Are asteroids cosmic refugees?

A new study suggests that the asteroid belt is a cosmic refugee camp, populated by objects that grew across the Solar System but not in the belt itself. Asteroids travelled the voids of space before settling on stable orbits in the belt.

The asteroid belt is a vast swath of Solar System real estate that extends between the orbits of Mars and Jupiter. The belt is nearly empty, containing a total of less than 1/1000th of the mass of Earth, and more than half in the four largest asteroids (Ceres, Vesta, Pallas and Hygiea). The belt is segregated: its inner parts are dominated by dry “S-types” and the outer parts by water- and carbon-rich “C-types”, which have very different chemical properties.

Models of Solar System formation have traditionally assumed that the asteroid belt was born massive, but that all but a sprinkling of that material was later removed. Classical models struggle to match the terrestrial planets and asteroid belt. However, some new ideas such as the “Grand Tack” model have succeeded by invoking other processes such as large-scale orbital excursions (“migration”) by Jupiter, which efficiently clears the asteroid region but requires a specific evolution of the giant planets.

Researchers at the Laboratoire d’Astrophysique de Bordeaux (France) and Universidade Estadual Paulista (Brazil) have proposed a new model in which the asteroid belt was born empty then later re-filled. In their model, planetesimals – planetary building blocks – formed in the terrestrial planet- and giant planet regions but not in the asteroid belt in between. This picture is consistent with high-resolution images of planet-forming disks in which dust tends to clump into rings rather than be smoothly distributed.

In this new model the asteroids originated from different reservoirs. The researchers showed that, in the process of forming the rocky planets, planetesimals from the inner Solar System are naturally implanted into the asteroid belt with the right total abundance and distribution. These may represent the S-types. The C-types were injected from the Jupiter-Saturn region (and beyond) as a byproduct of the giant planets’ growth. Combining these two effects produces an asteroid belt with the properties that we see, starting from a completely empty belt.

This new idea does not settle the issue but rather reinvigorates the debate regarding the origin of the Solar System.

Sean Raymond’s blog: