Can we satisfactorily construct models of the formation of planetary systems based on a study of just our solar system? A decisive step forward has been taken thanks to high angular resolution techniques capable of surveying planetary systems being formed around stars other than our Sun.

In 2005 the JENAM (Joint European and National Astronomy Meeting), which brings together astrophysicists from all over the world, took place at the University of Liège. There the astronomers Olivier Absil and Dimitri Mawet, from the ULg’s Extragalactic astrophysics and space observation group, presented an overview of the interferometers used in astrophysics and thus attracted the attention of the editors of the Astronomy and Astrophysics Review. It was the birth of an ambitious editorial project which has just been published (1) and in which our two Liège researchers take stock of 10 years of observing planetary systems being formed, using different high angular resolution techniques such as interferometry or coronagraphy.

Angular resolution is the capacity of an instrument to distinguish two objects which appear very close to each other in the sky. In effect for one and the same physical separation two celestial bodies will appear all the more closer to one another the more distant they are from the observer. Thus the further away and/or smaller the astrophysical sources are, the more the instruments which observe them must have available high angular resolution power. The techniques which allow access to high angular resolution are relatively recent: they only started to supply results at the beginning of the years 2000, but the harvest is already an abundant one...

The solar system, for the lack of anything else

Up until the end of the 1990s, our Sun and its cortege of planets was (practically) the only observable and observed planetary system. It was thus on this model that leaned the theoretical models of planetary formation whose physical foundations are still accepted today, at least in terms of the broad outlines: a planetary system is formed around a young star, in the remnants of the protostellar nebula at the origins of the star. By rotating on itself this residue of gas and dust flattens out to the point where it forms a disk of sufficient density to allow collisions between different dust grains and thus enables their clustering together. It is the distance from the star which determines the type of planet formed: a planet which forms close to its star will be small and rocky, whilst if it is further out it will be a giant and gaseous. ‘If this model has not dated,’ specifies Olivier Absil, ‘it has been refined since the beginning of the years 2000, thanks to the discovery of several hundreds of exoplanets. We now know that the planets do not necessarily stay in the place where they were formed and that they can migrate:  a massive planet formed far from its star can come closer to it (hot Jupiters) and the opposite is also true.’

(1) Absil O., Mawet D., 2010. Formation and evolution of planetary systems : the impact of high angular resolution optical techniques, A & A Review 18, 317-382. DOI 10.1007/s00159-009-0028-y