Astronomers have revealed that the asteroid Hygiea could be classified as a dwarf planet, which would make it the smallest in the Solar System – dethroning Ceres.
As an object in the main asteroid belt, Hygiea right away satisfies three of the requirements to be classified as a dwarf planet: it orbits around the Sun, it is not a moon and, unlike a planet, it has not cleared the neighbourhood around its orbit.
The final requirement is that it has enough mass for its own gravity to pull it into a roughly spherical shape – and new observations using the European Southern Observatory’s SPHERE instrument at the Very Large Telescope (VLT) show that to be the case.
A research team led by Pierre Vernazza, from the Laboratoire d’Astrophysique de Marseille in France, also used the observations to constrain Hygiea’s size, putting its diameter at just over 430 kilometres. Pluto, the most famous of dwarf planets, has a diameter of nearly 2400 kilometres, while Ceres is close to 950.
Surprisingly, the observations also revealed that Hygiea lacks a very large impact crater.
As it is the main member of one of the largest asteroid families, with close to 7000 members that all originated from the same parent body, astronomers expected to find a large, deep mark on Hygiea. Instead, there were just two unambiguous craters.
“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-kilometre-sized object,” says co-author Miroslav Brož from Charles University in Prague, Czech Republic. “They are too small.”
Using numerical simulations, the team deduced that Hygiea’s spherical shape and large family of asteroids were likely the result of a major head-on collision with a large projectile of diameter between 75 and 150 kilometres.
Their simulations show this violent impact, thought to have occurred about 2 billion years ago, completely shattered the parent body. Once the left-over pieces reassembled, they gave Hygiea its round shape and thousands of companion asteroids.
The findings are presented in the journal Nature Astronomy.