected at the convention, and since they had already set the precedent of breaking the rules they continued blindly breaking the rules even more. So they made up the definition during the assembly, giving nobody anywhere else in the world any time to digest it, giving nobody the choice to come to the meeting to present a well reasoned and well-researched case to sway the outcome, and they forced a vote. They got a deep split in the votes, proving there really is no consensus, but one side won so they declared that the deed was done. That became highly politicized and caused people to take sides and get emotionally involved, so now most people have an attitude about the whole thing and it has poisoned the ability to readdress the question,” Metzger said.

Perhaps you’ve noticed a pattern: the main objection that Pluto planetary advocates have against the IAU decision is that it didn’t follow a scientific process. Instead, it was based on a subjective, bureocratic process.

“Nobody is creating a theory about how objects that fail to clear their orbits are fundamentally different than those that do clear their orbits or those that are satellites of another planet. Nobody has ever proposed differences in geology, geochemistry, atmospheres, oceans, the emergence of life, mineralogy, etc., that are consistent one way in bodies that clear their orbits versus another way in bodies that do not clear their orbits. On the other hand, the term “planet” actually is being used to compare different planets across different dynamical states. So a paper might discuss Pluto, Triton, and Mars, calling them all “planets”, even though one cleared an orbit, one did not, and one was captured by Neptune to become a satellite,” Metzger said.

“Summary: the IAU definition does not match how scientists actually use the planet concept in doing real, reductionist science. The definition they created was actually designed to keep the number of planets small so school kids could memorize the planets, but having a small number is not the concern of science. The IAU abandoned science in order to create a cultural definition instead,” he added.

Wait, does that mean that the Moon and other satellites are planets too? Exactly.

“The idea that satellites are a distinct category with no overlap in the “planet” category is actually a recent invention. For almost all scientific history, moons were planets. What scientists used to say before the 1920s is that planets orbiting the sun are “primary planets” while planets that orbit another planet are “secondary planets” or just “satellite” or “moon” for short (but they were still known to be planets). Being a satellite was just a dynamic relationship that a planet could have with another planet.   When we decided that most asteroids are too small to be planets, in the 1960s, then we should have decided at the same time that the smaller satellites are also too small to be planets for the exact same reason, while the larger, round satellites are still planets. Unfortunately, astronomers had already become confused about satellites by the 1920s and forgot the meaning of the word “planet”, switching over to a cultural “folk taxonomy” that actually has its roots in astrology and geocentrism. The public was abandoning geocentrism in the late 1700s and early 1800s, and astrology was still very influential, and this led them to develop the idea that satellites are not planets. I am currently writing a manuscript that shows this. It was similar to the general public’s idea that green beans are a vegetable rather than a fruit, although biologists say a green bean is a fruit. The public develops a “folk taxonomy” that is human-centric and not scientifically reductionist, in contrast to the scientific taxonomy that is designed to align along the natural divisions of reductionist theory, thus providing deep explanatory power about nature,” Metzger said.

“So although the public is still unaware of this fact, it is true that planetary scientists are referring to moons as “planet”. Many of us do not even realize we are doing this. We say Titan has a planetary core, a planetary crust, a planetary radius, etc. They are “planetary” because they are characteristic of planets. Being a satellite has nothing to do with it. But if having these things are characteristic of planets, and bodies that are in both primary and secondary orbits have them equally, then every time we say “planetary” we are acknowledging that the type of orbit a body has is irrelevant to whether it is a planet or not.”

If Pluto is reinstated as a full-fledged planet, then the other four dwarf planets should join it too, bringing the total number of planets in the solar system to 13. However, by Metzger’s account of what constitutes a planet by scientific taxonomy, there ought to be at least 150 planets in the solar system.

“Most of them are in the Kuiper Belt. Ceres, which is in the asteroid belt, is also a planet. But also, large moons (satellites) are planets,” he said.

Another vocal supporter of Pluto’s planetary status is Alan Stern, principal investigator of NASA’s New Horizons mission, which flew by Pluto in 2015, revealing the icy body in unprecedented details, including a stunning heart-shaped nitrogen-ice plain.

Last year, Stern hosted an online debate for the Philosophical Society of Washington, in which he asked people to vote on whether or not Pluto should be reinstated as a planet. Before voting was opened, Stern explained their argument that a planet should be defined by its geophysical properties — if a body is massive enough to assume a nearly round shape but not massive enough to trigger nuclear fusion in its interior like a star, then that’s a planet. The poll closed with 130 votes in favor of making Pluto a planet again and 30 against it.

During his presentation, Stern went on to describe New Horizons’ findings, which show that Pluto has mountains, glaciers, avalanches, a liquid ocean beneath its icy crust, and a complex atmosphere. These are all hallmarks of planetary processes.

What’s more, the third criteria for planetary eligibility — that a planet has to clear its orbit of other bodies like asteroids — is not fair for bodies orbiting so far away from the sun. The more you move away from the sun, the harder it is to clear away small objects because the orbit is much slower than that of objects closer to the sun. This means that a planet needs to be increasingly massive in the outer reaches of the solar system in order to clear objects. In fact, Stern makes the assertion that not even Earth would qualify as a planet if it was on Pluto’s orbit. 

“The IAU definition is not aligned with useful reductionism. They voted to maintain the folk taxonomy of the public. Not only did they vote that moons are not planets, but they added an “orbit clearing” requirement to keep the number of planets small, so the set of planets would continue to be similar to the set that was known from geocentrism and astrology. Galileo rejected that set. He called the moons of Jupiter “planets”. He did not think dynamics should have anything to do with the definition of a planet. He did not think the number should be kept low to match the ancient geocentrism and astrology. It is really too bad that astronomers rejected Galileo and restored some of the ideas he fought to reject,” Metzger said.

“The basic problem is that modern astronomers have failed to realize that taxonomy, and evolving concepts like “planet”, are crucial to the program of science. They thought there was no harm in adopting a folk taxonomy from culture. They did not see any reason not to. This is where the most work needs to be done to repair the damage. The astronomers who believe the IAU’s planet definition is good need to get a much broader view of the functional role of taxonomy in science, to understand how important and beneficial taxonomy can be. If they think the IAU’s definition is good and beneficial, it is only because they have such a low concept of how much better things could be. Biologists understand this very well, so they demand that scientists retain “taxonomical freedom” and that taxonomy should never be a matter of regulation or restraint. Astronomers need some growth in this area,” he added.

More than 14 years after Pluto’s historic demotion to a dwarf planet, this debate is far from over. Perhaps Pluto might rejoin the ranks of the solar system’s roster of planets, but until then spirits remain high.

Bottom line: Pluto’s classification as a dwarf planet is arbitrary rather than the product of a scientific process.

Pluto’s white-peaked mountains are actually covered in methane frost

Although Pluto has been demoted to a dwarf planet, the distant world close to the rim of the solar system has glaciers, avalanches, a liquid ocean beneath its icy crust, a complex atmosphere, and tall mountains. In fact, flybys of Pluto performed in 2015 by NASA’s New Horizons probe revealed snow-capped chains of mountains that could have very well been mistaken for a landscape on Earth.

While it’s true these mountains are covered in snow, it’s not the kind of snow you’re familiar with. Instead of water ice, these formations are covered in methane frost, according to a new study published in Nature Communications by astronomers at NASA’s Ames Research Center in California.

The weirdest peaks in the solar system

Pluto’s Pigafetta Montes and Elcano Montes and the Alps on Earth in a side-by-side comparison. The resemblence is uncanny. Credit: Thomas Pesquet/ESA/NASA.

Being more than 3.6 billion miles (5.8 billion kilometers) away from the sun, Pluto is an icy world where temperatures typically dip as low as minus 387 degrees Fahrenheit (-232 degrees Celsius). Bearing these frigid conditions in mind, it’s no wonder that Pluto is mostly covered in ice and snow. But due to the planet’s unique atmospheric composition, which includes nitrogen, methane, and carbon dioxide, the snow appeared red — with one exception: its white mountain peaks.

The peaks examined by the researchers for their new study lie at the edge of the iconic heart-shaped glacier at Pluto’s equator, in a region known as Cthulhu. It’s the only place other than Earth in our solar system that’s known to have white-peaked mountains.

