Hygieia the new dwarf planet of the solar system

Hygiea
SPHERE/VLT

As astronomers learn more details about the objects that populate our solar system, we must redefine the image we have created about them and become a little more specific. 

Take Pluto for instance, in 2006 Pluto was no longer considered a ‘planet’ and became known as a ‘dwarf planet’ -joining the ranks of other objects such as Ceres, Vesta and Pallas. 

Hygieia is the new dwarf planet and is much smaller than its predecessors, so small in fact that it has stolen the title ‘as the smallest dwarf planet in the solar system’ from Ceres. 

What is a dwarf planet?

In order for you to be able to identify what is a dwarf planet and what isn’t, you must understand what requirements an object in the solar system must fulfil in order to be considered a dwarf planet.

There are four criteria that need to be met, the dwarf planet:

- must orbit around the Sun.

- is not a moon (natural satellite of another planet)

- has not cleared its orbital environment of other objects (as a planet like Venus, Mars or Earth would).

- its mass is enough for gravity to 'push' it into a spherical shape.

When Hygieia was discovered, it met the first three requirements, but was missing the last one to win the dwarf planet title. 

Today, astronomers using ESO’s ESFERA instrument on the Very Large Telescope (VLT) have observed Hygieia, for the first time, at a resolution high enough to study its surface and determine its shape and size. This new instrument has allowed them to determine that it is indeed spherical. 

Image Credit: SPHERE/VLT

A statement from the European Southern Observatory picks up the words of the principal investigator, Pierre Vernazza, from the Masella Astrophysics Laboratory in France: 

"Thanks to the unique capacity of the SPHERE instrument in the VLT, which is one of the most powerful imaging systems in the world, we were able to determine the shape of Hygieia, which turned out to be almost spherical. Hygieia can be reclassified as a dwarf planet, until now the smallest in the solar system."

There are no scars from its formation.

The asteroid belt is proof of the violent events that took place during the birth of our solar system. 

Hygieia is one of the 7,000 members of this ring of frozen rocks that orbits the sun, and is located between Jupiter and Mars. 

Scientists believe that had it not been for Jupiter's enormous gravity, all this debris could have formed one or more rocky planets.

These violent events that we know took place five billion years ago led astronomers to think that Hygieia would have some kind of impact character, some scar, so to speak. But they found no trace of it: "This result was a real surprise, since we expected the presence of a large impact basin, as it happens to Vesta," says Vernazza.

However, astronomers looked at Hygieia's surface with 95% coverage, so they could only identify two unequivocal craters.

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

Hygieia formed after an unusual event

The team decided to investigate further. Using numerical simulations, they deduced that the spherical shape of Hygiea and the large family of asteroids are probably the result of a large frontal collision with a large projectile between 75 and 150 km in diameter. 

Their simulations show that this violent impact, which is believed to have occurred some 2 billion years ago, completely destroyed the parents' bodies.

Once the remaining pieces were brought back together, they gave Hygiea its round shape and thousands of accompanying asteroids. 

"Such a collision between two large bodies in the asteroid belt is unique in the last 3 or 4 billion years," according to Pavel Ševeček, Ph.D., a student at Charles University Astronomical Institute, who also participated in the study.

The fact that we can now know in such detail the characteristics of our neighbouring bodies of the solar system, and we can catalogue them with more precision, is because of the advances in numerical computation and more powerful telescopes, such as the VTL, which allow us to obtain images with unprecedented resolution.

A basin-free spherical shape as an outcome of a giant impact on asteroid Hygiea, Nature Astronomy (2019). DOI: 10.1038/s41550-019-0915-8 , https://nature.com/articles/s41550-019-0915-8.

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