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An asteroid orbiting between Mars and Jupiter could be a new dwarf planet

The asteroid, named Hygiea, is the fourth largest asteroid that likely originated two billion years ago. 

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Bengaluru: Astronomers have discovered that an asteroid, named Hygiea, orbiting between Mars and Jupiter is spherical in shape — making it a candidate for being reclassified as a dwarf planet. If done so, Hygiea would be the smallest dwarf planet in the solar system.

A team of astronomers, using European Southern Observatory’s (ESO) Very Large Telescope (VLT), also discovered that Hygiea, which wasn’t studied in such high resolution before, appears to be similar in composition to Ceres — the largest asteroid and smallest dwarf planet orbiting in the same asteroid belt. The findings were published in Nature Astronomy Monday.

Dwarf planets in the solar system

Dwarf planets became a common subject in popular science consciousness since the reclassification of Pluto 12 years ago from planet to dwarf planet.

For any object to be classified as a planet, it needs to fulfill three criteria: It must orbit the sun, be massive enough for its own gravity to keep its shape spherical, and should have “cleared its neighborhood”.

To clear its neighbourhood, a celestial body must ensure that it is the most gravitationally powerful object in its orbit and no other bodies of comparable size or mass can be in the area.

Since bodies like Pluto haven’t cleared their neighborhood, but satisfy the other criteria, they are classified as dwarf planets.

Planetary definitions are decided by the International Astronomical Union (IAU), a global consortium of astronomers, which has laid down three guidelines to officially declare any celestial body a planet. There are 13,697 members in the IAU, representing 82 countries that vote on major decisions, including names of astronomical bodies.

There are currently five dwarf planets — Pluto, Eris, Haumea, Makemake and Ceres. All, except Ceres, orbit beyond Neptune.

Also read: Water vapour discovered on exoplanet 110 light years away

The case for Hygiea

Hygiea, the fourth largest asteroid in the asteroid belt, seems to have likely originated together with Ceres 2 billion years ago. It is slightly oblong in shape, with a mean diameter of 431 km. It orbits the sun in five-and-half-years and rotates around its own axis in 27.6 days.

The asteroid was discovered in 1849, and named after the eponymous Greek goddess of cleanliness and hygiene. It was initially called Bourbon Hygieia, but in 1852 was simply changed to Hygiea, with a single ‘i’. Despite its relatively large size, it is very dim upon observation due to its dark surface.

A surprising find from the astronomers’ research is the lack of a giant impact crater, like the shape-bending one found on a fellow asteroid, Vesta. This is surprising because Hygiea is the biggest asteroid of one of the largest asteroid families, with nearly 7,000 bodies, which all formed from the same parent body. Typically, most large chunks from the parent body carry the mark of the impact in the form of a large crater.

“This result came as a real surprise as we were expecting the presence of a large impact basin, as is the case on Vesta,” said lead researcher Pierre Vernazza from Laboratoire ’Astrophysique de Marseille in France in an accompanying statement.

The team observed up to 95 per cent of the asteroid’s surface and identified only two small craters.

The researchers then concluded that it was likely that Hygiea’s spherical shape and large family of fellow asteroids resulted from a major, direct head-on collision with another large body. This second body would have had a diametre anywhere from 75 km to 150 km. Simulations showed that this apocalyptic impact completely smashed to smithereens the parent body.

“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.

After the collision and the shattering, the orbiting debris collected together, and reassembled just like the moon did after impacting with proto-earth. Eventually, its own mass gave it a strong gravity, which in turn pulled Hygiea into its round shape.

Also read: NASA chief may call it anything, Pluto remains a dwarf planet 


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