These mountains are about 2.5 miles (4 km) in height, which is really tall for Pluto’s diameter — If Earth was the size of a nickel, Pluto would be about as big as a popcorn. Intriguingly, these are mountains made of water ice which is as hard as rock on Earth due to the extremely cold temperature.

However, the bright-white peaks have a different composition. Based on computer simulations performed by Tanguy Bertrand and colleagues at NASA, the bright frosts are mostly made of methane-rich ice.

White peaks that form backwards

This diagram shows how the formation of snow caps on Pluto is basically the opposite of that on Earth. Credit: Tanguy Bertrand et al.

Here on Earth, temperature decreases the higher up in altitude you climb, causing water carried by moist winds to condense and create snow-capped peaks. Not on Pluto, though — the opposite process occurs there.

As you ascend Pluto’s mountain peaks, the temperature increases because its surface is warmed by the sun. So, oddly enough, temperature on Pluto rises with altitude.

Since Pluto has more methane gas at its warmer, higher altitudes, the gas becomes saturated and freezes directly onto the mountain peaks. At Pluto’s surface, at lower altitudes, the methane concentration is too low for it to condense.

NASA scientists believe the same process may be responsible for the formation of other methane-rich deposits across Pluto, as well as on other bodies in the solar system, such as Neptune’s moon Triton.

Pluto may have had a ‘hot start’ harboring liquid oceans when it first formed

Although Pluto orbits the sun at an average distance of 3.7 billion miles (5.9 billion kilometers), some studies claim that the icy dwarf planet may have a liquid ocean under its surface. According to a new study, the accretion of new material during Pluto’s early geological history may have generated enough heat to sustain the formation of this ocean, which would go on to continuously freeze over billions of years.

Extensional faults (arrows) on the surface of Pluto may have been created by the freezing of a subsurface ocean. Credit: NASA/Alex Parker.

Pluto is so far away from the sun that it lies inside the Kuiper Belt, a group of rocks and ice left over from the formation of the solar system. It’s really no wonder that today the planet looks like a barren ball of ice and rock.

However, detailed images of Pluto’s surface taken during flybys by NASA’s New Horizons spacecraft have revealed intriguing geological features that can tell us many things about the dwarf planet’s past.

According to Francis Nimmo, a professor of Earth and planetary sciences at UC Santa Cruz, Pluto’s surface shows evidence of both ancient and modern extensions of its surface. This suggests that Pluto may have been ‘hot’ during its early days.

To understand why this matters, we have to make a brief tangent to talk about how water freezes. Most liquids shrink when they are cooled because molecules are moving slower, making them less able to overcome the attractive intermolecular forces that draw them closer together. When the freezing point is reached, the substance solidifies in a tightly packed crystalline matrix.

Water is an exception to this chemical behavior. Liquid water also contracts when cooled, but only until it is chilled to approximately 4 degrees Celsius. If you cool water past this threshold, it will actually expand slightly. Cool it further to its freezing point and the water will expand by about 9%.

So, water expands when it freezes and contracts when it melts — and this has important implications for Pluto’s history.

“If Pluto had a subsurface ocean shortly after it formed, that ocean would have been continuously freezing over solar system history. That would cause Pluto to be expanding and that would result in specific geologic features we can look for. In contrast, if Pluto started with a cold ice shell, that ice would have warmed and melted slowly from the heat of radioactive decay forming an ocean. That ocean would have been refreezing in more recent times as the heat from radioactive decay waned. This would lead to early compressional tectonics followed by later extension,” Carver Bierson, UCSC graduate student and co-author of the new study, told ZME Science.

If Pluto had a ‘cold start’, compression on its surface would have occurred early on, followed by more recent extensions. However, if it had a hot start, then extension would have occurred throughout Pluto’s history — and this seems to be the case, current observations suggest.

“When we look at the surface of Pluto we don’t see any clearly compressional features even on the oldest terrains. We do see many extensional features, most of which are recent. The oldest tectonic features we see look to be extensional, but are hard to interpret because they are so eroded. Taken together we think this favors Pluto’s ocean already being present very early in Pluto’s history,” Bierson added.

Pluto’s hot past

Where could all this energy have come from? Pluto is too far from the sun to be of consequential difference, which leaves us with only two main possible sources of energy.

One is the heat released through the decay of radioactive elements in the rock, the other is gravitational energy released during impact with asteroids and other cosmic bodies. Early in the solar system’s history, it was quite common for planets to be bombarded by asteroids and meteorites.

Calculations performed by the researchers suggest that if all of that gravitational energy was retained as heat, Pluto could have supported an initial liquid ocean.

In reality, however, we know that is simply not possible — some of that energy will escape into space.

“How Pluto was put together in the first place matters a lot for its thermal evolution,” Nimmo said in a press release. “If it builds up too slowly, the hot material at the surface radiates energy into space, but if it builds up fast enough the heat gets trapped inside.”

The researchers found that if Pluto formed over a period of less than 30,000 years, then it would have most certainly been hot. If it took a couple million years for accretion of proto-Pluto to take place, then a hot start would only be possible if a large impactor buried its energy deep beneath the surface.

“If Pluto’s ocean was there early on it raises another question, “What was the heat source to form that ocean?” Today Pluto sits in an extremely cold part of the solar system. Its surface temperature is about 45 Kelvin (-480 Fahrenheit). When Pluto was forming new material would have been coming in and impacting its surface. Each impact is like an explosion that will warm the nearby area. If Pluto formed slowly, the surface would cool between each impact and generally stay very cold. If however Pluto formed quickly you have impact on top of impact and the surface doesn’t have time to cool. We calculate that if Pluto formed in less than 30000 years the heat from these impacts could have been sufficient to lead to an early ocean,” Bierson said.

There are a lot of assumptions and ‘ifs’ in this study, but if the findings are confirmed, other large objects in the Kuiper belt likely started out hot, possibly harboring oceans billions of years ago.

What’s intriguing is that these oceans could persist to this day in the largest objects in the belt, such as the dwarf planets Eris and Makemake, shielded under a blanket of thick ice.

“Pluto is the first Kuiper Belt object we visited and what we found was amazing. There is no reason to think that Pluto’s neighbors (Eris, Makemake, Haumea) are any less interesting. From this work, we suggest that they also should have formed with oceans, but we don’t know if they would have completely refrozen over solar system history. Up to now, we haven’t been able to see those worlds as more than points of light in the night sky. This is hard because the Kuiper belt is very very far away. It took New Horizons 9 years from launch to Pluto and it was one of the fastest spacecraft ever launched. Still, I am optimistic that with new telescopes and maybe a future mission we can keep unlocking their secrets,” Bierson concluded.

The findings were reported in the journal Nature Geoscience.

Not blue nor red: here’s what Pluto actually looks like

The New Horizons missions showed us Pluto in unprecedented detail. Far from a boring, frozen landscape, Pluto revealed intriguing geology and remarkable features. This new information sparked renewed interest for Pluto and astronomy in general.

But here’s the thing: Pluto doesn’t really look as you see it above — that’s a false-colour image. False-color images (or enhanced-color images) are used by astronomers to detect differences in the composition and texture of Pluto’s surface, and it works: you can easily see many of Pluto’s geological features. The image above was widely circulated, sometimes without making the mention that it’s not in real color.

So here’s what Pluto actually looks like:

Still beautiful in true color. Image credits: NASA.

Pluto’s real colors

Even this image is not exactly a true simple photo in the sense we understand it on Earth (although it does represent Pluto in real color). It is a combination of 4 images taken with New Horizons’ Long Range Reconnaissance Imager (LORRI). The images were then combined with color data acquired by another instrument, creating a global image of Pluto.

The images were taken when the spacecraft was 450,000 kilometers (280,000 miles) away, but they show features as small as 2.2 km (1.4 miles) small. Combining the images offers twice the resolution of a single image.

However, keeping in mind what Pluto’s real colors look like, there’s no reason we can’t appreciate the blue-red image as well.

“This high-resolution, false color image of Pluto is my favorite. The New Horizons flyby of Pluto on July 14, 2015 capped humanity’s initial reconnaissance of every major body in the Solar System. To think that all of this happened within our lifetime! It’s a reminder of how privileged we are to be alive and working at NASA during this historic era of space exploration,” says Laurie Cantillo, NASA Planetary Science Public Affairs.

