Asteroid Hygiea May Be the Smallest Dwarf Planet in Our Solar System

Hygiea is the 10th asteroid discovered in the Asteroid Belt and the fourth-largest asteroid by mass. While one of the largest asteroids, its dark surface made it difficult to distinguish from Earth using telescopes of the day. Ceres was discovered in 1801, but Hygiea (formal designation: 10 Hygiea) wasn’t located until 1849.

We actually discussed Hygiea in passing recently, when we revisited the “Is Pluto a planet?” question. The note that accompanied the discovery of 10 Hygiea in the 1850 Annual of Scientific Discovery is reprinted below:

Unlike Ceres and Vesta, Hygiea has never been visited by a probe, so our knowledge of it is comparatively limited. Upon taking additional measurements of Hygiea with the SPHERE instrument attached to the VLT (Very Large Telescope) in Chili’s Atacama Desert, the scientists have revised some of our metrics and measurements for the dark asteroid — and in the process, raised the idea that it may be the smallest dwarf planet in the solar system.

The debate over dwarf planets actually touches directly on the entire question of whether Pluto is a planet. Dwarf planets are a new classification created to describe objects like Ceres and Pluto. These objects are rounded under their own gravity and are in orbit around the Sun, but they have not cleared their own orbits (this definitionally applies to the asteroid belt). Observing Hygiea with ground-based equipment showed that the asteroid looks almost as round as Ceres, with no sign of the mammoth impact craters that have deformed objects like Vesta.

That’s significant because Vesta is almost a dwarf planet in its own right. Some of the definitions of a planet that the IAU considered back when it was reclassifying Pluto would have actually made Pluto and Vesta both planets, but Vesta isn’t round anymore. While it may have been at one point, massive impacts smashed into the asteroid, knocking its shape out of true and leaving it just outside the acceptable range for a dwarf planet.

The new estimates on Hygiea show that its day is about half the length previous estimated and it’s a bit smaller than thought, at 430km in diameter. Ceres and Pluto are 2400km and 950km in diameter, respectively, so a round Hygiea would be a significantly lower bar for the minimum size of any dwarf planet. But Hygiea is also the known source for a large asteroid family, and an impact large enough to create the observed asteroids should have left Hygiea with far more damage than it appears to have (Vesta’s massive craters are dimly visible in ground-based measurements).

This video shows how the impact that created the Hygiea family of asteroids might have played out. After a head-on collision, most of the debris created by the impact are pulled back down on to Hygiea by gravity, effectively reassembling some of the smashed material. This impact would have obliterated Hygiea’s parent body. It would explain why the resulting planetoid is round, however, given that we already know the Hygiea impact also seeded an entire asteroid family into the asteroid belt.

If true, there are some interesting implications for Hygiea itself. The asteroid would have a notably different composition than Vesta or Ceres and might resemble a rubble pile covered with a layer of regolith rather than a true asteroid. It’s not clear if these findings will persuade NASA to devote more resources to puzzling out this mystery; a dwarf planet designation would definitely be a bump for Hygiea’s overall profile. NASA has an upcoming mission to 15 Psyche planned, so it’s possible that we might be able to steal a look at Hygiea depending on their respective orbital positions when the Psyche mission deploys.

Feature image by NASA. Sorry folks. That’s the potato we have, not the potato we wish we had.