Hygiea could be the smallest dwarf planet in our solar system, say scientists

Hygiea's transition from an asteroid to a dwarf planet could be attributed to the discovery of Hygiea's shape


                            Hygiea could be the smallest dwarf planet in our solar system, say scientists

Hygiea may no longer just be one among the million asteroids circling between Mars and Jupiter in the asteroid belt. It may soon be entering the league of dwarf planets - a league, so far, occupied by only five objects in our solar system, according to a group of astronomers. 

This transition from an asteroid to a dwarf planet could be attributed to the discovery of Hygiea's shape: it is found to be spherical, the only condition it had to fulfill to get an entry into the list of dwarf planets. Earlier, the object met three of the four conditions: it orbits around the Sun, it is not a moon and, it has not cleared the neighborhood around its orbit, unlike a planet. With this, Hygiea could potentially be the smallest dwarf planet in the solar system, say scientists.

Hygiea is part of the asteroid belt, which is home to about 1.1 and 1.9 million asteroids larger than 1 kilometer (0.6 mile) in diameter, and millions of other smaller ones, according to NASA. Of these objects in the belt, Ceres is the only dwarf planet in the inner solar system - and the smallest known dwarf planet in the whole solar system, until now.

Changing Hygiea's fate is the planet hunter named Spectro-Polarimetric High-contrast Exoplanet REsearch instrument or SPHERE on European Southern Observatory's Very Large Telescope (VLT). "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 lead researcher Pierre Vernazza from the Laboratoire d'Astrophysique de Marseille in France. "Thanks to these images, Hygiea may be reclassified as a dwarf planet, so far the smallest in the Solar System."

Previously, many scientists, including Vernazza, did not see Hygiea fitting the bill. "We weren’t expecting such a spherical shape as we knew that Hygiea had suffered a giant impact", Pierre Vernazza told MEA WorldWide (MEAWW).

Hygiea. Credits: ESO/P. Vernazza et al./MISTRAL algorithm (ONERA/CNRS)

To get a glimpse of Hygiea's past, the scientists set out to dig deeper. And the team was in for a surprise. According to their observations, Hygiea lacked the very large impact crater that scientists expected to see on its surface. But they could see two unambiguous craters. "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", explained co-author Miroslav Brož of the Astronomical Institute of Charles University in Prague, Czech Republic.

With further modeling and analysis, the team predicts that Hygiea and other asteroids were in one piece, together - parent body. About 2 million years ago, they say, the parent body was violently hit by a large projectile of diameter between 75 and 150 km, completely shattering it.

The aftermath of the head-on collision saw a lot of leftover pieces. They predict that the left-over pieces reassembled to give Hygiea its shape and thousands of companion asteroids. "Such a collision between two large bodies in the asteroid belt is unique in the last 3-4 billion years", said Pavel Ševeček, a PhD student at the Astronomical Institute of Charles University, who also participated in the study.

The study also finds evidence to prove that Hygiea could dethrone Celes as the smallest dwarf planet. Hygiea's diameter is at just over 430 km and Ceres is close to 950 km in size. On the other hand, Pluto, the most famous of dwarf planets, has a diameter close to 2400 km.

"Hygiea could be the first dwarf planet candidate among those with a diameter greater than 400 km of a long series. Indeed, there are many with diameter falling in between 400-800 km sized bodies in the trans-Neptunian region so we should expect many more dwarf planet candidates to be discovered in the future", Vernazza told MEAWW. 

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