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High-resolution telescope images reveal the asteroid Hygiea's nearly spherical shape, meaning that it might be considered as a dwarf planet. ESO/P. VERNAZZA ET AL./MISTRAL ALGORITHM (ONERA/CNRS)

Hygiea, a candidate to become the smallest dwarf planet of the Solar System

Published on 07.11.2019

Hygiea is the fourth largest asteroid in the Solar system and might now be the smallest dwarf planet, as the recently published images show a basin-free spherical shape, meeting the final criterion for being considered as a dwarf planet.

An international team of researchers publishes in a recent article in Nature Astronomy Letters new images of the basin-free spherical shape of the asteroid Hygiea. The article publishes for the first time direct observations of Hygiea’s shape, and proposes an explanation of how such an object forms in order to reach hydrostatic equilibrium.

The article provides for the first time direct observations of the Hygiea asteroid, obtained using the SPHERE instrument on the Very Large Telescope VLT, in Chile. Researchers captured the observations at twelve different epochs between 2017 and 2018, and combined them with existing photometric images – from normal telescopes - in order to see the hidden angles of the direct observations. This is the first time that researchers observe Hygiea through adaptive optics, a technique that further defines the edges of the shapes and provides a direct measurement of the body’s diameter.

A new candidate for a dwarf planet

In order to be considered as a dwarf planet, objects must meet four main criteria: to orbit around the Sun, to share its orbital neighborhood with other objects, to not be a satellite and to be in hydrostatic equilibrium and nearly spherical shaped. All of the Main Belt asteroids, including Hygiea, meet the first three criteria. The VLT-SPHERE observations allowed the researchers to study the fourth requirement. They saw that Hygiea’s shape is nearly spherical, which led them to calculate the sphericity of the asteroid. They obtained a high value of sphericity, 0.9975, close to the maximum value of 1, meaning Hygiea’s shape is in hydrostatic equilibrium and nearly spherical.

With the latter criteria accomplished, researchers propose that Hygiea might be re-classified as a dwarf planet. Nevertheless, the official designation might take a while, as it has to be stated by the International Astronomical Union. If accepted, Hygiea would join the handful of dwarf planets in our solar system, joining Pluto and Ceres.

A basin-free shape

Hygiea is the main remnant of an impact, which separated a fraction of its parent body more than 2 billion years ago, creating an asteroid family. However, when the team evaluated the images, they could not see any sign from the impact. Although the calculations showed a large-scale topography similar to the one of Ceres, they did not reveal the impact basin. Using SPH computer simulations, they argue that a possible explanation is that the formation process was the result of reaccumulation after the original impact. Using computer simulations, the astronomers showed that a space rock of approximately 100 kilometers across impacted and fragmented Hygiea’s parent body. When most of the remnants clumped back together into the space rock now known as Hygiea, they formed the smooth, spherical body seen today.

Large campaign of exoplanet observations

The project is part of the European Southern Observatory large program of exoplanet observations. More than 20 researchers from 13 different countries form the group, which was led by Professor Pierre Vernazza from the Laboratoire d’Astrophysique de Marseille. Our researcher in Solar System and Minor bodies Toni Santana-Ros is one of the members of the team. Santana-Ros collaborated in the observation campaign, developing the object’s photometry light curve. Those images were used to obtain the outcomes of the Hygiea 3D model.

Researchers also state than several new dwarf-planet candidates will be defined once the 3D model of observations becomes available for trans-Neptunian farther objects.

You can read the whole article via Nature Astronomy.