Pluto through the years

Indeed, it’s crazy to think that just a couple of decades ago, our best image of Pluto was just a few pixels wide. Here’s how this image evolved through the years:

Pluto’s evolution through the years, from the Hubble images of the ’90s to the New Horizons days. Credits: NASA.

Now, thanks to NASA’s New Horizons mission, we can see Pluto in stunning detail. We can see its frozen canyons, which tell an ethereal story of geological diversity and reveal varied compositional features:

Long canyons run vertically across the polar area, seemingly indicating a period of past tectonic activity on the planet. Image credits: NASA.

We can see its majestic mountains and foggy hazes rising from its frozen valleys:

A near-sunset view of the rugged, icy mountains and flat ice plains extending to Pluto’s horizon. Image credits: NASA.

We can even see Pluto’s highlands and its jagged, Earth-like relief:

Also an enhanced color image. Credits: NASA.

Whether or not we deem it a planet, we live in a time when we are building geological maps of Pluto — and that alone is worth celebrating!

Pluto’s volcanoes might spew out liquid water, new study finds

Pluto’s volcanoes appear to have spewed out ammonia-rich watery fountains into space, hinting at a complex (and quite possibly, organic) chemistry of Pluto’s subsurface sea.

“This was a huge surprise to all of us about Pluto,” says planetary scientist Dale Cruikshank of the NASA Ames Research Center in Moffett Field, Calif. “It means there are lots of surprises waiting to be uncovered in that part of the solar system.”

Pluto just got a lot more interesting. Image credits: Z.L. Doyle/SWRI/JHU-APL/NASA.

Before the New Horizons mission, our understanding of Pluto was very limited. The interplanetary space probe whooshed by the planetoid, offering an unprecedented view of Pluto’s surface and that of its satellites, showing that the entire planetary system is much more complex than previously anticipated. This sparked a renewed interest for studying Pluto — and data from New Horizons keeps coming in.

Now, a new analysis of spectral images obtained with the New Horizons spacecraft indicates the presence of ammonia on Pluto’s surface. These spectral data (essentially different wavelengths of light) can reveal the chemical signature of different substances. Cruikshank and his colleagues analyzed the images and found a variety of ice on top of a bedrock of water ice — Pluto’s surface temperature is about minus 390 degrees Fahrenheit (minus 270 degrees Celsius), which is cold enough to form mountains from water ice. It’s not this water that was surprising — researchers have suspected for a while that Pluto’s dense core surrounded by a mantle of water ice, and they’ve also known that Pluto’s surface hosts mountains of water ice.

Instead, it’s the ammonia that was surprising.

Ammonia isn’t able to survive for a long time on the surface of a body like Pluto. Without an atmosphere to protect it, ammonia is destroyed with relative ease, in as many as 400,000 years (which, at a planetary scale, is the blink of an eye). So the presence of ammonia suggests that Pluto has a young or renewed surface.

“If you find it at all, it suggests that it has been put there fairly recently,” Cruikshank says. “There is really no limit [to how recently], as far as I can see in the geology.”

A section of Virgil Fossae. Arrows point to topographic features that are muted in form by an apparent mantling by material ejected in a cryovolcanic event within the fossa main channel. Below, the same region shown with water ice distribution: darker shade of blue indicates greater concentration. It’s stunning that we can see features on Pluto in such detail. Image credit: NASA, Johns Hopkins University, Southwest Research Institute.

This isn’t the only possibility though. Ammonia might also survive in a more durable chemical form such as a hydrated or an ammoniated salt, which are much more resilient. However, the location of the newly-discovered ammonia signatures is also intriguing: it appears to be clustered around a geological feature called Virgil Fossae — essentially a large ditch-like feature, probably a fissure associated with ice volcanism (also called cryovolcanism).

Ammonia is also an excellent antifreeze, researchers write. In sufficient quantities, ammonia can lower water’s freezing point by 100 degrees Celsius. If Pluto’s subsurface mantle ocean contains ammonia, that would help explain how the water remains liquid despite sub-freezing temperatures. A recent study also shed more light on Pluto’s liquid water, suggesting that it is kept in place by a thin layer of gas.

The observations revealed another surprise: a red material that Cruikshank’s team thinks represents complex organic material. This is consistent with the existence of ammonia molecules and could suggest that Pluto’s ocean also has organic molecules. This was not confirmed, and even if this is the case, it doesn’t mean that Pluto can host life — it merely suggests that the chemical precursors to life can emerge even in such inhospitable places.

Distribution of red-tinted water ice exhibiting the spectral signature of ammonia in Virgil Fossae and surrounding terrain.
(B) is an image illustrating the uniquely bright red coloring of Virgil Fossa, while (C) showcases the geographical distribution of water ice and ammonia. Image credit: NASA, Johns Hopkins University, Southwest Research Institute.

Not everyone is convinced, however.

Marc Neveu, an astrobiologist at NASA Goddard Space Flight Center in Greenbelt, Md., who was not involved in the study, says that the quality of these images is not sufficient to draw a definite conclusion. Also, without a direct chemical analysis, it’s very difficult to assess the exact chemistry of Pluto’s surface.

Also, there’s no indication so far as to whether this was a singular eruption or a regular phenomenon. Ammoniated ice was spread as far as 200 kilometers from the fissure, but this could have been produced by a singular eruption. If this was the case, the eruption would have shot water at speeds of over 300 meters per second (1080 km/h), with the water freezing midair before falling down around Virgil Fossae.

At any rate, the Virgil Fossae complex is part of a tectonic system, which seems to include some form of cryovolcanism. Researchers make a good case for supporting this idea, suggesting that these eruptions also bring ammonia from Pluto’s mantle up to the surface. While the case isn’t settled yet, it’s yet another indication of how intriguing Pluto’s geology really is.

Journal Reference: C. M. Dalle Ore et alDetection of ammonia on Pluto’s surface in a region of geologically recent tectonismScience Advances. Published online May 29, 2019. doi:10.1126/sciadv.aav5731.

PIA21863 Pluto.

Pluto’s ocean might be held in place by a thin layer of gas

This might also mean that there are many more oceans in the universe than previously thought.

PIA21863 Pluto.

Digital rendering, with exaggerated relief and color of Pluto. Based on close-up images taken by NASA’s New Horizons spacecraft in 2015. Credits NASA/Johns Hopkins University Applied Physics Laboratory.

It’s almost shocking to think how much we know about Pluto. The tiny ex-ex-planet was studied in unprecedented detail by the New Horizons missions that brushed right by it. The images sent by the shuttled showed Pluto’s topography in great detail, revealing some unexpected details. For instance, scientists were surprised by a white-colored ellipsoidal basin named Sputnik Planitia, located near the equator and roughly the size of Texas. The basin appears to be very thin, which would suggest the presence of a subsurface ocean beneath it. However, if this was true, researchers would have expected it to be long-frozen by now, which does not seem to be the case.

Now, researchers believe that they have an idea what happened.

Shunichi Kamata and colleagues from several Japanese universities propose that an “insulating layer” of gas hydrates exists beneath the icy surface of Sputnik Planitia. Gas hydrates are a crystalline solid formed from water and gas. They look and behave very much like ice, but contain high amounts of methane gas. They also have very low thermal conductivity, which means that they can essentially act as an insulator.

Since actually going to Pluto and drilling it on-site is not really an option, researchers tested their hypothesis with computer simulations. They generated a model of Pluto and its subsurface ice starting 4.6 billion years ago, when the solar system began to form.

The simulations confirmed that without an insulator, the subsurface ice would have long frozen over.

It also showed that the thermal and structural evolution of Pluto’s interior and the time required for a subsurface ocean to freeze and for the icy shell covering it to become uniformly thick. They simulated two scenarios: one where an insulating layer of gas hydrates existed between the ocean and the icy shell, and one where it did not.

The study has been published in Nature Geoscience. DOI: 10.1038/s41561-019-0369-8

Global LORRI mosaic of Pluto in true colour. Credit: NASA.

Pluto is a planet after all, say planetary scientists

Global LORRI mosaic of Pluto in true colour. Credit: NASA.

