There are a number of larger bodies in the Main Asteroid Belt that have refuted the traditional classification. The largest of them is Ceres, followed by Vesta, Palace and Higeia. Until recently, Ceres was thought to be the only object in the Main Belt large enough to undergo hydrostatic equilibrium – where the object was massive enough that its gravity made it collapse to a roughly spherical shape.
However, it now appears that there is another body in the Main Belt that has earned the name 'dwarf planet'. Using data from the Very Large Telescope Spectrum-Polarimetric High Contrast Exoplanet (SPHERE) investigator, an international team of astronomers found conclusive evidence that Hygeia is actually circular, making it the smallest dwarf planet in the solar system.
Even before it was released, Hygeia satisfied most of the qualifications for designation as Dwarf Planet – which were adopted by the IAU General Assembly in 2006. In accordance with these qualifications and the definition of "Dwarf Planet" is:  "[A] a celestial body, (a) orbiting the Sun, (b) having sufficient mass for its self-gravity to overcome the solid forces of the body so as to acquire a hydrostatic equilibrium (almost circular) shape, (in ) has not cleared the neighborhood around its orbit and (d) is not a satellite. "
Hygeia has already fulfilled three of these requirements, since it orbits the Sun, is not a satellite of a larger body and does not have a clear neighborhood in its orbit. With this latest data showing that it may actually be circular, Hygeia officially meets all qualifications. Pierre Vernaca, a leading researcher on the Laboratoire d & # 39; Astrophysique de Marseille (LAM) team, explained in an ESO press release:
" Thanks to the unique ability of the VLT SPHERE tool, which is one of the most powerful image systems in the world, we could resolve the shape of Hygiea, which turns out to be almost spherical . Thanks to these images, Hygia can be classified as a dwarf planet, by far the smallest in the solar system. ”
Using SPHERE, Vernazza and his colleagues were also able to come up with more precise restrictions on the size of Hygeia. According to their estimates, Hygia measures just over 430 km (~ 270 miles) in diameter, where Ceres measures a relatively robust diameter of 950 km (590 miles) and Pluto is 2,400 km (1,490 miles). As noted, this makes Hygeia the smallest dwarf planet to date.
Interestingly, the observations of the team also show that Hygeia does not have a very large impact crater on its surface, which scientists expect to have there. This is due to the fact that Hygia is a major member of one of the largest asteroid families in the Main Belt. This family contains almost 7,000 dark carbon species (C and B-type) and is located in the outer zone of the Main Belt.
Because of their similarities, it is believed that all these asteroids originate from the same parent body (in this case Hygeia), which would have been created by mass effects in the past. As such, astronomers were expecting to find a large, deep imprint on the surface of Hygeia that would be caused by this event.
Although it was able to monitor 95% of the surface of Hygeia, the team was able to identify only two craters operating with a mill. "Neither of these two craters could be caused by the impact originating from asteroids of the Hygia family, whose volume is comparable to that of a 100 km site. They are too small, ”says Miroslav Broz, co-author of the study at the Charles University Astronomical Institute in Prague.
To further explore this mystery, the team conducted numerical simulations to determine how Hygeia could become spherical while generating a family of asteroids. They determine that they are probably the result of a major collision with an object between 75 and 150 km in diameter (~ 45 to 90 miles) about 2 billion years ago.
According to their simulations (see video below), this violent effect would completely disrupt the parent body. Over time, many fragments merged to give Hygeia their circular shape, while the remains were destroyed as asteroids. This kind of event, where two large bodies collide in the Asteroid Belt, would be a unique event in the last 3-4 billion years.
In this regard, this latest study not only revealed another candidate for the Dwarf planet, but also provided additional clues as to how the asteroid belt evolved over time. These types of detailed studies of asteroids are now possible thanks to the emergence of more powerful telescopes and advances in numerical calculations. As Vernaca concludes:
"Thanks to VLT and the new generation of SPHERE Adaptive Optical Instrument, we now see major belt asteroids of unprecedented resolution, closing the gap between Earth mission observations and interplanetary observations ." ] The international team is made up of astronomers by the National National la la Recherche Scientifique (CNRS), the Carl Sagan Center by the SETI Institute, the ESA European Space and Technology Center (ESTEC), NASA JPL, the European Southern Observatory (ESO), MIT and multiple obs Observatory and universities. The study describing their findings recently appeared in the journal Nature Astronomy .
Further reading: ESO