Paris, 9 June 2011
Why is Mars, the planet closest to us, ten times less massive than the Earth? This question remained unanswered for a long time, since it was extremely difficult to reproduce the mass of Mars in simulations. In 2009, recent progress in this field enabled Brad Hansen, an American researcher, to propose a model reproducing the initial conditions of terrestrial planets' formation. These could have been created from a 0.3 astronomical unit-wide disk of material (1 AU is the Earth-Sun distance), believed to extend from 0.7 to 1 AU. The centre of this disk, where the material was concentrated, may have contained the building bricks for Venus and the Earth, the largest terrestrial planets in the solar system. The outer and inner edges could have generated Mars and Mercury, respectively. However, this model does not take into account the existence, within the solar system, of planetary material beyond Mars, where the asteroid belt (between 2 and 4 AU) and the four giant planets (Jupiter, Saturn, Uranus and Neptune, between 5 and 30 AU) are located.
The study of exoplanets has revealed that certain giant planets can migrate near to their star. On the basis of this observation, Alessandro Morbidelli and his colleagues have proposed the hypothesis that the giant planets of our solar system (Jupiter and Saturn) migrated within the solar system before the formation of the terrestrial planets. The researchers based their study on Hansen's work to envisage the following scenario: before the formation of Saturn, Jupiter could have migrated towards the Sun up to the present position of Mars (1.5 AU from the Sun). It could then have pushed aside or ejected all the material in its path, leading to the formation of a “truncated” 0.3 AU-wide disk of material, with an outer edge at 1 AU (according to the work of Hansen). Saturn, once formed, may in turn have migrated towards the Sun. Under its “influence”, Jupiter could have “veered off track” and migrated until it reached its current position (around 5 AU from the Sun), beyond the asteroid belt.
Using numerous digital simulations, the scientists have demonstrated that the migrations of Jupiter and Saturn are compatible with the formation of the asteroid belt between Mars and Jupiter. In addition, they have succeeded in explaining the coexistence of two types of asteroids in this belt: some very dry, others with high water contents. According to the “gas-driven migration" scenario, Jupiter could have intercepted two populations of small bodies during its migrations. Those now situated in the inner part of the asteroid belt could have come from the zone between 1 and 3 AU from the Sun, whereas those located in its outer part could have come from a separate region, beyond 5 AU.
“This model implies that the giant planets of our solar system underwent substantial radial migration, just like the planets observed around other suns”, explains Sean Raymond. Another important aspect is that this new model provides an explanation for the very first millions of years of our solar system, a history made of numerous hitherto unexplained enigmas. The team is now seeking to include the formation of Uranus and Neptune in this scenario.
1 - In France, the laboratories involved were: Laboratoire “Cassiopée Astrophysique, Sciences Mécaniques et Analyse des Données” (CNRS/Université de Nice) at the Observatoire de la Côte d'Azur and the Laboratoire d'Astrophysique de Bordeaux (CNRS/Université Bordeaux1).
A low mass for Mars from Jupiter's early gas-driven migration. K.J. Walsh, A. Morbidelli, S.N. Raymond, D.P. O'Brien, A.M. Mandell. Nature, Published online on the 5 June 2011 (the actual paper is scheduled to be published in July 2011).
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