Global LORRI mosaic of Pluto in true colour. Credit: NASA.

In 2006, the International Astronomical Union (IAU) set forth a new set of guidelines for what constitutes a planet. In a decision that surprised and even infuriated a lot of people, the commission announced that Pluto could no longer be considered a planet, in light of these new guidelines — and, ever since, the scientific community has debated Pluto’s place and status in the solar system.

If it looks like a planet, moves like a planet, then it’s a…

According to a new study, the IAU definition for a planet does not make much sense because astronomers have not used it in their work when discussing planets.

The IAU has been responsible for the naming and nomenclature of planetary bodies and their satellites since the early 1900s. Right at the end of the 2006 Prague General Assembly, members voted that the definition of a planet in the Solar System would be as follows:

A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

Pluto orbits the sun in a super-crowded region called the Kuiper Belt, which is jam-packed with asteroids, debris, and other so-called trans-Neptunian objects (TNOs) such as Eris or Sedna. Because it doesn’t clear its orbit, the IAU considered that Pluto didn’t fit criterion (c) and demoted to it to a ‘dwarf planet’ — objects that only meet the first two criteria.

Philip Metzger, a planetary scientist at the University of Central Florida, and one of the main architects of the New Horizons mission that reached Pluto for the first time, is one of the loudest critics of the IAU’s definition.

“It’s a sloppy definition,” said Metzger, who is the lead author of the new study. “They didn’t say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit.”

Metzger and colleagues reviewed a myriad of studies published over the past 200 years looking for any instance where the clearing of the orbit was used as a requirement for defining a planet. The authors found only one such mention — a study published in 1802 — which was based on flawed, now-disproven reasoning.

Instead, the researchers found over 100 recent examples where researchers had used the word planet in a way that would violate the IAU definition. “They are doing it because it’s functionally useful,” Metzger said.

“The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be a defined on the basis of a concept that nobody uses in their research,” Metzger says. “And it would leave out the second-most complex, interesting planet in our solar system.”

Moons such as Saturn’s Titan and Jupiter’s Europa have been called planets by scientists since the time of Galileo. It was in the early 1950s, when Gerard Kuiper published a study that made the distinction between planets and asteroids based on how they formed, that scientists started paying more attention to the division.

However, even this reason is no longer considered a factor that determines if a celestial body is a planet, Metzger says.

And since clearing orbit is obviously not a standard in the scientific literature, it shouldn’t have ever been used in the IAU’s controversial 2006 definition, the authors argue.

“We showed that this is a false historical claim,” said co-author Kirby Runyon, with Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “It is therefore fallacious to apply the same reasoning to Pluto.”

If that’s the case, when is an object worthy of being called a planet? The authors of the new study recommend classifying a planet based on its intrinsic properties rather than extrinsic ones that are subject to change, such as the dynamics of a planet’s orbit. Today, the debris and asteroids that fall into Pluto’s path are many, but they may disappear a billion years from nowso they shouldn’t be fundamental to describing a body.

Metzger says that a fundamental criterion to classifying a planet should be whether it’s large enough to allow gravity to mold the object into a sphere.

“And that’s not just an arbitrary definition,” Metzger says. “It turns out this is an important milestone in the evolution of a planetary body, because apparently when it happens, it initiates active geology in the body.”

Modern observations, such as those performed by the New Horizons mission, revealed that Pluto has an active underground ocean, a multilayer atmosphere, organic compounds, and multiple moons.

“It’s more dynamic and alive than Mars,” Metzger says. “The only planet that has more complex geology is the Earth.”

Certainly, this isn’t the last word, but the lively debate is valuable and useful. At the end of the day, even if we might have hurt Pluto’s feelings, we’ll come out of this with a much clearer picture of what a planet looks like.

The findings appeared in the journal Icarus.

Finger-print like pattern observed near al-Idrisi Montes mountain on Pluto. Credit: NASA.

Pluto has frozen ‘sand’ dunes made from methane ice

Finger-print like pattern observed near al-Idrisi Montes mountain on Pluto. Credit: NASA.

Finger-print like pattern observed near al-Idrisi Montes mountain on Pluto. Credit: NASA.

For some time, scientists have been intrigued by strange, regularly spaced ridges, that stuck out of Pluto’s cold, dark plains like thumbprints pressed into ice. Now, a new study suggests that these formations are actually dunes made out of methane “sand,” which is striking considering the dwarf planet’s extremely thin atmosphere.

“When we first saw the New Horizons images, we thought instantly that these were dunes but it was really surprising because we know there is not much of an atmosphere. However despite being 30 times further away from the sun as the Earth, it turns out Pluto still has Earth-like characteristics. We have been focusing on what’s close to us, but there’s a wealth of information in the distant reaches of the solar system too,” said Dr. Jani Radebaugh, Associate Professor in the Department of Geological Sciences at Brigham Young University.

Earth is not the only place in the solar system with dunes. Previously, scientists had spotted them on Mars, Titan, and even on a freaking comet. Now, Pluto, a place few expected could harbor such formations, joins the list of dune-bearing worlds.

“We knew that every solar system body with an atmosphere and a solid rocky surface has dunes on it, but we didn’t know what we’d find on Pluto,” said Dr Matt Telfer, Lecturer in Physical Geography at the University of Plymouth, lead author of the new study published in Science.

The dunes were first spotted by NASA’s New Horizons spacecraft in 2015 when it found methane mounds nestled beside the massive glacier that makes up the western half of Pluto’s Sputnik Planitia, the famous heart-shaped region. The mounds stretch for more than 12 miles and occupy an area equivalent to twice that of Utah Lake.

Pluto's Sputnik Planitia. Credit: NASA.

Pluto’s heart-shaped Sputnik Planitia. Credit: NASA.

The frozen sand whose features resembles earthly dunes is shaped by winds blown on the glacial plain from the direction of a mountain range found along Planitia’s border. Scientists know this because of the shape of the piles of sand whose dark streaks allowed them to retrace the direction from which the wind blew.

This was a surprising observation. With an atmosphere 100,000 times thinner than on Earth and an average temperature hovering at about -230°C, it’s difficult to imagine the wind blowing anything there. The surface patterns recorded by New Horizons don’t lie, though.

Researchers modeled the conditions they saw and concluded that the ‘sand’ is made of nitrogen or methane ice. There’s plenty of nitrogen ice adorning the glacier nearby from the dunes while methane is likely sourced from the snowcaps of nearby mountains like al-Idrisi Montes, which drifts down into the plane.

Once airborne, the particles are pushed by winds that blow between 18 and 24 miles an hour. The winds may be powered by the sublimation of surface ice which turns from solid directly into gas when sunlight hits. The upward force is what drives the piles of particles at the surface.

“On Earth, you need a certain strength of wind to release sand particles into the air, but winds that are 20% weaker are then sufficient to maintain transport. The considerably lower gravity of Pluto, and the extremely low atmospheric pressure, means the winds needed to maintain sediment transport can be a hundred times lower. The temperature gradients in the granular ice layer, caused by solar radiation, also play an important role in the onset of the saltation process. Put together, we have found that these combined processes can form dunes under normal, everyday wind conditions on Pluto,” said Dr Eric Parteli, Lecturer in Computational Geosciences at the University of Cologne.

The researchers also think that the dunes are young, rather than ancient, and might still be active. For instance, Pluto’s heart-shaped central plain has a peculiar polygonal pattern which resembles the surface of gently boiling water. This feature is the result of slow convection in the thick layer or nitrogen and other ices over hundreds of thousands of years, according to one hypothesis. If this is true, the dunes have to be more recent than the action of this convection otherwise they’d be churned apart by it.

Personally, I find it remarkable every time we get to see familiar Earthly features on alien worlds, be them tall mountains, vast deserts, or undulating canals. Sights like these, including these most recent findings on Pluto, remind us that, in some respects, our world isn’t that special at all.

Just 15 minutes after its closest approach to Pluto on July 14, 2015, NASA's New Horizons spacecraft looked back toward the sun and captured a near-sunset view of the rugged, icy mountains and flat ice plains extending to Pluto's horizon. Credit: NASA.

Pluto might have been formed by a billion comets

Pluto might not officially be a planet anymore, but that doesn’t make it’s any less interesting.

