The Formation and Evolution of Planetary Systems

Ever since humans understood that stars are other suns, we have wondered whether planetary systems similar to our own might exist around other stars, and whether the origin and evolution of life might have occurred also on other planets in addition to the Earth. The reason it is very difficult to discover if other planets exist around nearby stars is the large distance to astronomical objects, which imply that a planet next to a nearby star would be extremely faint and very close to its host star, making it extremely challenging to observe the planet directly owing to the glare of the star.

The first method used to discover the so-called exoplanets was the method of radial velocities. The first planets were discovered with a radio telescope around a pulsar, which is a neutron star. These planets are believed to have formed after a supernova explosion and should be very different from the ones in our Solar System. However, the majority of the planets discovered after that were found by measuring very accurate radial velocities of stars similar to the Sun with spectrographs on large telescopes. As of today, nearly 300 planets are known. We have learned that planetary systems are highly diverse, and remarkably different from the Solar System. At present it is only possible to discover rather massive planets that are not too distant from their host star, because the smaller planets produce velocities of their host stars that are too small to be detected, and distant planets have orbital periods that are longer than the time over which we have been making these measurements.

The exoplanets tend to have rather large orbital eccentricities compared to our Solar System. It seems that planetary systems undergo an evolution from the time of their formation from a gas disk around a young star, in which their orbits are perturbed by their mutual gravitation. Probably many planets are ejected from the system during this evolution, and others are left around their star but on orbits that have substantial eccentricities. There also examples of planets on orbits of small eccentricity like in the Solar System. We have also found that planets are more frequently found in stars of a high content of heavy elements (the so-called metals in astronomy).

Recently I am investigating planet formation in a scenario where there would be two populations of planets: one formed in the standard mechanism, from a gas disk in which planetessimals would grow into a seed that would later accrete gas and become a Jovian planet, and another one where they would form directly from the fragmentation of the protostellar cloud. This second mechanism should lead also to the formation of brown dwarfs.

In this scenario, planets formed by fragmentation at large distances from their stars would sometimes be perturbed among themselves or by nearby stars and molecular clouds, and these perturbations would shift their orbits to very high eccentricity. When the planets cross the disk of gas around the star, they could be slowed down in the collision and be captured by the disk. In this way their orbit would gradually circularize and become coplanar with the disk, and in the end the planet would be left in a small orbit looking as if it had been formed from the disk like other planets. These planets would then be found in short period orbits from the method of radial velocities and be among the present sample.

This work has been carried out with my collaborator Ignasi Ribas at ICE (Barcelona), and my graduate student Andreu Font-Ribera.

Publications related to this project :

  1. I. Ribas and J. Miralda-Escudé 2007, ``The Eccentricity-Mass Distribution of Exoplanets: Signatures of Different Formation Mechanisms?'', A. & A., 464, 779.
    You can also find the astro-ph preprint .
  2. A. Font-Ribera, J. Miralda-Escudé , and I. Ribas 2009, Ap. J., in press ``Protostellar Cloud Fragmentation and Inward Migration by Disk Capture as the Origin of Massive Exoplanets'' A. & A., 464, 779.
    You can also find the astro-ph preprint .