World.- Until now Ceres was considered to be the smallest dwarf planet in the solar system, but one instrument of the European Southern Observatory has revealed that there could be another minor: Hygiía, a spherical object of the main asteroid belt. The International Astronomical Union has the final say.
The SPHERE/VLT instrument has revealed that Hygía meets the missing requirement to be considered a dwarf planet: that it has enough mass to have its own gravity, thus generating a more or less spherical shape. / ESO/P. Vernazza et al./MISTRAL algorithm (ONERA/CNRS)
Hyshis is an object of the main asteroid belt that meets three of the four requirements to be classified as a dwarf planet: it orbits around the Sun, is not a moon, and, unlike a planet, has not cleared the surroundings of its orbit.
The final condition in which the International Astronomical Union (IAU, the international association that officially names celestial objects) is set is that it has enough mass to have its own gravity, thus generating a more or less spherical shape.
With the images obtained from Hygiah with the SPHERE instrument, this spherical ‘asteroid’ can be reclassified as a dwarf planet
Compliance with the latter requirement has just been confirmed with the SPHERE instrument, installed in the Very Large Telescope (VLT) that has the European Southern Observatory (ESO) in Chile. Details are published this week in the journal Nature Astronomy.
«Thanks to the SPHERE instrument we have been able to solve the form of Hygiía, which turns out to be almost spherical,» says lead researcher Pierre Vernazza of the Laboratory of Astrophysics in Marseille, France, who stresses: «With the images obtained, Higía can be reclassified as a dwarf planet, for now the smallest of the solar system.»
The final decision will be made by the IAU, but the discovery that Higía is spherical can dethrone Ceres as holder of the title of smallest dwarf planet in the solar system.
The team also used SPHERE’s observations to restrict the size of Hygía, estimating its diameter at just over 430 km. Pluto, the most famous of the dwarf planets, has a diameter of about 2,400 km, while Ceres is about 950 km in size.
The surprise of not finding a crater
Surprisingly, observations also revealed that Higía lacks the large impact crater that scientists expected to see on its surface. It is the main member of one of the largest asteroid families, with about 7,000 members emerging from the same main body. Astronomers hoped that the event that led to the formation of this crowded family would have left a large and profound mark on Hygiah.
«This result was a real surprise, as we expected the presence of a large impact basin, as with Vesta,» vernazza confirms. Although astronomers have observed the surface of Hygiah with 95% coverage, they were only able to identify two possible inconclusive craters.