One of the most important objects of inquiry regarding the distant dwarf planet has to do with its formation. Now, thanks to unprecedented observations carried out by spacecraft like New Horizons and Rosetta, scientists are coming close to understanding how Pluto came to be. One of the craziest, but still plausible, Pluto formation theories was recently unveiled by researchers at the Southwest Research Institute (SwRI). They suggest that Pluto might have been formed by one billion comets that came together.

Just 15 minutes after its closest approach to Pluto on July 14, 2015, NASA's New Horizons spacecraft looked back toward the sun and captured a near-sunset view of the rugged, icy mountains and flat ice plains extending to Pluto's horizon. Credit: NASA.

Just 15 minutes after its closest approach to Pluto on July 14, 2015, NASA’s New Horizons spacecraft looked back toward the sun and captured a near-sunset view of the rugged, icy mountains and flat ice plains extending to Pluto’s horizon. Credit: NASA.

The solar system‘s planets formed by accretion of material from enormous discs that surround the early Sun. The matter inside the rapidly spinning disk around the parent star simply gathers and forms clumps, steadily accumulating until these turn into asteroids, comets, planets, and moons.

Pluto was always thought to have formed much in the same way. However, geochemist Christopher Glein and colleagues at SwRI’s Space Science and Engineering Division found it odd how the dwarf planet and the famous Comet 67P/Churyumov-Gerasimenko — the one where ESA landed a probe in 2014 — share so much of their chemical makeup. For instance, Pluto’s nitrogen-rich Sputnik Planitia is unnaturally similar to Comet 67P.

Just as water is the main driving force that shapes Earth’s surface, so is nitrogen for Pluto. Due to its low viscosity, nitrogen flows like glaciers on Earth, eroding the bedrock and, in the process, altering the landscape. And there’s so much nitrogen on Pluto. Earth’s atmosphere is made up of 78% nitrogen, but Pluto’s, which is far colder, is 98% nitrogen. That’s an unusual proportion of nitrogen, both at the surface and in the atmosphere, which is not easily explained by the conventional planetary formation theory. Instead, what seems to connect the dots is a huge comet frenzy.

“We’ve developed what we call ‘the giant comet’ cosmochemical model of Pluto formation,” Glein said in a statement.

“We found an intriguing consistency between the estimated amount of nitrogen inside the glacier and the amount that would be expected if Pluto was formed by the agglomeration of roughly a billion comets or other Kuiper Belt objects similar in chemical composition to 67P, the comet explored by Rosetta.”

Real photo of 67P/Churyumov-Gerasimenko. Credit: NAVCAM/ROSETTA/ESA.

Real photo of 67P/Churyumov-Gerasimenko. Credit: NAVCAM/ROSETTA/ESA.

The researchers got their data from the Rosetta probe that reached comet 67P and NASA’s New Horizons mission, which flew by Pluto in July 2015. In the new ‘giant comet’ cosmochemical model, Pluto’s initial chemical makeup is inherited from comet building blocks but was later changed by liquid water.

But this is far from being the last word on the matter. Another competing model suggests that Pluto coalesced from cold ices with a chemical composition closer to that of the sun

“This research builds upon the fantastic successes of the New Horizons and Rosetta missions to expand our understanding of the origin and evolution of Pluto,” said Glein.

“Using chemistry as a detective’s tool, we are able to trace certain features we see on Pluto today to formation processes from long ago. This leads to a new appreciation of the richness of Pluto’s ‘life story,’ which we are only starting to grasp.”

The new study is set to appear in the journal Icarus. It is already available online on the pre-print server arXiv.

These NASA researchers think Pluto is planet, but our definitions are ‘bullshit’ — so they want to change them

One team of NASA scientists wants to set an old wrong right — by making Pluto, among other cosmic bodies, a ‘planet’ again.

Pluto haze.

Haze on Pluto at far higher altitudes than expected, sighted by the New Horizons mission.
Image credits NASA / JPL.

Back in 2006, the International Astronomical Union (IAU) announced that the word “planet” is getting a new definition. The new interpretation excluded many celestial bodies, including Pluto. Something which many people, including yours truly, considered to be a great injustice and plain Not Cool. Luckily, other people agree with me — more to the point, a group of NASA planetary scientists agrees with me.

And what they’re planning might turn Pluto back into the planet it never ceased to be.

A geophysical definition of planets

“It’s bullshit,” is how Alan Stern, principal investigator of NASA’s New Horizons mission to Pluto, sumarrises its exclusion from the rank of ‘official’ planet.

Stern is leading a team of NASA researchers who are proposing a new definition of planets — one that goes further than simply re-instating icy Pluto. The proposal’s goal is to redefine what we consider ‘a planet’, and tie that definition to very simple, easily observable factors.

In NASA fancy-speak, they want any “sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters” to be considered a planet. In common English, they basically want to define planets as the bodies that are big enough to become spherical-ish under their own gravitation, aren’t so big they’ll ignite into a star, and orbit around something — anything, really.

“Why do we say this? We are planetary scientists, meaning we’ve spent our careers exploring and studying objects that orbit stars. We use “planet” to describe worlds with certain qualities,” Stern wrote for the Washington Post.

“When we see one like Pluto, with its many familiar features – mountains of ice, glaciers of nitrogen, a blue sky with layers of smog – we and our colleagues quite naturally find ourselves using the word “planet” to describe it and compare it to other planets that we know and love.”

Some of you may already suspect that the definition this group is proposing also covers a lot of moons — even Earth’s own. The team says this isn’t an oversight, rather, it’s an intended extension of the definition, one that acknowledges the practical realities of day-to-day planetary science.

“Moon refers to the fact that they orbit around other worlds which themselves orbit our star,” Stern continues “but when we discuss a world such as Saturn’s Titan, which is larger than the planet Mercury, and has mountains, dunes and canyons, rivers, lakes and clouds, you will find us – in the literature and at our conferences – calling it a planet. This usage is not a mistake or a throwback. It is increasingly common in our profession and it is accurate.”

The proposed definition also patches some of the gaps in the current IAU planetary classification system:

  1. Under the current system, only planets orbiting our Sun are ‘planets’. Those orbiting other stars, those orbiting freely in the galaxy (rogue planets) aren’t considered to be real ‘planets’.
  2. Secondly, the IAU system requires that planets “clear their neighborhood” (also known as zone-clearing) — that during formation they become large enough for their gravitational pull to bring in and ‘clear’ all matter in their proximity. The problem with this criterion is that “no planet in our Solar System” can satisfy it, the team notes, since there is a number of small cosmic bodies constantly flying through planetary orbits; Earth’s included.
  3. Finally, what the team considers “most severe”, is that zone-clearing is size-dependent: in other words, the criterion becomes completely arbitrary because you can define its “zone” any way you want.

The team agrees that given the breakneck rate at which new planets and new planetary types have been discovered in recent decades, it made sense to ask which kind of objects should be classed as planets. But they also contend that the “process […] was deeply flawed and widely criticized even by those who accepted the outcome.” During the 2006 conference, where the standards were adopted, “the few scientists remaining at the very end of the week-long meeting (less than 4 percent of the world’s astronomers and even a smaller percentage of the world’s planetary scientists) ratified a hastily drawn definition that contains obvious flaws,” Stern adds.

The team is confident that the IAU will reconsider the flaws in its definition sooner or later. And even if that doesn’t happen, Stern says that eventually, this officially-sanctioned definition will have to align itself to “both common sense and scientific usage.”

“The word “planet” predates and transcends science. Language is malleable and responsive to culture,” he concludes.

“Words are not defined by voting. Neither is scientific paradigm.”

The proposal “A geophysical planet definition” has first been presented last March at the annual Lunar and Planetary Science Conference in Houston, Texas.

NASA’s New Horizons probe snaps farthest image from Earth

As the interplanetary New Horizons probe woke up from its hibernating slumber, it turned its telescopic camera toward a field of stars and took a picture — making history.

For a brief period of time, this New Horizons Long Range Reconnaissance Imager (LORRI) frame of the “Wishing Well” star cluster, was the farthest image ever made by a spacecraft, breaking a 27-year record set by Voyager 1. About two hours later, New Horizons later broke the record again. Image credits: NASA/JHUAPL/SwRI.

