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The rocks that remained

An alternative model suggests the asteroid belt originated in leftovers from the formation of the planets in the solar system

NASA The asteroids 433 Eros (left) and 253 Mathilde: 433 Eros is an S-type asteroid, rich in silica and typical of the interior of the belt; 253 Mathilde is a C-type asteroid containing carbon, more commonly found in the outer portionNASA

Billions of irregularly shaped masses, most the size of a stone but a few hundreds of kilometers in diameter, revolve around the Sun in the region between the orbits of Mars (the last of the four rocky planets) and Jupiter, the largest planet in our system. This group of orbiting rocks comprises what is conventionally known as the asteroid belt. The origin of this agglomeration of asteroids is a mystery, but most widely accepted ideas assume that there was much more of this material in this region in the early days of the solar system, and that 99% of it was expelled for some reason. Astrophysicists André Izidoro, of the Orbital Dynamics and Planetology Group at São Paulo State University’s Guaratinguetá campus, and Sean Raymond of the University of Bordeaux in France, proposed a new model based on computational simulations to explain the origin of the asteroid belt which is in sharp contrast with more traditional ideas. In the September 13 issue of Science Advances they published an article containing the details of their alternative model.

According to Izidoro and Raymond, the region where the belt is located today had been a large void when the solar system was born approximately 4.5 billion years ago, rather than a place where matter was concentrated, as other better-known models have maintained. In this model, the current configuration of the belt is not the result of enormous losses of matter throughout the history of the system, but rather a modest gain in matter. “The most external portion of the asteroid belt originated as a by-product of the formation of the solid nucleus of the gas giant planets, Jupiter and Saturn,” explains Izidoro. “And the innermost part emerged from the residues of planetary embryos that were the origins of the terrestrial planets, Mercury, Venus, Earth and Mars.” According to this hypothesis, agglomerations of materials that were not part of the composition of the gaseous or the terrestrial planets, particularly Jupiter and Mars, were expelled to the area which at that time was empty, where the solar system would arise, due to gravitational interactions and the dragging action of gas present in space. This led to the asteroid belt, a “cosmic refugee camp,” the metaphor Raymond uses to describe this area filled with stones of different sizes.

The new model also provides an explanation for the arrangement of the two main types of asteroids within the belt. The area farthest from the Sun is where C-type asteroids concentrate; they are dark and rich in carbon, and comprise 75% of the objects in the belt. Within the more internal section, most objects are S-type asteroids, brighter and with a high concentration of silica, accounting for 17% of the belt’s bodies. According to Izidoro and Raymond’s model, C-type asteroids (which are also called wet asteroids) originated from material remaining from the formation of gas giant planets. “Water on Earth may also have come from these asteroids that eventually collided with our planet when it was still forming,” says Izidoro, who together with Raymond addressed this issue in another recent article published on June 30 in the scientific journal Icarus. S-type asteroids, which are considered dry, are what remains of the materials which were used in the formation of Mars and other terrestrial planets.

For months, Izidoro and Raymond ran more than 200 computer simulations to see how the planets in the solar system may have formed and how the asteroid belt consequently emerged. The simulations assumed that no primal material was present between Mars and Jupiter, and the researchers were able to virtually reproduce the current constitution of the belt. “Our next step is to test each of the existing models, our own and others, to explain the asteroid belt and see what we can learn about the formation of the solar system,” says Raymond.

Low density 
For astrophysicist Jorge Meléndez of the Institute of Astronomy, Geophysics, and Atmospheric Sciences at the University of São Paulo (IAG-USP), the simulations by Raymond and Izidoro are very interesting and provide a new vision of the solar system. “The study shows that at the beginning of the system, a much more massive asteroid belt was not necessary,” says Meléndez. “One of the problems with the current model is explaining how this belt [which was supposedly so large early in its development] lost so much mass.” Currently, the mass of the belt is no more than 4% that of the Moon and is more than a thousand times less than the Earth. Although the asteroids within it spin around an enormous swath of the solar system, the belt itself has a low density of objects in relation to its area. Only one celestial body, the dwarf planet Ceres, has the same mass as one third of the entire asteroid belt.

Planetary formation and dynamics: From the solar system to exoplanets (No. 16/12686-2); Grant Mechanism Junior Researcher; Principal Investigator André Izidoro (UNESP); Investment R$178,755.00.

Scientific articles
RAYMOND, S. N. and IZIDORO, A. The empty primordial asteroid belt. Science Advances. September 13, 2017.
RAYMOND, S. N. and IZIDORO, A. Origin of water in the inner Solar System: Planetesimals scattered inward during Jupiter and Saturn’s rapid gas accretion. Icarus. June 30, 2017.