An article (link to the PDF file) published in the journal “Nature Astronomy” reports a study of asteroid Hygiea that shows its roughly spherical shape, one of the requirements to be cataloged as a dwarf planet. A team of researchers led by Pierre Vernazza of the Laboratoire d’Astrophysique of Marseille, France, used the SPHERE instrument mounted on ESO’s VLT in Chile to obtain detailed images of one of the largest objects in the asteroid belt between Mars and Jupiter. If Hygiea were reclassified, it would be the smallest dwarf planet with a diameter that is less than half that of Ceres.
Asteroid Hygiea, officially cataloged as 10 Hygiea, is the fourth largest object of the asteroid belt after Ceres, Vesta and Pallas. The first two were studied closely during NASA’s Dawn space probe’s mission so their characteristics are well known. Ceres is one of the objects classified as dwarf planets at the time of the creation of that category. Vesta is larger than Hygiea and is most likely a protoplanet but its shape has various irregularities, also caused by significant impacts, so it wasn’t reclassified as a dwarf planet.
The SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch instrument) instrument mounted on the VLT (Very Large Telescope) and activated in June 2014 has the main purpose of searching for exoplanets. Its characteristics make it useful for other types of astronomical research as well. In this case, it allowed to obtain high quality images of asteroid Hygiea, which offered some surprises.
The top image (ESO/P. Vernazza et al./MISTRAL algorithm (ONERA/CNRS)) shows asteroid Hygiea observed using the SPHERE instrument. The bottom image (ESO/P. Vernazza et al., L. Jorda et al./MISTRAL algorithm (ONERA/CNRS)) shows asteroids Hygiea and Vesta together with a part of the dwarf planet Ceres observed using the SPHERE instrument.
According to astronomers, asteroid Hygiea and about 700 more asteroids that are much smaller make up a family called Hygiea precisely after the main member that originated from a progenitor body that lost many fragments following an impact. If this hypothesis is correct, asteroid Hygiea got deformed by that impact but the SPHERE instrument detected no large craters. That’s surprising because the researchers expected Hygiea to be more like Vesta with obvious signs of ancient impacts.
One possibility that emerged thanks to computer simulations is that the progenitor object was struck by another asteroid about 2 billion years ago and destroyed. Over time most of the pieces may have coalesced again in an object with an approximately spherical shape with small craters. It would be a unique collision in the asteroid belt in the last 3-4 billion years.
Asteroid Hygiea orbits the Sun, it’s not a moon and hasn’t cleaned up its orbit. These are three of the four requirements for an object to be classified as a dwarf planet. This new study shows that Hygiea also satisfies the fourth requirement, the roughly spherical form. The International Astronomical Union (IAU) is the entity that decides the classification of these objects and in the near future might include Hygiea among the dwarf planets.