Launched in 2006, the New Horizons mission stayed true to its name. It performed an unprecedented flyby study of the Pluto system in 2015, offering a dramatic view of the dwarf planet and its satellites. Now, the shuttle is currently on its way to study one or more other Kuiper belt objects.

Most of the time, New Horizons is sleeping — hibernating, to save energy. But every once in a while, it wakes up, and it snaps a few photos which it then beams back to Earth. With its latest wake-up, it broke the record for the farthest image humanity has ever taken away from Earth. Two hours later, it broke it again.

“New Horizons has long been a mission of firsts — first to explore Pluto, first to explore the Kuiper Belt, fastest spacecraft ever launched,” said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado. “And now, we’ve been able to make images farther from Earth than any spacecraft in history.”

New Horizons has observed several Kuiper Belt objects (KBOs) and dwarf planets at unique phase angles, as well as so-called Centaurs — former KBOs in unstable orbits that cross the orbits of the giant planets on the edge of our solar system. These December 2017 false-color images of KBOs 2012 HZ84 (left) and 2012 HE85 are, for now, the farthest objects from Earth ever captured by a spacecraft. They’re also the closest-ever images of Kuiper Belt objects. Image credits: NASA/JHUAPL/SwRI

When you’re traveling through 700,000 miles (1.1 million kilometers) of space each day, nothing stands still. New Horizons is one of only five shuttles which has managed to reach the escape velocity required to exit the solar system. It’s not just taking awesome photos on its path, but also carrying measurements of the plasma, dust and neutral-gas environment along the way, enabling astronomers to better understand the outskirts of our solar system. Now, it’s heading towards the Kuiper Belt, the circumstellar disk surrounding our solar system.

The Kuiper Belt is similar to the asteroid belt but is far larger: 20 times as wide and 20 to 200 times as massive. Specifically, New Horizons is targeting 2014 MU69, a mysterious object (or pair of two objects) which Alan Stern, mission principal investigator from the Southwest Research Institute (SwRI), has called “provocative” and a “scientific bonanza.”

New Horizons is reportedly healthy and everything is functioning as planned. The next awakening is scheduled for June 4. When that happens, it will break the record again.

You can follow New Horizons’ epic adventure here.

Plutonian landscapes in twilight, under a hazy sky. Credit: NASA/JHU APL/SwR.

Layers of hydrocarbon haze could explain why Pluto’s so super-cold

Plutonian landscapes in twilight, under a hazy sky. Credit: NASA/JHU APL/SwR.

Plutonian landscapes in twilight, under a hazy sky. Credit: NASA/JHU APL/SwR.

Orbiting the sun over 40 times farther away than the Earth, it’s no wonder that Pluto is so incredibly cold. But when New Horizon made its historic flyby of the dwarf planet, NASA scientists were dumbfounded to find Pluto was even colder than expected — about 30 degrees Celsius (86 degrees Fahrenheit) colder. A new hypothesis suggests that a haze of solid hydrocarbons might be regulating Pluto’s temperature. If this hypothesis is confirmed, it would signify a new regime of planetary climate; something that’s never been witnessed before.

Pluto might not be a planet but at least it’s still cool

Researchers at the University of California, Santa Cruz, led by planetary scientist Xi Zhang, believe they’ve found the culprit for Pluto’s anonymously cold atmosphere. The New Horizons spacecraft revealed that the dwarf planet is surrounded by 20 or so layers of haze arranged like the skin of an onion. The haze is made of soot-like solid particles.

Scientists had known for decades that Pluto has an atmosphere and that it might be hazy, but it wasn’t until recently that they began to comprehend this stunningly complex atmospheric mechanism.

Zhang and colleagues devised a mathematical model that investigated whether or not the nanoparticles could be influencing Pluto’s atmospheric temperature. Indeed they can, the model suggested, whose results were almost a perfect match with New Horizon’s empirical observations.

The particles are no bigger than 150 nanometers in diameter and are thought to consist of hydrogen cyanide, acetylene, and other organic compounds, similar to the ones found around Saturn’s moon Titan. These particles might have a significant cooling effect on Pluto by absorbing infrared radiation, thereby reducing atmospheric temperature.

 “Basically, we needed a strong coolant to explain why Pluto is so cold,” Zhang told New Scientist. “We found that the abundant haze particles can strongly cool the atmosphere by re-emitting infrared radiation to space, a process not considered in previous theories.”

“In the infrared range of radiation, a slightly larger amount of energy is radiated back to space by the haze particles, cooling the atmosphere overall,” he added.

From six billion miles away, Earth-based detectors aren’t sensitive enough to identify Pluto’s near-infrared radiation, which could be one of the reasons why the haze wasn’t discovered sooner. Fortunately, the James Webb Space Telescope, which is scheduled to launch in 2019, will be equipped with the proper tools to investigate Pluto’s infrared radiation.

If the researchers’ hypothesis holds water, this would make Pluto the only solar system planetary body whose temperature isn’t principally controlled by gases.

Scientific reference: X Zhang, D F Strobel and H Imanaka, Nature, 2017, DOI: 10.1038/nature24465.

Proposed ‘geophysical definition’ re-instates Pluto as a planet — and adds 100 new ones in the Solar System

Pluto, along with a host of more than 100 other bodies in the Solar System should be planets, says Kirby Runyon from the John Hopkins University.

Pluto in its frozen beauty, captured by the New Horizons craft.
Image credits NASA.

Eleven years ago, the International Astronomical Union demoted Pluto, who had done nothing wrong and just kept on orbiting as always, to the status of “non-planet”. Many people, scientists and laymen and I, cried bloody murder at the ruling — we had just lost one of the nine Solar planets. And we were not happy.

Kirby Runyon from the Johns Hopkins University would see justice returned to icy, tiny Pluto, and the debate about what a planet actually is finally settled. Runyon is the lead author of a paper making a case for re-instating Pluto as a planet which will be presented next week at a scientific conference in Texas. Even more, he says that a host of moons (including Europa and the Moon) and other bodies (like these guys here) in the Solar System should be planets, too.

What does a planet make

Back in the glory days, Pluto was the smallest of the nine recognized planets, with a diameter just under three-quarters of the Moon’s, and roughly one fifth of Earth’s. This small size eventually led the IAU to demote Pluto in 2006, but Runyon says it shouldn’t have been the case — Pluto “has everything going on on its surface that you associate with a planet. […] There’s nothing non-planet about it.”

Runyon, whose doctoral dissertation focuses on changing landscapes on the moon and Mars, led a team of six researchers from five institutions in drafting a proposed new definition of “planet”, and a justification for the new system of classification, which will be presented at the Lunar and Planetary Science Conference’s poster session. The authors were all members of the science teams part of the New Horizons missions — the spacecraft became the first man-made object to fly by Pluto, and also captured the first close-up images of the former planet.

The one point that led to Pluto’s demotion was that the IAU standard required all planets and their satellites to move alone through their orbits — and Pluto is too tiny to completely accrete all the matter it passes by on its orbit. The other two criteria — that it orbits a star and is kept round by gravity — were met. Principal investigator for the New Horizons mission Alan Stern of the Southwest Research Institute in Boulder, Colorado, has argued that the IAU definition also excluded Earth, Mars, Jupiter, and Neptune, which also share their orbits with asteroids, so if we demoted Pluto why not these four planets as well?

If it looks like a planet and quacks like a planet…

Charon, Pluto’s largest satellite, can also be considered a planet under the new system.

To settle the debate, the team argues that the qualities intrinsic to a celestial body should carry more weight than external factors (such as orbit) when deciding it if is a planet or not. They settled on the definition of a planet as being “a sub-stellar mass body that has never undergone nuclear fusion” and has enough gravitational pull to maintain a more-or-less round shape.

This geophysical definition goes against the grain of the three-element astronomical system the IAU currently employs in that it makes no reference to the body’s surroundings, so deciding if a planet is officially a planet should become much easier and immediately apparent under Runyon’s system. Most planetary scientists are also generally trained as geoscientists more than astrologists, so a geophysical definition might suit them better than the old classification.

