A new image of Hygiea, which could be the solar system’s smallest dwarf planet so far found ESO/P. Vernazza et al./MISTRAL algorithm (ONERA/CNRS)

It is time to add a new dwarf planet to the docket. The best images yet of the asteroid Hygiea show that it appears to be nearly perfectly spherical, the main requirement to be upgraded to the designation of dwarf planet. But the way it formed may cause a rethink of that definition.

Hygiea was discovered in 1849, but we have never before been able to take high-resolution pictures of it. In the data that we did have, it was impossible to tell to what extent the changes in the light reflecting off Hygiea were caused by variations in its shape versus variations in the brightness of its surface. Now, new images taken using the Very Large Telescope in Chile have revealed its true shape for the first time, and it is much more spherical than we originally thought.

In order to be a dwarf planet, an object must satisfy four criteria: it has to orbit the sun, it cannot be a moon, it must not have swept up all the smaller objects in its orbit, and it must have enough mass that its own gravity has pulled it into a spherical shape. We already knew that Hygiea satisfied the first three, and the new images taken by Miroslav Brož at Charles University in Prague and his colleagues show that it satisfies the fourth.


Smallest dwarf

At a bit more than 430 kilometres across, that would make Hygiea the smallest dwarf planet we have found, less than half the diameter of the next-smallest dwarf. According to simulations by the researchers, it seems as if it also formed differently from the others – in an enormous collision that completely broke apart a larger body, allowing the fragments to coalesce into a sphere, rather than by slowly changing shape over time.

“To make a body like the dwarf planet Vesta, you usually need something like millions of years to create this round shape and solidify,” says Brož. “In this case, it’s a much more violent event, because it’s an impact event and it takes only a few hours, not millions of years.”

Hygiea won’t officially be a dwarf planet until it is certified by the International Astronomical Union. Brož says that this new formation mechanism may prompt a rethink of the definition of dwarf planets, because although this asteroid technically satisfies all the criteria, its violent formation makes it different from the other dwarf planets, which formed more peacefully.

Journal reference: Nature Astronomy, DOI: 10.1038/s41550-019-0915-8