Tag Archives: Hygiea

Meet Hygiea, the Smallest Dwarf Planet in Our Solar System

Around 2 billion years ago, two large rock bodies hit each other in the main asteroid belt, a region between the orbits of Mars and Jupiter populated by fragments of rocks of various sizes. The impactor, with a size ranging from 75 to 150 kilometers in diameter, hit a body at least 4 times larger. Astronomers have known about this impact for a long time because it created a whole family of asteroids in the main asteroid belt, formed by the celestial body Hygiea and almost 7,000 smaller asteroids that have similar orbits.

The new image of Hygiea obtained by SPHERE reveals its round shape, which makes Hygiea the smallest dwarf planet found to date. Credit: ESO/P. Vernazza et al./MISTRAL algorithm (ONERA/CNRS)

Hygiea itself has been considered an asteroid since it was discovered in 1849 by Italian astronomer Annibale de Gasparis. With a diameter just over 430 kilometers, it is the fourth-largest object in the main asteroid belt. New observations obtained with the Very Large Telescope (VLT), located in Chile and operated by the European Southern Observatory, have revealed that Hygiea is also round.

Determining the shape of Hygiea doesn’t have any practical implications for its orbit or behavior, but it’s enough to propel Hygiea from asteroid to dwarf planet, according to current scientific classifications.

There are four conditions that solar system objects must meet to be classified as dwarf planets: They must orbit the Sun, not be a satellite orbiting another body, not be massive enough to clear their orbit from other objects, and have a round shape due to their own gravity.

Fulfilling all the requirements makes Hygiea the smallest dwarf planet in the solar system, as researchers report in Nature Astronomy, taking the position from Ceres, which has a diameter of 950 kilometers. Pluto is the largest dwarf planet, with a diameter of 2,400 kilometers. For reference, our own Moon has a diameter of 3,474 kilometers, larger than any of the dwarf planets.

Observations with SPHERE

These new observations benefited from a new instrument, the Spectro-Polarimetric High-contrast Exoplanet Research instrument (SPHERE), installed in 2014 on one of the four 8.2-meter individual telescopes that form the VLT. SPHERE was designed to detect and study new giant exoplanets orbiting nearby stars, but it can also be used to observe small objects within our solar system with unprecedented resolution, as this study has shown.

“The revolution with SPHERE is that it works at the refraction limit of the telescope,” said Pierre Vernazza, an astronomer at the Laboratoire d’Astrophysique de Marseille in France and first author of the new study. “The combo of SPHERE and VLT is currently the most powerful imaging system in the world. Objects that were just a few pixels across before become really visible, and we can see craters that are just 30 kilometers in size and do geology from the ground thanks to this improvement.”

The team also showed that Hygiea rotates with a period of 13.8 hours, half the previously accepted value.

“It is solid work showing off amazing capabilities from Earth that we previously thought could only be possible from space,” said Richard Binzel, a planetary scientist at the Massachusetts Institute of Technology who wasn’t involved in the study. “It is a combination of large telescope aperture, clever optical design, and high-speed computing to cancel out the blurring effects of Earth’s atmosphere, much like how noise-canceling headphones deliver a clear sound in the hubbub of an airport.”

A Smooth Surface Reveals a Violent Past

What really came as a surprise, Vernazza said, is that Hygiea’s surface is smooth, lacking signs of large impacts. Researchers were expecting to find some sort of a large impact basin, as is the case on the asteroid Vesta, which bears the mark of a massive impact on its south pole. Vesta also has its own family of asteroids, albeit smaller than Hygiea’s.

What Hygiea’s smooth surface reveals, according to the new study, is that the impact that created Hygiea was so powerful that it completely shattered its parent body. The fragments then coalesced again, assuming a round shape, and a fraction of the mass was ejected to form Hygiea’s asteroid family.

“Most of the mass was reaccreted to form Hygiea, and the reaccreting body behaves as a fluid during a few hours,” Vernazza said, “and that’s what allowed it to acquire a shape that is roughly spherical.”

To test this hypothesis, the team created a series of computer simulations that show that the most likely scenario involved a fast-traveling impactor between 75 and 150 kilometers in diameter and that the whole process probably took just a few hours to complete.

