Within the Main Asteroid Belt, there are a number of larger bodies that have defied traditional classification. The largest among them is Ceres, which is followed by Vesta, Pallas, and Hygeia. Until recently, Ceres was thought to be the only object in the Main Belt large enough to undergo hydrostatic equilibrium – where an object is sufficiently massive that its gravity causes it to collapse into a roughly spherical shape.
However, it now seems that there is another body in the Main Belt that has earned the designation of “dwarf planet”. Using data from the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument at the Very Large Telescope (VLT), an international team of astronomers found compelling evidence that Hygeia is actually round, making it the smallest dwarf planet in the Solar System.
Even before this came to light, Hygeia satisfied most of the qualifications for being designated as a dwarf planet – which were adopted by the IAU General Assembly in 2006. In accordance with these qualifications and definitions, a “dwarf planet” is:
“[A] celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.”
Hygeia already fulfilled three of these requirements, since it orbits the Sun, is not the satellite of a larger body, and has not cleared the neighborhood of its orbit. With this latest data that indicates that it is might actually be round, Hygeia officially meets all the qualifications. Pierre Vernazza, the team’s lead researcher from the Laboratoire d’Astrophysique de Marseille (LAM), explained in an ESO press release:
“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. Thanks to these images, Hygiea may be reclassified as a dwarf planet, so far the smallest in the Solar System.”
Using SPHERE, Vernazza and his colleagues were also able to come up with more accurate constraints of Hygeia’s size. According to their estimates, Hygiea measures just over 430 km (~270 mi) in diameter, where Ceres measures a comparatively robust 950 km (590 mi) in diameter and Pluto measures 2400 km (1490 mi). As noted, this makes Hygeia the smallest dwarf planet discovered to date.
Interestingly, the team’s observations also indicated that Hygeia does not have a very large impact crater on its surface, which scientists were expecting to be there. This is due to the fact that Hygiea is the main member of one of the largest families of asteroids in the Main Belt. This family contains almost 7,000 dark carbonaceous (C-type and B-type) members and is located in the outer region of the Main Belt.
Because of their similarities, all these asteroids are believed to have originated from the same parent body (in this case, Hygeia), which would have been created by a massive impact event in the past. As such, astronomers were expecting to find a large, deep mark on Hygeia’s surface that would have been caused by this event.
Despite being able to observe 95% of Hygeia’s surface, the team could only identify two run-of-the-mill 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,” said Miroslav Brož, the study’s co-author from the Astronomical Institute of Charles University in Prague.
To investigate this mystery further, the team conducted numerical simulations to determine how Hygeia could have become spherical while also giving rise to a family of asteroids. They determined that these were likely the result of a major head-on collision with an object measuring between 75 and 150 km (~45 to 90 mi) in diameter roughly 2 billion years ago.
According to their simulations (see the video below) this violent impact would have completely shattered the parent body. Over time, many of the fragments coalesced to give Hygeia its round shape while the remnants stayed disrupted as asteroids. This kind of event, where two large bodies collided in the Asteroid Belt, would have been a unique occurrence during the last 3-4 billion years.
In this respect, this latest study not only revealed another dwarf planet candidate, but also provide additional clues as to how the Asteroid Belt evolved over time. These sorts of detailed asteroid studies are now possible thanks to the advent of more powerful telescopes and advances in numerical computations. As Vernazza concluded:
“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.”
The international team was made up of astronomers from the Centre National de la Recherche Scientifique (CNRS), the SETI Institute’s Carl Sagan Center, the ESA’s European Space Research and Technology Centre (ESTEC), NASA JPL, the European Southern Observatory (ESO), MIT, and multiple observatories and universities. The study which describes their findings recently appeared in the journal Nature Astronomy.
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