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Ceres may have formed in the outer solar system

A map of Ceres' northern hemisphere by the Dawn spacecraft's Gamma Ray and Neutron Detector, GRaND. Credit: NASA/JPL-Caltech/UCLA/ASI/INAF

A map of Ceres’ northern hemisphere by the Dawn spacecraft’s Gamma Ray and Neutron Detector, GRaND. Credit: NASA/JPL-Caltech/UCLA/ASI/INAF

Dwarf planet Ceres may be located in the asteroid belt between Mars and Jupiter, but its composition and large water content suggest it formed in the outer solar system where other dwarf planets orbit.

Visited by NASA’s Dawn spacecraft in 2015, Ceres differs from all other objects in the asteroid belt in several ways.  It has a diameter of 588 miles (946 kilometers) and is spherical due to being rounded by its own gravity.  Only two other objects in the belt, Vesta and Pallas, even come close to being round.

While asteroids are mostly collections of rock held together loosely by chemical bonds, Ceres has the shape and complexity of a planet, and its mass equals one third of the total mass of the asteroid belt.

Like the terrestrial planets, Ceres is differentiated into core, mantle, and crust. Its crust, or surface, which Dawn mapped, is a mix of water along with clay and carbonates. Its mantle is composed of water ice while its core is made up of rock.

Beneath its surface, Ceres may harbor remnants of an underground ocean, much like those that exist on Pluto and on icy moons such as Europa and Enceladus.

Ceres’ surface also contains high levels of ammonia, a substance not found on other objects in the asteroid belt but far more common in the outer solar system. Heat from the Sun typically evaporates ammonia in the region between Mars and Jupiter. More distant regions like the Kuiper Belt are colder, so volatiles like ammonia are much more common.

With a low density of 2.2 grams per cubic centimeter and low reflectivity, Ceres somewhat resembles carbonaceous chondrites, also known as C-type asteroids.  These are the most common asteroids in the belt, and many are located relatively close to Ceres.

However, C-type asteroids have nowhere near the amount of water Ceres has and do not contain ammonia.

Noting these differences, a team of scientists led by Rafael Ribeiro de Sousa of the University of Sao Paulo in Brazil propose Ceres originated somewhere beyond the orbit of Saturn in a paper recently published in the journal Icarus.

According to one model of solar system evolution, the gas giant and ice giant planets formed closer to the Sun and migrated outward during the solar system’s early, chaotic years. This movement displaced smaller, distant objects like Ceres, some of which migrated inward while others were destroyed in violent impacts and still others were completely ejected from the solar system.

This same outward migration of the gas and ice giants could have led to Neptune’s capture of Triton, its largest moon, which is thought to have once orbited the Sun on its own.

“The results of our numerical simulations support the idea that some Ceres-sized planetestimals might have been implanted from the trans-Saturnian region into the asteroid belt by a combination of close encounters with giant planets, with other protoplanets, gas drag, chaotic diffusion, and mean motion resonances with Jupiter,” the researchers note in the paper.

According to computer simulations run by the researchers, as many as 3,500 small planets of Ceres’ size had to exist in the outer solar system in order for even one to survive the many collisions of the solar system’s earliest years and end up in Ceres’ location.


Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.

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