As astronomers learn more details about the objects that populate our solar system, we must redefine our conception of them, to be a little more specific. An example is Pluto, which ceased to be considered a ‘planet’ in 2006 to be called a ‘dwarf planet’, swelling the list along with other objects such as Ceres, Vesta and Pallas.
Now, another frozen body has passed the test and has earned itself to be part of this category: Hygiea is the new dwarf planet, and it is smaller than its predecessors, which could take the title of the smallest dwarf planet to Ceres. Of the solar system.
But what is a dwarf planet?
To begin with, we must know what requirements an object in the solar system must have to be considered a dwarf planet . These requirements are four:
– orbits around the Sun.
– not a moon (natural satellite of another planet)
– has not cleared its orbital environment of other objects (as a planet such as Venus, Mars or Earth would do).
– its mass is enough for gravity to ‘push’ it into a spherical shape
When Hygiea was discovered, it met the first three requirements, but was missing the last of them to obtain the title of dwarf planet. Now, astronomers using ESO’s SPHERE instrument on the Very Large Telescope (VLT) have observed Hygiea for the first time at high enough resolution to study its surface and determine its shape and size. And they have determined that it is indeed spherical. All the data have been published in the journal Nature Astronomy.
Image Credit: SPHERE / VLT
We know that Hygiea is spherical and that it is the fourth largest object in the asteroid belt after Ceres, Vesta and Pallas, but also that, as we have explained, it would remove Ceres the crown of the smallest dwarf planet in the solar system. being smaller than all of them.
A statement from the European Southern Observatory includes 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. world, we were able to determine the shape of Hygiea, which turned out to be almost spherical. Hygiea can be reclassified as a dwarf planet, so far the smallest in the solar system. “
The team also used SPHERE’s observations to figure out the size of Hygiea: its diameter is just over 430 km. To get an idea, Pluto, the most famous of the dwarf planets, is close to 2,400 km in diameter , while Ceres is close to 950 km in size.
We also know that Hygiea is not alone, but has a ‘family of companions’, debris that occupies a place next to him in the asteroid belt.
The video reveals the position of Hygiea in the asteroid belt / ESO.
No scars from their formation
The asteroid belt is proof of the violent events that took place during the birth of our solar system. Hygiea is one of the 7,000 members of this ring of icy rocks that orbits the sun, and is located between Jupiter and Mars. Scientists believe that, were it not for Jupiter’s enormous gravity, all of 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 Hygiea would have some impact character, some scar. But they found no trace of it: “This result was a real surprise, as we expected the presence of a large impact basin, like Vesta, ” says Vernazza.
However, the astronomers observed the surface of Hygiea with 95% coverage, so they could only identify two unambiguous craters. ” Neither of these two craters could have been caused by the impact caused by the Hygiea 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 the Astronomical Institute of the Charles University in Prague, Czech Republic.
Hygiea was 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 likely the result of a large head-on collision with a large projectile between 75 and 150 km in diameter. Their simulations show that this violent impact, believed to have occurred around 2 billion years ago, completely destroyed the parents’ bodies.
Once the leftover pieces were put 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 3-4 billion years,” according to Pavel Ševeček, Ph.D. student at the Astronomical Institute of Charles University, who also participated in the study.
The fact that we can now know in such detail the characteristics of our neighboring bodies in the solar system, and we can catalog them with more precision, is thanks to advances in numerical computation and more powerful telescopes, such as the VTL, which allow obtaining images with an unprecedented resolution.
More information:
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.
