A publication in Nature Astronomy

Hygiea, the smallest dwarf planet in the Solar System

Crédit: ESO/P. Vernazza et al./MISTRAL algorithm (ONERA/CNRS)

Astronomers using ESO’s SPHERE instrument at the Very Large Telescope (VLT) have revealed that the asteroid Hygiea could be classified as a dwarf planet. The object is the fourth largest in the asteroid belt after Ceres, Vesta and Pallas. For the first time, astronomers have observed Hygiea in sufficiently high resolution to study its surface and determine its shape and size. They found that Hygiea is spherical, potentially taking the crown from Ceres as the smallest dwarf planet in the Solar System. Astronomers from ULiège participated to this study using both TRAPPIST telescopes.


o be officially considered as a dwarf planet, a body of the solar system must fulfill, according to the International Astronomical Union, four conditions. It must be in orbit around the sun; but not around a planet (otherwise it would be a moon); it must not have "cleaned up" the neighborhood of its orbit as the main planets did; and it must have a mass sufficient to reach the hydrostatic equilibrium, that means to have a more or less spherical shape.

The solar system contains many bodies that meet the first three requirements, including the thousands of asteroids in the asteroid Main Belt between Mars and Jupiter. But the new observations of one of the largest asteroids in this region by an international team of astronomers, including astronomers from the ULiège STAR Research Institute show that it could also respect the last condition and enter the restricted circle of dwarf planets.

Its name is Hygiea and it is the fourth largest asteroid, behind the dwarf planet Ceres (945 km in diameter) and the asteroids Vesta (525 km) and Pallas (512 km) in the Main Belt. Hygiea orbits at an average distance of 470 million km from the Sun, three times farther than the Earth.

Until now, Hygiea had been poorly studied - it was considered as a huge rocky body roughly 500 km long by 350 km large with a giant impact crater. But the new observations using the Very Large Telescope (VLT) from the Southern European Observatory in Chile and the two TRAPPIST telescopes from ULiege have revealed its true form and origin.

Thanks to the unique capabilities of the VLT's SPHERE instrument, one of the most powerful imaging systems in the world using the adaptive optics technique, we have been able to solve the Hygiea form, which to everyone's surprise is almost spherical, "says astronomer Pierre Vernazza of the Astrophysics Laboratory of Marseille in France and lead author of this study.

"Thanks to these images, Hygiea should be reclassified as a dwarf planet, and to this day the smallest of the solar system, thus dethroning Ceres, which is twice as big”.

According to the new measurements, Hygiea has a diameter of just over 430 km and a rotation period which has been revised downwards by a factor of about two to only 13.8 hours. "We already knew that its surface had a composition and density similar to that of the dwarf planet Ceres, but the new observations show that Hygiea is also almost spherical," says Emmanuel Jehin, FNRS Senior Research Associate at ULiege STAR Institute, "We used the two TRAPPIST telescopes for a total of more than a hundred hours to accurately measure its rotation period, an important parameter to rebuild the 3D model, and we found that the period listed was wrong since years! This confirmed the spherical shape model built by the high-resolution images of the VLT and quite different from what we had expected!”

The absence of a large impact crater is also a surprise" says Marin Ferrais, PhD student of ULiège and today in thesis in the Marseille team. "Indeed, Hygiea is the largest representative of a family of about 7,000 asteroids that share orbits and very similar compositions. An important impact should have been at the origin of these thousands of Asteroids in link to Hygiea and we only found two small craters.

To solve this puzzle, the team performed many simulations. The scenario that best explains the spherical shape of Hygiea and the associated asteroid family involves a massive collision that occurred about 2 billion years ago, with an asteroid of a diameter between 75 and 150 km moving at high speed that completely pulverized the parent body. Debris were ejected off forming smaller asteroids, but the main mass of melted and hot pieces collapsed. By cooling and hardening, they reformed an almost perfect sphere.

"If Hygiea is officially classified as a dwarf planet, it will expand the range of sizes at which these objects can be found, which means there could be many more dwarf planets than we thought in the solar system!, Emmanuel Jehin concludes."


Impact simulation explaining the origin of Hygiea’s round shape

Computational simulation of the fragmentation and reassembly that led to the formation of Hygiea and its family of asteroids, following an impact with a large object. While changes in the shape of Hygiea occur after the impact, the dwarf-planet candidate eventually acquires a round shape.


Scientific reference

P. Vernazza, L. Jorda, P. Ševeček, M. Brož, M. Viikinkoski, J. Hanuš, B. Carry, A. Drouard, M. Ferrais, M. Marsset, F. Marchis, M. Birlan, E. Podlewska-Gaca, E. Jehin, P. Bartczak, G. Dudzinski, J. Berthier, J. Castillo-Rogez, F. Cipriani, F. Colas, F. DeMeo, C. Dumas, J. Durech, R. Fetick, T. Fusco, J. Grice, M. Kaasalainen, A. Kryszczynska, P. Lamy, H. Le Coroller, A. Marciniak, T. Michalowski, P. Michel, N. Rambaux, T. Santana-Ros, P. Tanga, F. Vachier, A. Vigan, O. Witasse, B. Yang, M. Gillon, Z. Benkhaldoun, R. Szakats, R. Hirsch, R. Duffard, A. Chapman & J. L. Maestre , “A basin-free spherical shape as an outcome of a giant impact on asteroid Hygiea”, Nature Astronomy, 28 oct. 2019

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the ESO press release


JEHIN Emmanuël I ORCA Laboratory I STAR Research Institute

GSM 0495/23.72.98 I ejehin@uliege.be

FERRAIS Marin I Laboratoire d'Astrophysique de Marseille


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