It orbits the Sun, it is not a moon, it has not cleared its orbit and … it has turned out to be spherical. It has been named Hygeia.
To be officially a dwarf planet there are four requirements that a space rock must meet, according to the International Astronomical Union. It has to orbit around the Sun; not around a planet (but it would be a moon); it cannot have cleared the neighborhood from its orbit, and it must have enough mass to have hydrostatic equilibrium — that is, have a spherical shape.
There are many bodies in the Solar System that meet the first three requirements, such as the asteroid belt between Mars and Jupiter. But now new observations of that belt show that one of the largest pieces of rock that compose it could meet all four.
Its name is Hygeia, and it is the fourth-largest asteroid in the belt, behind the dwarf planet Ceres (945 km in diameter), and the asteroids Vesta (525 km) and Pallas (512 km).
Hygeia has been little studied, and it was thought that it was only a space rock of 350 kilometers on one side and 500 on the other, with a giant impact crater. However, new observations made through the Very Large Telescope (VLT) have totally kicked the board.
“Thanks to the unique capacity of the SPHERE instrument of the VLT, which is one of the most powerful imaging systems in the world, we have been able to observe in detail the shape of Hygeia, which turned out to be almost spherical,” explains astronomer Pierre Vernazza del Laboratoire d’Astrophysique de Marseille, France. «Thanks to these images Hygeia could be reclassified as a dwarf planet, the smallest of the Solar System»
According to the new measurements, Hygeia is 430 kilometers in diameter, with a rotation time close to 13.8 hours. And although it was known that it had a surface composition and density similar to that of Ceres, the new data shows that it is almost as spherical as its older cousin.
Likewise, the lack of a large impact crater is another interesting novelty, especially given that the new dwarf planet candidate shares his orbit with his family: some 7,000 smaller objects of similar composition. It is thought that this group of bodies was formed 2,000 million years ago, during an impact event. Something similar happened with the asteroid Vesta, whose impact scars present clear evidence of how his family was formed.
However, after scrutinizing the VLT images, the research team only found two small and insignificant craters in Hygeia; nothing similar to giant wounds in Vesta.
“Neither of these two craters could be caused by the impact caused by the Hygiea asteroid family, whose volume is comparable to those of a 100 km object. They are too small, ”said astronomer Miroslav Brož of the Astronomical Institute of Charles University in the Czech Republic.
To solve the mystery, the team resorted to computational simulations. This is how they deduced that both the spherical shape of Hygiea and the origin of his “family” are probably the result of a large frontal collision with a large “projectile” between 75 and 150 km in diameter (see video).
The simulations show that the violent impact completely shattered the “bodies” of the parents. Once the resulting pieces were rejoined, Hygiea acquired its spherical shape and the rest of the collision debris shaped its “family” of thousands of accompanying asteroids. “A collision of two large bodies in the asteroid belt is something that has not been seen for 3 or 4 billion years,” said Pavel Ševeček, another participant in the study.
Hygeia is almost half the size of Ceres, snatching him the position of the smallest known dwarf planet. And it is possible that it will also be the smallest body of the Solar System to have achieved hydrostatic equilibrium, which provides us with a more detailed understanding of the phenomenon — with 396 km of moon diameter Saturn’s Mimas is not large enough, so Hygiea follows closely.
If officially designated as a dwarf planet, it will expand the range of sizes in which these objects can be found, which means there could be more dwarf planets out there than we think, beyond the orbit of Neptune.
The research has been published in Nature Astronomy.