Tag Archives: Oort Cloud

What is the Oort cloud: the very edge of the solar system

Every system needs a boundary, and the Solar System is no different. Although we haven’t been able to physically reach and see it, we have a theory about what this edge looks like. And its name is the Oort cloud.

The hypothetical structure of the Oort cloud: an inner, disk-shaped cloud surrounded by a spherical outer structure. Image credits Pablo Carlos Budassi via Wikimedia.

Ask anyone on the street where the Solar System ends, and they’re likely to mention Pluto. To a certain extent, it wouldn’t be wrong; Pluto really is one of the farthest planets / dwarf planets from the Sun; as of early 2021, the farthest object in the Solar System is Farfarout . But if we want to be all sciency about it — and we do — the Solar System arguably ends where our star’s gravitational influence becomes too weak to capture and hold objects. In other words, it spans over all the space where the Sun is the dominating tidal force (Smoluchowski, Torbett, 1984). Exactly what constitutes the edge of the Solar System is still up to some debate, however, and some sources — including this post by NASA — consider the space beyond the heliosphere as being ‘interstellar space’.

For the purposes of this post, however, we’ll take the volume where the Sun’s gravity reigns supreme as being the Solar System. The point where that influence ends is far, far away from Earth. So far away, in fact, that we’ve never been able to actually see it, and, realistically speaking, there’s no way humanity will reach there while any of us reading this are still alive. But we do have some theories regarding what goes on out there.

The boundary of the Solar System is marked by a hypothetical structure known as the Oort cloud. We estimate that it is a truly vast expanse filled with varied clusters of ice, from innumerable tiny chunks up to a few billion planetesimals of around 20 kilometers (12 mi) in diameter. There are likely a few rocky or metallic asteroids here, as well, but not many in number. The material in the Oort cloud was likely drawn to its current position through the gravitational influence of the gas giants — Jupiter, Saturn, Neptune, and Uranus — in the early days of the Solar System.

All in all, it is one of the most exciting places humanity has not yet reached.

So what is it?

The thing to keep in mind here is that the Oort cloud is a hypothetical structure. We haven’t yet seen it, nor do we have any direct evidence of it being real. But its existence would fit with other elements and phenomena we see in the Solar System, and it does fit our theoretical understanding of the world around us, as well.

The Oort cloud is a vast body. Since it’s a theoretical structure, there’s quite a bit of uncertainty in our estimates of its size. Still, we believe it stretches from around 0.03 to 0.08 light-years from the Sun, although other estimates put its outer boundary at 0.8 light-years from the Sun. There are also estimates that place it between 1.58 and 3.16 light-years away from the Sun. Needless to say, we don’t have a good handle on exactly where it is, and how large it is.

“It is like a big, thick-walled bubble made of icy pieces of space debris the sizes of mountains and sometimes larger. The Oort Cloud might contain billions, or even trillions, of objects,” NASA explains.

But to give you a rough idea of the distances involved, however, we’ll use Voyager 1, the fastest-going probe we’ve ever sent to space, and the one currently farthest away from Earth. On its current course and acceleration, Voyager 1 would reach the Oort cloud in around 300 years; it would take it some 30,000 years to pass through the cloud (depending on its actual dimensions).

Voyager 1. Image credits NASA via Global Panorama / Flickr.

Still, don’t get too excited. None of the space probes humanity has launched so far will still be operational by the time they reach the Oort cloud; despite being powered by RTGs, a type of nuclear battery, all of these crafts will run out of power far before they reach the Oort clour.

Why do we think it’s a thing?

The concept of the Oort cloud was first suggested in the 1930s by Estonian astronomer Ernst Öpik. The idea was cemented in the 1950s when its existence was independently suggested a second time by Jan Oort, a Dutch astronomer. Because of this dual origin, it is sometimes referred to as the Öpik–Oort cloud.

The existence of this cloud was proposed mainly due to comets — long-period and Halley-type comets, to be precise. Since comets coming close to the Sun lose part of their volatile contents (for example water) under the influence of solar radiation, logic dictates that they must form away from the star. At the same time, gravitational influences would see them either collide with a planet or star or be ejected from the Solar System eventually — meaning that their ‘lifespan’ is limited. Since there are still comets zipping around the Sun, this means that there must be a reservoir of comets to be drawn towards our star.

