Hygiea is still qualified as the fourth largest rock in the Asteroid Belt between Mars and Jupiter. But recent observations reveal a more spherical world.

According to the International Astronomical Union, there are four requirements that an object has to meet in order to count as a dwarf planet. It has to be in orbit around the Sun, but not a moon orbiting another planet; it has to clear other objects out of its orbit; and it should be large enough to have become rounded by its own gravity.

The entire main asteroid belt meets the first three requirements. But it looks like Hygiea meets all four now.

Recent high-resolution images taken by the European Southern Observatory’s (ESO) Very Large Telescope (VLT), reveal Hygiea has a spherical shape. So, this qualifies it as a dwarf planet.

“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 astronomer Pierre Vernazza from the Laboratoire d’Astrophysique de Marseille in France.

Asteroids come in a variety of shapes. But the spherical shape proves dwarf planets had enough mass for their gravity to pull them into a spherical rock.

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Until now, scientists thought Hygiea is a roughly oblong chunk of rock with diameters of 350 kilometers one way and 500 kilometers the other way, and with a giant impact crater scooped out of one side.

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

But new measurements estimate its diameter is just over 267 miles (430 kilometers). Hygiea is less than one-fifth the width of its cousin, Pluto, which has a diameter of about 1,491 miles (2,400 km).

Ceres held the title of the smallest dwarf planet until now, with a diameter of almost 590 miles (950 km). 

Another surprise is that astronomers were expecting to find a huge impact crater on the surface of Hygiea. But found none.

The impact was thought to be the result of an asteroid impact that created more space rocks to share Hygiea’s orbit. There are around 7,000 small objects of similar compositions sharing Hygiea’s orbit and because Hygiea is the largest body in this group, the space rocks are collectively called the Hygiea family of asteroids. Scientists think the family formed about 2 billion years ago when a large asteroid broke Hygiea up into thousands of pieces.

Something similar happened to Vesta to create the Vesta family, and Vesta has the impact scars to show for it.

However, before the International Astronomical Union can officially upgrade Hygiea to dwarf planet status, they must review all the evidence and take a vote.

The researchers published their new study today (Oct. 28) in Nature Astronomy.

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