A candidate for smallest dwarf planet
Hygiea
Hygiea is the fourth-largest body in the main belt, yet for more than a century and a half it remained a dark, anonymous dot. In 2019 images from the Very Large Telescope showed it to be almost perfectly spherical: enough to meet the last requirement it lacked to be called a dwarf planet, the smallest in the Solar System.
- Size
- 450 × 430 × 424 km
- Distance from Sun
- 472 M km
- Orbital period
- 5.6 years
- Rotation
- 13.8 hours
- Discovered
- 1849
- Type
- Carbonaceous (C)
Physical characteristics
Hygiea measures 450 × 430 × 424 km, with a mean diameter of about 430 km: less than half that of Ceres. It is a C-type, or carbonaceous, asteroid: its surface is as dark as asphalt and reflects about 7% of the light it receives. In composition it resembles Ceres and the most primitive meteorites, rich in carbon and in water-bearing minerals.
The surprise of 2019 was its shape. Hygiea is the parent of one of the largest asteroid families, nearly 7,000 fragments born of a colossal impact about two billion years ago, and astronomers expected to find a huge crater on it. There is none. Simulations indicate that the collision, with a body between 75 and 150 km wide, shattered it completely, and that the pieces, falling back together, formed a new, round body.
Orbit and rotation
Hygiea orbits in the outer part of the main belt, 472 million km from the Sun on average, or 3.15 astronomical units, and takes 5.6 years to complete one lap. Its orbit is only slightly inclined, 3.8° to the ecliptic, and mildly eccentric.
It spins once every 13.8 hours, slowly compared with the other large asteroids. Its distance and dark surface make it hard to observe: although it is the fourth largest, it never gets brighter than about magnitude 9, which is why it was only the tenth to be discovered, after bodies much smaller but closer and brighter.
Discovery and exploration
Annibale de Gasparis discovered it on 12 April 1849 from the Capodimonte Observatory in Naples: it was the first of nine asteroids found by the Italian astronomer. The observatory's director proposed calling it Hygiea Borbonica, joining the Greek goddess of health to the dynasty that ruled the Kingdom of the Two Sicilies; the adjective soon fell out of use.
No spacecraft has visited Hygiea. The images that revealed its shape were taken between 2017 and 2018 with the VLT's SPHERE instrument, which corrects for atmospheric turbulence and achieves from the ground a sharpness comparable to that of a space telescope. The study, led by Pierre Vernazza, was published in Nature Astronomy in October 2019.
Frequently asked questions
Is Hygiea a dwarf planet?
Not officially. Of the requirements set by the IAU in 2006, Hygiea also meets the last one, a shape rounded by its own gravity, as the 2019 images showed. The authors of that study proposed it as a dwarf planet, the smallest known, but the IAU has not ruled on it and in the catalogues it remains an asteroid.
Why is Hygiea so dark?
Because its surface is rich in carbon compounds, like that of the meteorites called carbonaceous chondrites. It reflects about 7% of sunlight, against more than 40% for Vesta. This is the most common case in the outer part of the main belt, where asteroids have preserved more primitive materials.
Who discovered Hygiea?
Annibale de Gasparis, on 12 April 1849, from the Capodimonte Observatory in Naples. It was the tenth asteroid known and the first he discovered; in the following years he found eight more, including Psyche.
Watch the whole Solar System in motion
Real orbits, moons and speeds rebuilt from NASA/JPL data, right in your browser. Free, nothing to install.
Launch the 3D simulation