Fourth-Largest Object in the Asteroid Belt, Hygiea, Could be Classified as a Dwarf Planet
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ESO Telescope Reveals What Could be the Smallest Dwarf Planet Yet in the Solar System
Astronomers using ESO's SPHERE instrument at the Very Large Telescope (VLT) have revealed that the asteroid Hygiea could be classified as a dwarf planet. The object is the fourth largest in the asteroid belt after Ceres, Vesta and Pallas. For the first time, astronomers have observed Hygiea in sufficiently high resolution to study its surface and determine its shape and size. They found that Hygiea is spherical, potentially taking the crown from Ceres as the smallest dwarf planet in the Solar System.
As an object in the main asteroid belt, Hygiea satisfies right away three of the four 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. This is what VLT observations have now revealed about Hygiea.
"Thanks to the unique capability of the SPHERE instrument on the VLT, 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."
The team also used the SPHERE observations to constrain Hygiea's size, putting its diameter at just over 430 km. Pluto, the most famous of dwarf planets, has a diameter close to 2400 km, while Ceres is close to 950 km in size.
Ceres, Vesta, Pallas, and Hygiea are the four largest asteroid belt objects. Saturnian moon Mimas is the smallest astronomical body known to be rounded due to self-gravitation, with a diameter of just 396 km.
Impact simulation video (34s).
Also at Ars Technica.
A basin-free spherical shape as outcome of a giant impact on asteroid Hygiea (open, DOI: 10.1038/s41550-019-0915-8) (DX)
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