Put together, both of these point to the existence of a cloud-like formation at the very edge of the Sun‘s gravitational influence populated with comet-like bodies — the Oort cloud.

Short-period comets orbit around the Sun every few hundreds of years; because of this short time, it’s generally accepted that they originate from structures closer to Earth, such as the Kuiper belt (a field of asteroids extending past Neptune). Long-period comets, however, can have orbits lasting thousands of years. The only source that could explain such huge spans of time is the Oort cloud. One exception to these rules is Halley-type comets. Although they are short-period comets, we believe they’re originally from the Oort cloud and that they’ve been pulled ever closer to the center of the Solar System under the gravitational effects of the Sun and inner planets.

What are we doing to study it?

The main impediment to our studying of the Oort cloud is distance. It’s simply too far away for our spaceships or probes to reach in any practical manner. There also haven’t been any direct observations of the Oort cloud.

Despite this, its existence is widely accepted in academic circles. Researchers rely on indirect methods of study to peer into the secrets of the Oort cloud. These revolve heavily around the study of comets and their properties. There is also a lot of effort being poured into developing devices and methods that can be used to spot individual bodies inside the Oort cloud. This is no easy feat, as they’re quite tiny by cosmological standards, and very far away.

Once we do have such tools at our disposal, however, astronomers will finally be able to confirm whether the Oort cloud actually exists. It’s very likely that it does, and it would fit with our current understanding of the Universe. But until we can see it, we won’t be able to tell for sure.

Saturn’s moon Titan may be older than Saturn itself

Titan is in the spolight again! After astronomers spotted a passing geological feature, now a joint team from NASA and ESA found evidence that the moon may have formed before its planet.

Generally, moons take shape after planets – but now, researchers have found convincing evidence that the nitrogen in Titan’s atmosphere originated in ancient conditions, in the cold birthplace of the most ancient comets from the Oort cloud — a spherical shell of icy particles that enshrouds the Solar System.

The evidence they found was isotopical. Isotopes provide valuable insight into the origin of things – be them planets or rock samples. Basically, in planetary sciences, there are many cases where the ratio of one isotope to another can provide crucial information regarding the age of planets – the ratio of isotope A to isotope B tells you how old it is (sort of).

Kathleen Mandt from the Southwest Research Institute in San Antonio and colleagues analyzed the ratio of nitrogen-14 (seven protons and seven neutrons) to nitrogen-15 (seven protons and eight neutrons) in Titan’s atmosphere.

“When we looked closely at how this ratio could evolve with time, we found that it was impossible for it to change significantly,” Mandt said in a press release. “Titan’s atmosphere contains so much nitrogen that no process can significantly modify this tracer even given more than four billion years of Solar System history.”

What they found was that the Solar System was simply not old enough for the isotope ratio to change like it has – which seems to indicate that Titan has its origin in the Oort Cloud. The Oort cloud is a cloud of predominantly icy debris believed to surround the Sun at up to 50,000 AU; if their results are correct, then Titan was created before the Earth and most of the Solar System as a dwarf planet and then was captured into orbit around Saturn.

“This exciting result is a key example of Cassini science informing our knowledge of the history of [the] Solar System and how Earth formed,” said Scott Edgington, Cassini deputy project scientist at NASA’s Jet Propulsion Laboratory.

The research was published this week in the Astrophysical Journal Letters.

 

Mysteriously well preserved Oort Cloud object cruises towards our solar system

It’s the outskirts of our solar system – spherical particles of rocks and ice, way beyond the familiar planets, and even the former planet Pluto, there lies the Oort Cloud – a spherical cloud of predominantly icy planetesimals that may lie roughly 50,000 AU from the Sun (AU = the distance from the Sun to the Earth).

The Holy Grail of astrophysics

oort cloud

Believed to be a remnant of the early solar system, there is still a lot of debate surrounding the Oort cloud – but imagine what astrophysicists could learn from studying it from close range. Unfortunately, we’re nowhere near being capable of sending a probe to such distances from the Sun; but there is still hope! If we can’t go to the Oort Cloud, maybe the Oort Cloud can come to us.

