Hygiea Is Not an Asteroid, but a Dwarf Planet, Research Shows

Pablo Tucker
October 30, 2019

Using the SPHERE instrument on ESO's Very Large Telescope, astronomers have observed Hygiea in sufficiently high resolution to determine its shape and size and study its surface. It's the fourth largest object in the belt, with Ceres, Vesta, and Pallas being the only bigger ones (of the three, onlyCeres is a dwarf planet).

According to criteria devised by the International Astronomical Union (IAU) in 2006, a celestial object needs to satisfy four requirements to earn its designation as a dwarf planet: it needs to be in its own orbit around the Sun, it can't be a moon, it has vacuumed-up other material in its immediate vicinity, and it has achieved "hydrostatic equilibrium"-in other words, it's mostly spherical in shape, having sufficient gravity to overcome a rigid, irregular form". The third: is not like a full planet, and it has not cleared space around its orbit. The final requirement is for it to have enough gravitational force to pull itself into a roughly spherical shape.

The team revealed that SPHERE observations constrain Hygiea's size, putting its diameter just over 430 kilometres.

In the latest observations, the researchers also measured a new rotation period for Hygiea of 13.8 hours; the updated time is half of the current value.


There are now five dwarf planets - Pluto, Eris, Haumea, Makemake and Ceres.

However, after scouring the VLT images, the research team only found two unambiguous impact craters on Hygiea; teeny-tiny piddling impact craters, nothing like Vesta's giant wounds. "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".

This video shows how the impact that created the Hygiea family of asteroids might have played out.

To investigate further, the researchers used computer simulations, which showed a major head-on collision between two large bodies - possibly one of the largest impacts in the history of the asteroid belt thought to have occurred about two billion years ago.


Outer body Gigeya from the main asteroid belt has all the options to be considered as a sixth dwarf planet.

As the debate rages on whether Pluto, now a dwarf planet, should be given back its planet status, it may soon be joined by an asteroid that could wind up being the smallest dwarf planet in the solar system. Ceres and Pluto are 2400km and 950km in diameter, respectively, so a round Hygiea would be a significantly lower bar for the minimum size of any dwarf planet. That's when Pluto was downgraded from its planet status to the one of a dwarf planet.

Discovered in 1849 and named after the Greek goddess of good health and hygiene, it orbits the sun every 5.5 years and has a mean diameter of 431 kilometers, although it's slightly oblong. "They are too small", said astronomer Miroslav Brož of the Astronomical Institute of Charles University in the Czech Republic. The left over pieces then reformed to produce a rounded Hygiea and thousands upon thousands of smaller close companions.

With advances in numerical computation, and more powerful telescopes, scientists are now more able to rewind the clock and piece together cosmic conundrums such as these.


"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 added.

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