This definition of a planet would certainly cast a wide net. While it isn’t extended to stars, black holes, asteroids, and meteorites, it does cover pretty much everything else in our solar system — the number of Solar planets would increase from eight to almost 110. And that whopping increase is actually a good thing, Runyon says, as he thinks it will engage the public in space exploration. The very word “planet” seems to carry a “psychological weight,” he adds, so more planets could help pique public curiosity and instill a yearning for exploration in people.

Io, Jupiter’s innermost satellite, would also become a planet.
Image credits NASA.

“I want the public to fall in love with planetary exploration as I have,” Runyon added. “It drives home the point of continued exploration.”

The team’s definition doesn’t require approval from a central governing body for scientists to start using it — in fact, it’s already been adopted by Planet Science Research Discoveries, an educational website founded by scientists at the University of Hawaii. If you’re one of those who feels the Solar System is the less for Pluto’s demotion, science says you can now have it back. Along with a hundred new members.

So what do you think of the new classification system? Is it a change for the better, a simplification of a cumbersome system — or complete and utter anarchy? Let us know in the comments.

The poster will be on view for a full day on March 21 at the conference sponsored by the Lunar and Planetary Institute, and Runyon will be on hand for at least three hours to answer questions about it according to Johns Hopkins University.

The other authors are Kelsi Singer of the Southwest Research Institute in Boulder, Colorado; Tod Lauer of the National Optical Astronomy Observatory in Tucson, Arizona; Will Grundy of the Lowell Observatory in Flagstaff, Arizona; Michael Summers of George Mason University in Fairfax, Virginia.

pluto heart

Pluto’s ‘heart’ might be filled with an ocean of liquid water

We’ve learned a lot from New Horizons’ flybys of Pluto, such as confirming it has an active geology, a climate, and almost certainly many more secrets waiting to be found. One of them might be an ocean of liquid water buried beneath Pluto’s thick nitrogen ice surface.

pluto heart


When New Horizons beamed back its best photos of the dwarf planet, it also showed to the world the largest geological feature on Pluto — a bright, heart-shaped Tombaugh Regio. In the western part of ‘the heart’ as it’s also called, lies the Sputnik Planum lobe, which is a 1000-km-wide plain of nitrogen and other ices.

Evidence suggests that Sputnik Planum was created by a giant impact with a cosmic body at least 200 kilometers across. Subsequently, this cataclysmic event should have displaced enough material to cause a negative gravitational anomaly because a crater is nothing but a big hole in the ground — less mass means less gravitational pull. At least, that’s what ought to have happened.

Instead, Sputnik Planum bears a positive gravitational anomaly which means something is filling that crater, and a team from Brown University think it’s liquid water.

“Thermal models of Pluto’s interior and tectonic evidence found on the surface suggest that an ocean may exist, but it’s not easy to infer its size or anything else about it,” said Brandon Johnson, who is an assistant professor in Brown’s Department of Earth, Environmental and Planetary Sciences. “We’ve been able to put some constraints on its thickness and get some clues about composition.”

Like Earth and the Moon, Pluto and its largest moon Charon are tidally locked, meaning the two always show the same face as they rotate. This happened over time and as Charon’s gravity pulled proportionately more on areas of higher mass, the two bodies became aligned on a tidal axis. Sputnik Planum sits directly on this tidal axis.

After the impact, the massive crater could have been filled with nitrogen with ice progressively layering on top of each other. But that couldn’t have been enough to cause the positive gravitational anomaly. Instead, what the Brown University researchers think happened was that liquid water, which is denser than ice, upwelled following the impact and evened the crater’s mass. Then, the nitrogen ice deposited on top of this liquid water would have been enough to explain the positive gravitational anomaly.

“This scenario requires a liquid ocean,” Johnson said. “We wanted to run computer models of the impact to see if this is something that would actually happen. What we found is that the production of a positive mass anomaly is actually quite sensitive to how thick the ocean layer is. It’s also sensitive to how salty the ocean is, because the salt content affects the density of the water.”

The computer models suggest this tentative layer of liquid ocean ought to be more than 100 kilometers thick, with a salinity of around 30 percent, as reported in Geophysical Research Letters

“What this tells us is that if Sputnik Planum is indeed a positive mass anomaly —and it appears as though it is — this ocean layer of at least 100 kilometers has to be there,” Johnson said. “It’s pretty amazing to me that you have this body so far out in the solar system that still may have liquid water.”

New NASA image shows first cloud on Pluto

A spectacular new image from the New Horizons image highlights what could very well be the first cloud on the dwarf planet.


The New Horizons space probe was launched back in 2006, and in late 2014, neared Pluto and snapped a trove of spectacular images of the former planet. However, because the probe can only send information at a rate of 38 kbps, it takes a very long time to receive all the information. New Horizons is sending back millions of photos and a new such intriguing photo was recently published by NASA.

It shows Pluto from a distance of about 13,400 miles, details the dark and rugged mountains as well as an area called “Pluto’s twilight zone.”

“The topography here appears quite rugged, and broad valleys and sharp peaks with relief totaling 3 miles (5 kilometers) are apparent,” NASA officials wrote.

But more intriguingly, the picture depicts something which looks like a cloud, shining through Pluto’s complicated layers of haze. If it really is a cloud, then it may very well be the only one ever spotted on the dwarf planet.

At the moment, we don’t know for sure if it is a cloud, and we probably won’t know for sure anytime soon. But if it looks like a cloud, and it can be a cloud… then it’s probably a cloud.

“Atmospheric models suggest that methane clouds can occasionally form in Pluto’s atmosphere,” said NASA in the same statement.

As for the rest of the picture, it’s quite useful itself, as NASA writes:

“These silhouetted terrains therefore act as a useful “anchor point,” giving New Horizons scientists a rare, detailed glimpse at the lay of the land in this mysterious part of Pluto seen at high resolution only in twilight. The scene in this inset is 460 miles (750 kilometers) wide.”

We’re only recently starting to learn just how complex and full of surprises Pluto really is. Other findings announced by NASA include:

  • Like Earth, Pluto has a long ion tail, that extends downwind at least a distance of about 100 Pluto radii (73,800 miles/118,700 kilometers, almost three times the circumference of Earth), loaded with heavy ions from the atmosphere and with “considerable structure.”
  • Pluto is geologically active.
  • Pluto’s obstruction of the solar wind upwind of the planet is smaller than had been thought. The solar wind isn’t blocked until about the distance of a couple planetary radii (1,844 miles/3,000 kilometers, about the distance between Chicago and Los Angeles.)
  • Pluto has a very thin boundary of Pluto’s tail of heavy ions and the sheath of the shocked solar wind that presents an obstacle to its flow.

Did Pluto’s moon Charon harbor an ancient ocean?

As NASA’s New Horizons shuttle zoomed past Pluto, it snapped awesome photos not only of the “ex-planet”, but also of its moons. Now researchers are analyzing those pictures and reporting surprising finds – such as an ancient ocean on Charon, Pluto’s moon.

Too big for its skin?

Pluto’s Moon Charon A close-up of the canyons on Charon, Pluto’s big moon, taken by New Horizons during its close approach to the Pluto system last July. Multiple views taken by New Horizons as it passed by Charon allow stereo measurements of topography, shown in the color-coded version of the image. The scale bar indicates relative elevation.

The side of Pluto’s largest moon viewed by NASA’s passing New Horizons exhibits feature of “pull apart” tectonic faults. Faults are basically discontinuities in large volumes of rock, with significant displacements. The mass of rock can move towards each other, away from each other or slide by each other in a parallel line. Pull apart faults are areas where rocks move away from each other, and they are usually expressed by ridges, scarps and valleys – all of which can be observed on Charon. Some valleys are actually 4 miles (6.5 kilometers) deep, which is extremely impressive when you consider that its entire mean radius is 606 kilometers.

However, Charon can’t have tectonics like Earth has, because it doesn’t have a hot core and mantle that circulates under the crust, so researchers propose a different mechanism. Charon’s outside is primarily water ice. It’s all frozen, but this might have not always been always the case. When Charon was young, this layer was probably kept warmer by heat provided by the decay of radioactive elements, as well as Charon’s own internal heat of formation. It was probably hot enough to melt some of the water, so Charon had a subsurface ocean. But as Charon cooled over time, this ocean would have frozen. When water freezes, it expands, lifting the outer layers and pulling them apart.