Called 2010 WG9, a trans Neptunian object never went close to the Sun, preserving its icy surface intact for an incredibly long period of time. If the theories are correct, it is basically an object that has maintained its structure since the dawn of the solar system.

“This is one of the Holy Grails of Planetary Science – to observe an unaltered planetesimal left over from the time of Solar System formation.”

Not even close to Uranus

But wait a minute, you may think – don’t comets come from the Oort cloud, shouldn’t we be worried? Well you’re right, that’s where they come from due to a gravitational perturbance – but there’s no real reason to worry. 2010 WG9 won’t ever get really close to the Sun – as a matter of fact, it won’t ever get closer than Uranus.

But things get even better! Comets are extremely hard to study, because they are surrounded by bright clouds of dust and gas, and they usually come closer to the Sun, with their original ice being evaporated and frozen again.

So while we have a really high number of objects from the Oort cloud, but we can’t really study them – and that’s when 2010 WG9 steps in! Astronomers at Yale University have observed 2010 WG9 for over two years, taking images in different filters, studying at different wavelengths. They analyzed it with four filters – B, V, R, and I, which means: blue, visible, red, and infrared wavelengths. What did they see? A quick change in color, over the cours of just a few days.

The source is what can only be described as a patchy surface – but let’s rewind a little bit. Say you have a blue filter, and you’re looking at the Earth through it – the oceans, seas, and everything blue will turn out much brighter, and anything else would appear much dimmer. So when the color is changing at 2010 WG9, that likely means that it has an exposed part, probably due to a relatively recent impact.

Rabinowitz was very keen to explain that 2010 WG9 has an unusually slow rotation: most such objects rotate every few hours, but this body from the Oort Cloud takes a stunning 11 days to rotate!The best reason for this discrepancy is that it exists in a binary system – it is tidally locked to another body, which gradually slows it. But the really valuable information is about the Oort Cloud.

“Very little is known about the Oort cloud – how many objects are in it, what are its dimensions, and how it formed,” Rabinowitz explained. “By studying the detailed properties of a newly arrived member of the Oort cloud, we may learn about its constituents.”

Scientific Article

Green visitor to pass through the solar system

Unfortunately for some, the green visitor is not an alien coming to greet us but something else, even though it also comes from far, far away. On the 24th of February the comet Lulin will pass within 61 million kilometers of Earth, thus being able to be seen with the naked eye from some locations.

This is the first time the comet is visiting the solar system, passing at a distance that is about 41% of the one between Sun and Earth and 160 times farther than the Moon. The Lulin comet was discovered by a Chinese student, Quanzhi Ye, in 2007 and it was named after the observatory where it was seen the first time.

The comet’s greenish cast is given by the two gases -cyanogen and diatomic carbon- from its Jupiter-sized atmosphere, which are lighted by the sun.

It is estimated that it will reach a maximum brightness of 4th or 5th magnitude, which means that it could be seen without using special instruments from some locations, but binoculars or a small telescope would certainly make it an easy target.

There is, however an element of unpredictability as the comet will lie close to Saturn in the constellation Leo. Saturn will certainly be able to be seen with the naked eye, however. The comet lulin is believed to have originated in the deep freeze of the Oort Cloud, which is at a distance thousands of times bigger than the one between Sun and Earth.

If this hypothesis is correct, then Lulin was bombarded  for eouns by cosmic rays, which are highly common in space, thus creating a thick crust of organic compounds. This crust might harden the process through which frozen water and other volatile ices turn into gas because of the heat of the sun, creating its famous tail. This may make the apparition of the comet less spectacular.

Several telescopes, including NASA’s Infrared Telescope Facility on Hawaii’s Mauna Kea, will be used to examine the polarization of light reflected from the comet’s halo, especially as the earth will be between it and the Sun, this rare configuration making the whole process easier. By this, it will be established what size and shape the dust particles from the tail have and also if they are smooth or rougher. Previos observations have shown that carbon monoxide and methane are part of the comet’s chemical composition.

However, Brian Marsden of the Harvard-Smithsonian Center for Astrophysics in Cambridge claims that the comet does not really deserve that much attention. By the time it will have got closer to the Earth, there will probably not be very much to see. So, if you have an insomnia and you are not really fond of astronomy you might as well watch TV.
Source: University of Leicester