Is Hygiea the smallest dwarf planet?

How ESO's Very Large Telescope revealed a round Hygiea in 2019, and why the IAU still hasn't changed its label.

Published 2026-10-23

Dwarf planets are usually associated with distant icy worlds like Pluto, plus Ceres in the asteroid belt. But in 2019, a team led by Pierre Vernazza suggested a new candidate much closer to home: 10 Hygiea.^[1]

The four tests

Under the IAU's definition, a dwarf planet orbits the Sun, is not a moon, has not cleared its orbital neighborhood, and has enough mass for its own gravity to pull it into a roughly round shape. As a main-belt asteroid, Hygiea passes the first three tests automatically.^[1] The question was its shape.

A round answer from the VLT

Using the SPHERE instrument on ESO's Very Large Telescope, astronomers observed Hygiea in enough detail to measure its shape and size for the first time. It turned out to be nearly spherical, with a diameter of just over 430 km. For comparison, Ceres is close to 950 km across and Pluto close to 2,400 km.^[1] If Hygiea qualifies, it would be the smallest dwarf planet known.^[1][3]

The missing crater

The team expected a giant impact basin, because Hygiea is the main member of a family of close to 7,000 asteroids that came from one parent body. Instead, they found only two unambiguous craters, both too small. Simulations pointed to a head-on collision with a body 75–150 km across about 2 billion years ago that completely shattered the parent. The pieces reassembled into a round Hygiea.^[1]

Officially… not yet

The IAU has not classified Hygiea as a dwarf planet.^[3] Interestingly, the IAU committee that drafted the 2006 planet definition had considered Hygiea as a possible dwarf planet if it proved round enough.^[3]

Hygiea is also strikingly similar to Ceres in its dark, carbon-rich, water-altered surface, which suggests the two may have similar origins.^[3]

Numbered citations link to the sources list on the Hygiea overview page.