This process makes a lot of geological sense, but the entire process must have been extremely violent. Charon exhibits one of the longest systems of chasms in the solar system,  running at least 1,100 miles (about 1,800 kilometers) long and reaching 4.5 miles (7.5 kilometers) deep. By comparison, the Grand Canyon is 277 miles (446 kilometers) long and just over a mile (1.6 kilometers) deep.


NASA finds vast quantities of frozen water on Pluto

New data provided by the New Horizons mission showed that water ice on Pluto is much more common than we thought.


Pluto turned out to be full of surprises. Not only does it have a thin atmosphere and is geologically active, but also hosts vast quantities of frozen water. The false color image above was derived from observations done with the Ralph/Linear Etalon Imaging Spectral Array (LEISA) instrument aboard New Horizons and shows where water is abundant.

According to this data, water seems to be considerably more widespread across Pluto’s surface than was previously known. Previous findings were deemed inaccurate because while they did detect ice, methane ice often masked water ice. The much more sensitive method used on the right involves modeling the contributions of Pluto’s various ices all together. This method has its own limitations in that it only highlights ices included in the model, but the model is constantly finessed.

“Water ice is Pluto’s crustal “bedrock,” the canvas on which its more volatile ices paint their seasonally changing patterns. Initial New Horizons maps of Pluto’s water ice bedrock compared LEISA spectra with a pure water ice template spectrum, resulting in the map at left,” NASA wrote.

Interestingly though, the data revealed little or no ice in the area called Sputnik Planum (the left or western region of Pluto’s “heart”) and Lowell Regio (far north on the encounter hemisphere). This could mean either that there is no water there, or that it is hidden beneath a thick blanket of methane ice (or something else). Sputnik Planum is a 1000-km-wide plain of frozen nitrogen and carbon monoxide ices, divided into polygonal cells which astronomers believe to act like convection cells.


New Horizons images suggest Pluto is geologically active

NASA released a stunningly colorful new image of the dwarf planet Pluto, the latest in a series of images that steadily trickle down from the New Horizons probe since it left the solar system this July. And it’s not only eye candy either; the features this picture reveals has left the smart guys at the agency scratching their heads.

Groovy. Image via NASA

Image via NASA

One of the most interesting features is the large, heart-shaped lobe on the western side of Pluto, named the Sputnik Planum. What’s striking about it is, ironically, what doesn’t seem to be striking it — there are no craters here. Researchers were left scratching their heads at this huge but crater-free plain; a lot of meteors hit the dwarf planet and have been doing so since before humans evolved, so why isn’t there any evidence of impact?

The only explanation is that the surface is new — NASA estimates that the Planum is only about 10 million years old. When compared with the humongous 4.5 billion-years estimated age of the solar system, it suggests that you can find the great heat and pressure needed to reshape rocks on such a huge scale on the dwarf, that Pluto is still geologically active.

“It’s a huge finding that small planets can be active on a massive scale, billions of years after their creation,” said New Horizons lead investigator, Alan Stern.

The photo was presented at the Division of Planetary Sciences of the American Astronomical Society meeting in National Harbor, MD on Monday, and the team also presented a number of other findings: information about Pluto’s thin and hazy atmosphere, and the discovery of what they believe to be an ice volcano on the surface.

Still, the lack of craters on Sputnik Planum remains one of the most stunning discoveries about the planet. The surface couldn’t have been exposed to the meteors and asteroids barreling through space for very long, meaning it was “added” quite recently. Other areas of Pluto are much older, some close to the 4 billion years mark.

Stern believes that the dwarf planet has been geologically active for much of it’s life. However, as the planet is so small, any heat it might have retained after its creation would have dissipated into space a long time ago — so the scientists don’t know what could be driving geological processes on Pluto.

But now, at least, they have a good starting point.



What makes a planet? Finally a simple formula that includes exoplanets as well


Image credit: Mark Garlick, space-art.co.uk.

There is only one clear set of official criteria by which we class or not a cosmic body as a planet. These criteria introduced in 2006 by the International Astronomical Union saw Pluto demoted to the status of dwarf planet. Oddly enough, though, these rules were made for classing planets only in our solar system, meaning the 5,000 exoplanets (what should be planets outside our solar system) identified so far are now in a “definitional limbo,” according to planetary astronomer Jean-Luc Margot at UCLA. Luckily, Margot has come up with a solution which seems to work marvelously.

What’s a planet?

Some scientists believe that if Pluto remains classified as a planet, then the dozens of Kuiper Belt Objects (KBO's) orbiting our Sun would also be classified as planets. Our solar system would have the 9 original planets, an additional 43 KBO's, and more as they are cataloged.

Some scientists believe that if Pluto remains classified as a planet, then the dozens of Kuiper Belt Objects (KBO’s) orbiting our Sun would also be classified as planets. Our solar system would have the 9 original planets, an additional 43 KBO’s, and more as they are cataloged.

The word “planet” comes from the Greek word “Planetes” – meaning wanderer. The ancient greek astronomers called this objects so since they found some points of light seemed to wander around the sky throughout the year. Prior to 2006, there was no scientific definition for the word planet, and people used the term loosely. Eventually, a consensus had to be reached given the increasing body of astronomical data and knowledge. The debate of what makes a planet got hotter once scientists discovered an object larger than Pluto within the Kuiper Belt, called Eris. In this dense cloud of ancient objects – remnants from the bygone early years of the solar system – there are still many large objects that remain unidentified. The solar system could have grown to dozen of planets, if we were to judge by size alone.

Astronomers of the International Astronomical Union (IAU) eventually voted that for an object to be classed as a planet, it needs to meet three strict criteria:

  • It must orbit the sun directly.  That excludes moons because although they indirectly orbit the Sun, they directly orbit a planet.
  • It must be big enough for gravity to squash it into a round, almost spherical shape. That excludes the asteroids.
  • it must have cleared other objects out of the way in its orbital neighborhood ( sweep up the trash). To clear an orbit, a planet must be big enough to pull neighboring objects into the planet itself or sling-shot them around the planet and shoot them off into outer space. This was eventually what caused Pluto’s demotion to dwarf planet status. This third criteria also became the object of intense debate following the 2006 ruling, which was made by less than 1% of all working astronomers (they’re not that many really).

While controversial, a strict definition of what makes a planet was long called for. However, the definition – as it was formulated – is strictly valid for objects inside the solar system only. What about the thousands of exoplanets discovered by Kepler and ground-based telescopes? Technically, they’re not planets it seems.

Margot is proposing to extend the planetary definition to planets outside the solar system, and he’s find a clever way too to circumvent the technical challenges. His method requires only estimates of the star’s mass and the planet’s mass and orbital period (all easily obtainable) to verify if the candidate planets verify the three criteria set forth by the IAU. “One should not need a teleportation device to decide whether a newly discovered object is a planet,” Margot said.

Margot’s test can be used to determine whether a body can clear a specific region around its orbit within a specific time frame, such as the lifetime of its host star. Margot says the test can be used to  classify 99 percent of all known exoplanets. When his mathematical computations are applied to the solar system, it clearly places the eight planets into one distinct category and the dwarf planets — Ceres, Pluto and Eris — into another.

“The disparity between planets and non-planets is striking,” Margot said. “The sharp distinction suggests that there is a fundamental difference in how these bodies formed, and the mere act of classifying them reveals something profound about nature.”

The next IAU general assembly is scheduled for 2018. Hopefully, Margot’s paper will be discussed at the meetings.

Astropicture of the Week: Pluto

We’ve seen Pluto in all its splendor recently thanks to the New Horizons mission that flew by the former planet / currently planetoid, but just when you thought it can’t surprise you anymore… something like this comes along. This is Pluto.


Recently, a paper was published based on the data that New Horizons sent back to Earth, including impressions about Pluto and its moons: Charon, Styx, Nix, Kerberos and Hydra. This is important not only to understand the Pluto system, but it can provide valuable information about our solar system in itself.

“All of the data about Pluto and its moons (even the little bitty moons) and 67P, and the next Kuiper Belt object we go to with New Horizons in a little over three years – all of this will feed into trying to understand how the Solar System formed,” said Prof Bill McKinnon from Washington University in St Louis, Missouri.