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Asteroid Hygiea Could Be the Smallest Dwarf Planet in the Solar System

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A new SPHERE/VLT image of Hygiea, which could be the solar system’s smallest dwarf planet yet. (Photo Credit: ESO / P. Vernazza et al. / MISTRAL algorithm - ONERA / CNRS)

Move over, Ceres: Astronomers using an ESO telescope revealed that asteroid Hygiea may be classified as the tiniest dwarf planet in our solar system.

Asteroid Hygiea, which was observed using ESO’s SPHERE instrument at the Very Large Telescope (VLT), is the fourth largest object in the asteroid belt after Ceres, Vesta, and Pallas, ESO said in a press release.

For the first time, astronomers analyzed asteroid Hygiea’s surface, shape, and size, which they detailed in a study published in Nature Astronomy on Oct. 28. The team determined that asteroid Hygiea is spherical and it may steal the smallest dwarf planet title from Ceres.

Here’s why asteroid Hygiea makes the cut: It meets three of the four requirements to be classified as a dwarf planet, because it orbits around the sun, is not a moon, and unlike a typical planet, it has not cleared the area around its orbit. The last requirement is that it has enough mass, so it’s own gravity can pull it into a spherical-like shape and the telescope’s observations have revealed this qualification.

“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 [asteroid] 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, [asteroid] Hygiea may be reclassified as a dwarf planet, so far the smallest in the solar system.”

The SPHERE observations revealed that asteroid Hygiea is small in size: its diameter is a little over 430 km (about 267 miles), which is tinier compared to Pluto (approximately 1,491 miles) and Ceres (roughly 590 miles).

Observations also showed that asteroid Hygiea doesn’t have the enormous impact crater that was expected to be seen on its surface, according to the study. Asteroid Hygiea is the main member of one of the biggest asteroid families, with close to 7,000 members that all came from the same parent body. Astronomers believe that the event that led to the formation of this mega-family impacted asteroid Hygiea.

“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 km-sized object,” explains study co-author Miroslav Brož of the Astronomical Institute of Charles University in Prague. “They are too small.”

Astronomers used numerical simulations to conclude that asteroid Hygiea’s spherical shape and big asteroid family were likely formed from a head-on collision with a large projectile between 75 and 150 km (about 47 to 93 miles) in diameter. Simulations demonstrated this event, which is thought to have occurred roughly 2 billion years ago. It completely shattered the parent body and once left-over fragments were put together again, they provided asteroid Hygiea’s round shape and asteroid neighbors.

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