Nomination of Prof. Michel Mayor, Dr. Garik Israelian and Dr. Nuno Santos for the Ambartsumian International Prize

The discovery 15 years ago of an extra-solar planet orbiting the solar-type star 51 Peg (Mayor & Queloz 1995, Nature 378, 355) has encouraged the launch of numerous search programs leading to a steadily increasing number of exoplanet detections. More than 400 other planetary companions have been found to orbit sun-like stars. Half of these planets were detected by a group in Geneva led by Prof. Michel Mayor.
This figure shows the planet mass as a function of the year of discovery. Red dots denote planets discovered with HARPS since 2004. Only planets discovered using the radial velocity method are displayed. Most of the known planets have been discovered using this method.

The growing number of exoplanets allows for the statistical analysis of their properties, as well as those of their host. One of the remarkable correlations that is also helping to understand the processes of planet formation is related to the stars hosting planets.

Swiss Academy of Sciences has nominated Prof. Michel Mayor and two members of his team for the Ambartsumian Prize. These researchers have made a fundamental contribution to this field.

Prof. Michel Mayor presently Emeritus Professor at the University of Geneva has created and led a group of Extrasolar Planets since the beginning of 1990s.

Dr. Garik Israelian has obtained his PhD in the Byurakan Observatory (1993) in Armenia. Dr. Israelian has lectured a PhD course on Stellar Atmospheres and Radiative Transfer at Geneva University in 2001. He is presently staff researcher at the Institute of Astrophysics of Canary Islands (Spain) and leader of the team Observational Tests of the Processes of Nucleosynthesis in the Universe.

Dr. Nuno C. Santos obtained his PhD degree in the University of Geneva (2001). He is presently researcher at the University of Porto and leader of the Origin and Evolution of Stars and Planets team at the Centre for Astrophysics.

The close collaboration between Prof. M. Mayor and two members of his team, Dr. G.Israelian and Dr. N. Santos, has led to several key investigations in the field of the discovery of extrasolar planets and the properties of their parent stars.

An overview of the results presented for the nomination

Given that planet formation is a by-product of the stellar formation process, a close look at planet-host stars is expected to provide important and unique clues about the formation of planetary systems.

The first detailed spectroscopic uniform studies, comparing stellar metallicities for planet hosts with those found for large comparison samples of field dwarfs (Santos, Israelian & Mayor 2001, Astronomy & Astrophysics, 373, 1019; Santos, Israelian & Mayor 2004, Astronomy & Astrophysics, 415, 1153), have clearly demonstrated that there is a strong correlation between the stellar metallicity (quantity of heavy elements) and the probability of finding a giant planet. These authors have proposed for the first time that planet host stars are metal rich because they were born in metal rich protoplanetary clouds. Confirmation of this result has been obtained by Fischer and Valenti (2005, Astrophysical Journal, 622, 1102) based on a different sample.

In this figure the frequency of giant planets is shown as a function of the amount of iron in the planet host stars. From Santos et al. (2005, Science, 310, 251) based on the results of Santos, Israelian and Mayor (2001, 2004).

The results obtained on the metallicity studies by Santos, Israelian and Mayor were absolutely crucial to constraint planet formation models. Indeed, the fact that the probability of finding a giant planet is a strong function of the stellar metallicity provided the first strong observational evidence that the core-accretion model as the main mechanism responsible for the formation of the observed giant planets population. The two papers mentioned above have now more than 500 citations.

Most spectroscopic studies on the chemical properties of stars with planets have concentrated on measuring the abundances of iron as a metallicity proxy. The authors have, however, explored with unprecedented detail the abundances of other species (CNO, Be, Mg, Si, Ti, Ca etc.) in dozens of refereed publications (Neves et al. 2009, Astronomy & Astrophysics, 497, 563, Ecuvillon et al. 2006, Astronomy & Astrophysics, 449, 809). Particular relevant were the results concerning the abundances of light elements Lithium and Beryllium (Santos et al. 2002, Astronomy & Astrophysics, 386, 1028, Santos et al. 2004, Astronomy & Astrophysics, 427, 1085).

The authors led a ground-breaking census of 500 stars observed in the context of the High Precision HARPS planet search program that has successfully linked a 60 years old “lithium mystery” observed in the Sun to the presence of planetary systems (Israelian et al. 2009, Nature, 462, 189). They have clearly demonstrated that like our own star, Sun-like stars that host planets have destroyed their initial lithium (the common isotope 7Li) much more efficiently than “planet-free” single stars. Furthermore, the authors have demonstrated that this trend is not related to any stellar property, like its mass or age (Sousa et al., 2010a, A&A, 512, L5). This finding shed light on the lack of lithium in our star and provides the first clear observational evidence of a chemical element (Lithium), which behaves in a different way in stars with exoplanets. The latest results by Sousa et al. (2010b, in The 16th Cambridge Workshop on Cool Stars, Stellar Systems and the Sun) provide additional support to these findings. The link between Lithium depletion and planet formation lies deep in the processes of angular momentum transfer in protoplanetary disks and rotational evolution of sun-like stars (Eggenberger, Maeder and Meynet, Astronomy & Astrophysics, 2010, Baraffe & Chabrier, 2010, Astronomy & Astrophysics, in press).

Furthermore, these authors have also shown that the rare Lithium isotope, 6Li, can be used as a tracer of accretion of planetary material (or entire planets) into the star. The well known “6Li test” proposed by the authors (Israelian et al. 2001, Nature, 411, 163) is a powerful tool to estimate the amount of matter accreted by a star following a process of planet formation. It can thus be used to obtain invaluable information about dynamical processes occurring during the early phases of planetary system formation and evolution. The “6Li test” could also be used to distinguish between giant planet formation theories (Sandquist et al. 2002, Astrophysical Journal, 572, 1012).

Lithium abundances plotted against effective temperature in solar-analogue stars with (red dots) and without (empty circles) detected planets. The red circle with the black point at its centre indicates the Sun. From Israelian et al. (2009, Nature, 462, 189)
Although the results mentioned above are linked to the presence of giant planets, the discovery of a growing population of Neptune-type planets and Super-Earths has already extended these studies. For example, the development of HARPS (Mayor et al. 2003, ESO Messenger, 114, 20) allowed the discovery of very low mass planetary systems such as mu Ara (a 4 planet system including a 10 Earth mass planet - Santos et al. 2004, Astronomy & Astrophysics, 426, L19), HD40307 (a system of 3 Super-Earths – Mayor et al. 2009a, Astronomy & Astrophysics, 493, 639), or the incredible system Gl581 containing the lowest known mass planet with only 2 times the mass of our Earth (Mayor et al. 2009b, Astronomy & Astrophysics, 507, 487).

Mass distribution of planets around solar-type stars showing a new population of low mass planets mostly discovered by HARPS. From Udry & Santos, 2007, Ann. Rev. Astron. & Astrophys. 45, 397, and partial based on the detections by Santos et al. 2004, Astronomy & Astrophysics, 426, L19, Mayor et al. 2009a, Astronomy & Astrophysics, 493, 639, Mayor et al. 2009b, Astronomy & Astrophysics, 507, 487)
Here again, the authors are leading the field with groundbreaking discoveries. For example, they have already found some hints that stars orbited by very low mass planets do not follow the same metallicity trends found for the giant planet hosts counterparts (Sousa, Santos et al. 2008, Astronomy and Astrophysics, 487, 373). This result can be confronted with results of planet formation synthesis models actively developed by different teams (D. Lin and S. Ida, and W. Benz groups).