Do planets form around small stars ?
1er décembre 2006
More than 200 exoplanets have now been detected around stars in the solar neighborhood, most of them through the measurement of tiny perturbations in the velocity of the star. The vast majority of these host stars are solar-type stars, while the most common stars in the Galaxy are small stars, less massive and less luminous than the Sun. Nonetheless, it was unknown till now whether or not planets form as frequently around these small stars as around solar-type stars. A team of astronomers from France, United Kingdom and Germany led by Jean-Francois Lestrade of Paris Observatory, just publishes in Astronomy & Astrophysics the first survey of low-mass stars to search for debris disks, which are the smoking gun for the existence of planets. They conclude that the presence of planets is equally probable around low-mass stars as it is around solar-type stars.
Searches for planetary systems around other stars are actively pursued by a variety of methods in astronomy. The most efficient technique so far detects tiny perturbations in the radial velocity of the parent star under the gravitationnal tug of the planets. The vast majority of the 200 extra-solar planets known today has been found by this technique around essentially solar type stars. However, the ability to form planets by stars different from the Sun is important to make a full census of planetary systems in the Galaxy. This is especially important for low-mass stars that are the most common in the Galaxy, they represent 70% of all stars. Formation of planets around low-mass stars could be strongly influenced by their very low luminosity, 1/10 to 1/10000 the solar luminosity. Theory of planet formation is still in its infancy and complex enough that it is yet impossible to predict whether or not this is a favorable factor.
- Figure 1 : Vue d’artiste d’un disque de débris, mélange de planétésimaux de glace et de roche et de poussière, entourant une naine rouge de faible luminosité (étoile de faible masse). Les instruments astronomiques n’ont pas encore la possibilité de faire une image si détaillée mais ils fournissent assez d’indices aux astronomes pour développer le concept représenté par cette image. (Credit : UCLA)
To answer this question, the international team of astronomers has tackled this issue by indirect observations. They have conducted the first survey of low-mass stars to search for debris that would have been left around these stars had planet formation occured. This method is an alternative to the radial velocity technique for these faint stars. These debris are in form of rocky/icy planetesimals, similar to comets, embedded in dust produced by their occasional collisions. These debris are left over in the periphery of planetary systems ; in our solar system this is the Kuiper Belt at 50 Astronomical Units from the Sun. The dust at such distance from the central star is very cold (-250 C) and can be detected by radiotelescopes observing at wavelengths shorter than about 1 millimeter. The team has used the UK/NL/CA radiotelescope JCMT in Hawaii and the FR/DE/ES radiotelescope IRAM near Granada in Spain to conduct observations of a sample of 32 low-mass stars. Analysis of the data and complementary data from the scientific literature have allowed to determine for the first time that between 7 and 21% of the low-mass stars are surrounded by debris disks. An interesting conclusion comes from the fact that this percentage is similar to the one found for the solar type stars and for the stars more massive than the Sun already searched for debris disks in previous surveys. Although, the percentage found needs to be refined by further observations, it indicates already that debris disks are equally prevalent across all stellar types. The main unanswered question is whether the detection rate of debris disks around stars (about 10%) is significantly underestimated. Could it be that debris disks exist around most of the stars and, correlatively, that planet formation is ubiquitous ? This question can be put into perspective by recalling that the debris disk of the Sun, the Kuiper Belt, has 10000 times less dust than debris disks that can be detected with present radiotelescopes. If these telescopes and a few astronomers were transfered onto the closest star, Proxima Centauri, their instruments would not be sensitive enough to detect the dust of the Kuiper Belt and they could not know that planet formation processes have occured around the Sun ! It is likely that our position is presently similar.
Reference : Search for Cold Debris Disks Around M-dwarfs J-F. Lestrade (Paris Obs), M.C. Wyatt (Cambridge Univ.), F. Bertoldi (Bonn Univ.), W.F.R. Dent (ROE, Edinburgh), K.M. Menten (MPI, Bonn), A&A, 2006, 460, 733-741
Contact Jean-Francois Lestrade (Observatoire de Paris, LERMA et CNRS )
Dernière modification le 4 mars 2013