As telescopes keep improving and astronomers are able to observe more distant objects, the list of dwarf planets is sure to grow. “There are a lot of candidates among trans-Neptunian objects, and as far as we know, there are more than a hundred bodies with diameters above 400 kilometers, and for sure most of these bodies will be roughly spherical,” Vernazza said. To be able to obtain direct imaging observations of these bodies, equivalent to what the VLT has done with Hygiea, astronomers will have to wait until the next generation of 30- to 40-meter telescopes is available at some point within the next decade.

“We’re still in the reconnaissance phase of exploring our solar system,” Binzel said.

—Javier Barbuzano (@javibarbuzano), Freelance Science Journalist

This article was originally published on Eos and has been republished here under a CC BY-NC-ND 3.0 license.

Researchers may have spotted the smallest dwarf planet in the solar system

The European Space Observatory’s SPHERE instrument has spotted what may be the smallest small planet in our solar system.

SPHERE/VLT image of Hygiea, which could be the Solar System’s smallest dwarf planet yet.
Image credits ESO/P. Vernazza et al. / MISTRAL algorithm (ONERA/CNRS).

The object christened Hygiea is currently considered an asteroid — but it might be classified as a dwarf planet. It’s the fourth largest body in the asteroid belt after Ceres, Vesta, and Pallas. The reclassification follows on the heels of new observations: for the first time, astronomers were able to look at Hygiea with a sufficiently-high resolution to study its surface and to determine that it is spherical (a condition necessary to be considered a planet).

Hygiea might thus officially become the smallest dwarf planet in our solar system — a title currently held by Ceres,

Small but significant

“Thanks to the unique capability of the SPHERE instrument on the VLT (Very Large Telescope), which is one of the most powerful imaging systems in the world, we could resolve Hygiea’s shape, which turns out to be nearly spherical,” says lead researcher Pierre Vernazza from the Laboratoire d’Astrophysique de Marseille in France.

“Thanks to these images, Hygiea may be reclassified as a dwarf planet, so far the smallest in the Solar System.”

Prior to this discovery, we already knew that Hygiea satisfied three of the four requirements to be considered a dwarf planet: it orbits around the Sun, it is not a moon, and it has not cleared the neighborhood around its orbit (like a proper planet would). The final requirement is for it to have enough gravitational force to pull itself into a roughly spherical shape. Thanks to new observations, we now know that Hygiea passes this criterion as well.

Based on the SPHERE data, the team estimated Hygiea’s size to be around 430 km in diameter. Ceres is closer to 950 km in diameter while Pluto, the largest dwarf planet, comes close to 2400 km.

One surprising find was that Hygiea lacks any large impact craters on its surface. The team really expected to find such a structure on its surface as Hygiea is the main member of one of the largest asteroid families (with around 7000 members) that all come from the same parent body. It was believed that Hygiea would have been left scarred by the event that led to that original body breaking apart. Although the astronomers observed Hygiea’s surface with a 95% coverage, they could only identify two relatively small craters.

“This result came as a real surprise as we were expecting the presence of a large impact basin, as is the case on Vesta,” says Vernazza.

“Neither of these two craters could have been caused by the impact that originated the Hygiea family of asteroids whose volume is comparable to that of a 100 km-sized object. They are too small,” explains study co-author Miroslav Bro of the Astronomical Institute of Charles University in Prague, Czech Republic.

Computer simulations suggest that Hygiea’s shape and the large number of members in its asteroid family were the result of a major head-on collision between the parent body and an object between 75 and 150 km in diameter around 2 billion years ago, The simulations showed that this violent impact completely shattered the parent body. Hygiea, the simulations suggest, is the product of left-over pieces that reassembled themselves into a round shape surrounded by companion asteroids.

“Such a collision between two large bodies in the asteroid belt is unique in the last 3-4 billion years,” says Pavel Ševeček, a PhD student at the Astronomical Institute of Charles University and paper co-author.

“Thanks to the VLT and the new generation adaptive-optics instrument SPHERE, we are now imaging main belt asteroids with unprecedented resolution, closing the gap between Earth-based and interplanetary mission observations,” Vernazza concludes.

The paper “A basin-free spherical shape as outcome of a giant impact on asteroid Hygiea” has been published in the journal Nature Astronomy.