Uncovering the Solar System’s Smallest Dwarf Planet

Using ESO’s VLT astronomers have discovered that the asteroid Hygiea could actually be classified as a dwarf planet — the solar system’s smallest so far.

A new SPHERE/VLT image of Hygiea, which could be the Solar System’s smallest dwarf planet yet. As an object in the main asteroid belt, Hygiea satisfies right away three of the four requirements to be classified as a dwarf planet: it orbits around the Sun, it is not a moon and, unlike a planet, it has not cleared the neighbourhood around its orbit. The final requirement is that it has enough mass that its own gravity pulls it into a roughly spherical shape. This is what VLT observations have now revealed about Hygiea. (ESO/P. Vernazza et al./MISTRAL algorithm (ONERA/CNRS)

Astronomers have used the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument (SPHERE) at the Very Large Telescope (VLT) to study the asteroid Hygiea in unprecedented detail-revealing its size, shape and surface detail. Their observations have revealed that Hygiea — the fourth largest object in the asteroid belt after Ceres, Vesta and Pallas — has a spherical shape.

This means that it could be classified as a dwarf planet. This would make Hygiea the smallest dwarf planet in the solar system — unseating Ceres as the holder of that title.

Astronomers using the SPHERE instrument on ESO’s Very Large Telescope have revealed that the asteroid Hygiea could be a dwarf planet. Find out more about this fascinating object in the new ESOcast Light. (ESO)

To be classified as a dwarf planet an object has to meet four criteria — it has to orbit around the Sun, it can’t be a moon, it can’t have cleared the neighbourhood around its orbit — the catch that saw Pluto demoted to a dwarf, and it has enough mass to enable its gravitational influence to pull it to a roughly spherical shape.

As an object in the main asteroid belt, Hygiea had already met the first three of these four criteria. But, these recent observations using the VLT have confirmed the fourth criteria has been met by Hygiea also.

“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,” says lead researcher Pierre Vernazza from the Laboratoire d’Astrophysique de Marseille in France, whose work is published in the journal Nature Astronomy.

“Thanks to these images, Hygiea may be reclassified as a dwarf planet, so far the smallest in the Solar System.”

The observations have also helped confirm Hygiea’s size, confirming that its diameter of around 430 km is much smaller than that of the solar system’s other dwarf planets — including Pluto, with a diameter of 2400 km and the smaller Ceres, which possesses a diameter of 950 km.

Clockwise from top left, the asteroids shown here are 29 Amphitrite, 324 Bamberga, 2 Pallas, and 89 Julia. These images were taken also by ESO’s SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) instrument, installed on ESO’s Very Large Telescope (VLT) at the Paranal Observatory, Chile. ( ESO/Vernazza et al.)

Aside from the shape of Hygiea, another aspect of the asteroid also took astronomers by surprise. The lack of a large impact crater that researchers had expected to see on its surface. As Hygiea is the main member of one of the largest asteroid families — with close to 7000 members that all originated from the same parent body — the event that led to the formation of this group was expected to have left a large, deep mark on Hygiea.

“This result came as a real surprise as we were expecting the presence of a large impact basin, as is the case on Vesta,” says Vernazza. Even with the surface of Hygiea observed with 95% coverage, the team could only identify two unambiguous craters — both of which are large enough to be a result of the same impact.

“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,” explains study co-author Miroslav Brož of the Astronomical Institute of Charles University in Prague, Czech Republic.

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. (P. Ševeček/Charles University)

Upon further investigation and by employing mathematical simulations, the team determined that Hygiea’s shape and family of asteroids are likely a result of a major collision with an object of a diameter of between 75 -150 km. The impact — which likely occurred around 2 billion years ago — completely fragmented the asteroids’ parent body. The remnants of this collision accumulated resulting in Hygiea’s spherical shape and its asteroid companions.

“Such a collision between two large bodies in the asteroid belt is unique in the last 3–4 billion years,” adds Pavel Ševeček, a PhD student at the Astronomical Institute of Charles University who also participated in the study.

If it does turn out to the smallest dwarf planet in the solar system, the question is, how can Hygiea hold on to the title? As studying asteroids in such increased detail has only recently been made possible by advances in both telescopes and the increasing power of numerical simulations such as the one employed in this study. This increases the chances of finding more dwarf planets masquerading as asteroids.

The ESO Very Large Telescope (VLT) during observations. (ESO/ Serge Brunier)

“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,” Vernazza concludes.