When Sicilian astronomer Giuseppe Piazzi spotted Ceres in 1801, he thought it was a planet. Astronomers didn’t know about asteroids at that time. Now we know there’s an enormous quantity of them, primarily residing in the main asteroid belt between Mars and Jupiter.

Ceres is about 1,000 km in diameter and accounts for a third of the mass in the main asteroid belt. It dwarfs most of the other bodies in the belt. Now we know that it’s a planet—albeit a dwarf one—even though its neighbours are mostly asteroids.

But what’s a dwarf planet doing in the asteroid belt?

A new research article provides the answer: Ceres didn’t form in the asteroid belt. It formed further out in the Solar System and then migrated to its current position. This isn’t the first study to reach that conclusion, but it adds weight to the idea.

The article is “Dynamical Origin of the Dwarf Planet Ceres,” and it’s published in the journal Icarus. The lead author is Rafael Ribeiro de Sousa, a physics professor at Sao Paulo State University in Brazil. Other co-authors come from the same university and France and the US.

(Note: Ceres is called a dwarf planet, a protoplanet, and sometimes an asteroid. No point getting hung up on it. It was officially classified as a dwarf planet in 2006.)

Ceres is one of three dwarf planets or protoplanets in the asteroid belt. The other two are Vesta and Pallas. A fourth large body, Hygiea, is 434 km in diameter and may also be a dwarf planet. These four largest bodies make up half the mass of the asteroid belt.

These are the four largest objects in the asteroid belt. Ceres is the only one massive enough for self-gravity to maintain a spheroid shape. Image Credit: ESO/M. Kornmesser/Vernazza et al./MISTRAL algorithm (ONERA/CNRS)
These are the four largest objects in the asteroid belt. Ceres is the only one massive enough for self-gravity to maintain a spheroid shape. Image Credit: ESO/M. Kornmesser/Vernazza et al./MISTRAL algorithm (ONERA/CNRS)

Most of what we know about Ceres comes from NASA’s Dawn mission. Dawn was the first spacecraft to visit two extraterrestrial bodies and the first to orbit a dwarf planet. Dawn visited both Vesta and Ceres before the spacecraft ran out of fuel in October 2018. Now it’s a derelict in a stable orbit around Ceres.

An artist's illustration of NASA's Dawn spacecraft approaching Ceres. Image: NASA/JPL-Caltech.
An artist’s illustration of NASA’s Dawn spacecraft with its ion propulsion system approaching Ceres. Image: NASA/JPL-Caltech.

The terminology and descriptions of the largest objects in the asteroid belt can be confusing, but Ceres stands apart from the other three. Ceres is the only body in the belt that’s massive enough to maintain a spheroid shape. Ceres also has a transient atmosphere called an exosphere. Sunlight sublimes water ice and ammonia ice into vapour, but the dwarf planet’s gravity is too weak to hold onto it. This is an important clue to Ceres’ origins because asteroids don’t typically emit vapour.

The presence of ammonia is also a clue.

Compounds like ammonia condense beyond the Solar System's frost line. Since Ceres contains ammonia, it likely formed beyond the frost line. Image Credit: NASA / JPL-Caltech, InvaderXan of http://supernovacondensate.net/.https://www.mansbrand.com/lisa-has-passed-a-key-review-phase-its-time-to-actually-design-the-final-mission/

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