Large Asteroid Could Be Reclassified as Dwarf Planet, Making It the Smallest in Solar System

Hygiea is seen in a telescope image from the European Southern Observatory. (Credit: ESO/P. Vernazza et al./MISTRAL algorithm (ONERA/CNRS) via CNN)

Hygiea is seen in a telescope image from the European Southern Observatory. (Credit: ESO/P. Vernazza et al./MISTRAL algorithm (ONERA/CNRS) via CNN)

A large asteroid could be reclassified as a dwarf planet — which could make it the smallest in the solar system — after new research revealed its shape, astronomers said on Monday.

Nestled in the asteroid belt between Mars and Jupiter is an object that may have been overlooked. They believe the asteroid Hygiea should actually be classified as a dwarf planet.

There aren’t many dwarf planets in our solar system. The five classified as dwarf planets include Pluto (although some still regard it as the ninth planet), Ceres in the asteroid belt, Makemake, Haumea and Eris.

Pluto is the most famous of the dwarf planets — long considered our solar system’s ninth planet until it was demoted in 2006 when the International Astronomical Union created a new definition for planets and decided Pluto did not fit the bill.

Hygiea is the fourth largest object in the asteroid belt behind Ceres, Vesta and Pallas. Vesta and Pallas are large asteroids. But new imaging and data provided by the European Southern Observatory’s SPHERE instrument on the Very Large Telescope have shown Hygiea to be spherical.

This is the first time astronomers have been able to study the surface, shape and size of Hygiea in high resolution. If it is deemed a dwarf planet, Hygiea would become the smallest dwarf planet in our solar system. The study published Monday in the journal Nature Astronomy.

In order to be classified as a dwarf planet, a celestial body must fulfill certain criteria. It must orbit around the sun and must not be a moon.

In addition to this, it must not “clear the neighborhood” around its orbit like a planet does, and must have been molded by its own gravity into a round or nearly round shape.

Hygiea checks many of those boxes.

“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,” lead researcher Pierre Vernazza from the Laboratoire d’Astrophysique de Marseille in France said in a statement.

“Thanks to these images, Hygiea may be reclassified as a dwarf planet, so far the smallest in the Solar System,” he added.

The new observations helped astronomers determine that Hygiea has a diameter of 267 miles across. For reference, Ceres is 590 miles in diameter and Pluto is 1,491 miles in diameter.

Astronomers weren’t expecting this when they turned their telescope to study Hygiea. Instead, they were expecting to find a large impact crater. Hygiea belongs to a large asteroid family in the asteroid belt, where 7,000 different asteroids all came from the same body that was once much larger. But Hygiea doesn’t show the telltale mark.

Instead, they only spotted two craters that don’t match up with the mark.

“This result came as a real surprise as we were expecting the presence of a large impact basin, as is the case on Vesta,” Vernazza said.

“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. They are too small,” said Miroslav Brož, study co-author at the Astronomical Institute of Charles University in Prague, Czech Republic.

This means Hygiea has a different origin story than expected. Based on their data, the astronomers were able to simulate the theory that Hygiea and its fellow asteroids are the result of a large head-on collision around two billion years ago. A projectile between 46 and 93 miles in diameter obliterated the parent object. Hygiea pulled together from the leftover pieces and everything else turned into thousands of asteroids that are found around Hygiea.

“Such a collision between two large bodies in the asteroid belt is unique in the last 3 to 4 billion years,” said Pavel Ševeček, study co-author and PhD student at the Astronomical Institute of Charles University.

Without powerful telescopes like the one used in this study, astronomers wouldn’t know these finer details of our solar system.

“Thanks to the VLT and the new generation adaptive-optics instrument SPHERE, we are now imaging main belt asteroids with unprecedented resolution, closing the gap between Earth-based and interplanetary mission observations,” Vernazza said.

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