Hygiea: Enormous Asteroid Is Actually a Tiny Dwarf Planet, Scientists Say

One of the biggest asteroids in the solar system is actually a dwarf planet, scientists have said. Hygiea, which is about the size of Iowa, with a diameter of 267 miles, is the fourth largest object in the asteroid belt that sits between Mars and Jupiter. It formed during a huge collision over two billion years ago—an event that led to the creation of one of the biggest asteroid families.

Despite being discovered in 1849, Hygiea has never been observed with a high enough resolution to establish its surface composition or shape. Using the European Southern Observatory's (ESO) Very Large Telescope, researchers led by Pierre Vernazza, from the Laboratoire d'Astrophysique de Marseille, France, have now found Hygiea is spherical and has a similar composition to Ceres—a dwarf planet that is the biggest object in the asteroid belt.

To be classified as a dwarf planet, an object must satisfy four requirements. It must orbit the Sun, it must not be a moon (meaning it orbits a planet) and it must not have cleared its local neighbourhood of other rocky fragments. Finally, it should be large enough to have become rounded by its own gravity.

Publishing their findings in Nature Astronomy, Vernazza and colleagues say their evidence indicates Hygiea satisfies this last requirement, therefore should be reclassified as a dwarf planet. If this goes ahead, it would make Hygiea the smallest dwarf planet in the solar system.

"By comparing Hygiea's sphericity with that of other Solar System objects, it appears that Hygiea is nearly as spherical as Ceres, opening up the possibility for this object to be reclassified as a dwarf planet," they wrote.

The team also found—unexpectedly—that Hygiea lacks impact craters on its surface, finding just two over 95 percent of the asteroid's surface. "This result came as a real surprise as we were expecting the presence of a large impact basin, as is the case on Vesta," Vernazza said in a statement.

Hygiea has been observed with the Very Large Telescope. Scientists say it is spherical in shape, so should be classified as a dwarf planet. ESO/P. Vernazza et al./MISTRAL algorithm ONERA/CNRS

The object that led to the formation of Hygiea is estimated to have been between 46 and 93 miles wide. This impact obliterated the asteroid's parent body, breaking it up into thousands of pieces. "Such a collision between two large bodies in the asteroid belt is unique in the last three to four billion years," study author Pavel Ševeček, from the Astronomical Institute of Charles University, the Czech Republic, said in a statement.

The International Astronomical Union (IAU) is responsible for reclassifying planetary bodies and their satellites. In 2006, a meeting of the IAU led to Pluto being downgraded from a planet to a dwarf planet because it did not fulfil the criteria set out in the definition of a planet. The announcement was met with considerable criticism and Pluto's status as a dwarf planet or planet is still being debated.

At a speech at the International Astronautical Congress last week, NASA chief Jim Bridenstine said: "I am here to tell you, as the NASA Administrator, I believe Pluto should be a planet."

Commenting on the potential reclassification of Hygiea, Robert Massey, from the U.K.'s Royal Astronomical Society, said the move would be "completely reasonable" given that it fits the criteria for a dwarf planet.

"It's an unusual situation, as Hygiea appears to be smaller than the minor planets Vesta and Pallas, but is more spherical," he told Newsweek. "The smoothness of worlds generally increases with gravitational pull, which itself increases with mass—so the Earth for example is far smoother for its size than any asteroid we know about. Apart from the quirk of introducing another dwarf planet to the Solar System, in a place where we probably didn't expect it, this must surely encourage planetary scientists to send a space probe to take a closer look."