The Smallest Dwarf Planet In The Solar System Has Been Hiding In the Asteroid Belt

As low resolution as this is, it is by far the most detailed image of the asteroid Hygiea, which based on its spherical appearance probably should be reclassified as a dwarf planet. ESO/P. Vernazza et al./MISTRAL algorithm (ONERA/CNRS)

When Pluto was controversially displaced from its planetary status, astronomers tried to soften the blow by creating a new class of object, known as a dwarf planet. This appears to be good news for one rather neglected object, Hygiea, which after being studied in high resolution to determine its shape and size may soon be promoted, making it no longer just another asteroid most people have never heard of but instead the smallest known dwarf planet yet.

There are some rules to be classified as a dwarf planet. They must orbit the Sun directly, rather than being a Moon of something else, and not be weighty enough to have cleared their orbit of smaller objects. They also need enough gravity to have forged themselves into near-spherical shapes. The point where this occurs depends not only on an object's size, but its composition.

Hygiea is the fourth-largest body in the main belt between Mars and Jupiter, but unlike its larger counterparts, Ceres and Vesta, has not been visited by probes. Nor had it been investigated by the modern generation of enormous instruments until the European Space Organization (ESO) turned the Very Large Telescope (VLT) on it. Consequently, its shape was unknown.

Now, a paper in Nature Astronomy based on the measurements taken with the VLT's aptly named SPHERE polarized light detector reports Hygiea is indeed rather round-looking.

"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," said Dr Pierre Vernazza from the Laboratoire d'Astrophysique de Marseille in France in a statement. "Thanks to these images, Hygiea may be reclassified as a dwarf planet, so far the smallest in the Solar System."

Before using the VLT there was a substantial uncertainty about Hygiea's size, but we now know it is 434 kilometers (270 miles) wide, which must be very close to the minimum a dwarf planet can be. By contrast, Ceres – the current officially recognized smallest dwarf planet – has more than twice the diameter, and Pluto is more than double that again. It’s 14-hour rotation period is half what rougher measures had anticipated.

Hygiea's spherical state is particularly surprising because the presence of many sufficiently similar smaller objects demonstrate it is the largest remnant of a larger object destroyed in a major collision. Vernazza had expected to see an enormous crater left behind by this impact, as is the case for Vesta.

The paper explains the finding as an indication the collision was so large it completely shattered the predecessor object, and Hygiea formed out of some of the debris. "Such a collision between two large bodies in the asteroid belt is unique in the last 3-4 billion years," says Pavel Ševeček, a PhD student at the Astronomical Institute of Charles University, Prague, although events like this were common earlier in the Solar System's past.

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