Entrevista com Michel Mayor

Michel Mayor é professor no Departamento de Astronomia da Universidade de Genebra e um dos pais da exoplanetologia. Em 1995, em conjunto com Didier Queloz, descobriu o primeiro planeta “normal” em torno de uma estrela de tipo solar, o 51 Pegasi b. Desde então tem liderado a designada “Equipa de Genebra”, da qual faz parte o português Nuno Santos, na descoberta de novos exoplanetas e no desenvolvimento de instrumentação para esse efeito. Ao longo da sua extensa carreira recebeu vários prémios pela excelência das suas contribuições para a ciência. Em particular, o seu papel fundamental no aparecimento e desenvolvimento da área da exoplanetologia valeu-lhe a atribuição de numerosos prémios internacionais como o “Prémio Marcel Benoist” (1998), o “Prémio Balzan” (2000), a “Medalha Albert Einstein” (2004) e o “Prémio Shaw em Astronomia” (2005). Aqui fica o registo de uma pequena entrevista que concedeu ao AstroPT.

[AstroPT] – How long had you been working on radial velocity follow-up of sun-like stars by the time you discovered, together with Didier Queloz, 51 Pegasi b ?

[M. Mayor] – This discovery was a direct consequence of a long and systematic development of spectrographs optimized to measure the radial velocity of stars. In the early 70ies I built a first echelle spectrograph called CORAVEL, in collaboration with André Baranne, optical engineer at Marseille Observatory. I have been using that instrument at Haute-Provence Observatory (HPO) for more than 20 years. During that time span we made a systematic search for double stars among the solar-type stars of the solar vicinity, with Antoine Duquennoy. At the end of the 80ies, HPO decided to build a new spectrograph using the same principle to determine stellar radial velocities. The Geneva Observatory joined the development of the new instrument and the result was not one but two, almost similar, spectrographs (ELODIE at HPO on the 1.93 meter telescope, and later CORALIE at La Silla Observatory on the 1.20 meter EULER telescope), again constructed with André Baranne as chief optician. ELODIE was put in operation in the first months of 1994, and I began, with A.Duquennoy and Didier Queloz, a search for exoplanets and brown dwarfs orbiting 142 nearby solar-type stars (closer than about 20 parsecs from the Sun).

[AstroPT] – Can you briefly reconstruct the events that led to the discovery of 51 Pegasi b and its announcement ?

[M. Mayor] – The stars in our program were selected as they appeared to be single from our previous CORAVEL study. But with the new ELODIE spectrograph the precision of the radial velocities was 20 times better (about 13 m/s). Antoine died in June 1994 and we continued the program, Didier Queloz (a graduate student at that time) and myself. We received 1 week of telescope time every 2 months. Already in the late Fall of 1994 a star (51 Pegasi) exhibited some apparently periodic variation. In january 1995 we had enough measurements to determine a first ephemeris, with a strange period as short as 4.23 days. We decided to wait until the next season to check the stability of the period, and the amplitude and phase of the variation (to eliminate the possibility of velocity variations linked with stellar variability). In July 1995 during one week at HPO, Didier and myself had the satisfaction to see that the variation was still there, unchanged … and we rushed to publish our discovery in Nature magazine.

[AstroPT] – Fifteen years have passed since that seminal work. How do you evaluate the progress that has been done in the field of exoplanetary science in this time lapse ? In your opinion what were the most spectacular discoveries so far ?

[M. Mayor] – Today more than 400 exoplanets have been discovered with masses from about 2 Earth-mass to 20 Jupiter-mass (the upper limit set to deuterium-burning is only an arbitrary definition). This is a range of masses of 3000. During the first years after the discovery of 51 Pegasi b, a few astronomers were still not convinced that these objects were planets. The discovery of the first transit (HD 209458b) provided the direct proof that this was really a gaseous giant planet, a crucial step. Another important discovery was that of multiplanetary systems. More recently, the discovery (made with the HARPS spectrograph) of a rich population of low mass planets (Super-Earths and Neptunes) orbiting solar type stars is impressive and also set constraints on formation mechanisms of these kind of planets. The spectacular efficiency of present experiments to detect transiting planets opens the possibility to do comparative planetology…and this only a limited list of the highlights in this young field of astronomy.

[AstroPT] – What is the focus of your current research ?

[M. Mayor] – Presently I am mostly interested in the detection of low mass planets and the study of the statistical properties of this population.

[AstroPT] – Recently, thirty new planets and two brown dwarfs were announced that were discovered under the HARPS GTO (Guaranteed Time Observations) program. Can you describe this program and its goals ?

[M. Mayor] – From 2000 to 2003 we built the HARPS spectrograph, an instrument optimized for detecting exoplanets. Our consortium was granted 100 observing nights per year during 5 years on the 3.6 meter telescope at La Silla. This guaranteed observation time (hence the GTO name) is given in compensation for having provided all the costs and manpower needed for the development of the instrument. Our Consortium did a rather comprehensive study of the field of exoplanets in the Southern Sky: (1) search for very low mass planets; (2) search for giant gaseous planets in an enlarged volume limited sample of solar type stars; (3) search for planets orbiting low mass stars (M dwarfs); (4) search for planets orbiting metal-deficient stars, and; (5) radial velocity follow-up of stars with detected photometric transits by the CoRoT satellite.

The goal of (1) was to explore this domain of mass and the result was outstanding with the discovery of the important population of low mass planets (Super-Earths) in tight orbits. The goal of (2) was to improve the statistical knowledge of the population of gaseous giant planets, important, for example, to put constraints on formation mechanisms. Many new planets have been discovered, some of them were announced recently at the conference in Porto. Some planetary systems of special interest have been detected. For example a system with 2 giant planets with resonant orbits studied by Alexandre Correia working at Aveiro university (a first example of a 3:2 resonance). Point (3) focused on M-stars. Many new planetary systems have been identified orbiting low mass stars. An important system, for example, is GJ 581 with 4 planets, one (GJ581e) being the lightest of any planets discovered up to now (1.94 Earth-mass) and another one (GJ581d) being located in the Habitable Zone of the star. The program (4), lead by Nuno Santos from Porto, aimed to search for planets orbiting metal-deficient stars. Only 3 gaseous planets have been detected ao far in a rather large sample of 120 stars. The host stars for these planets have metallicities larger than about Fe/H = -0.5. This value corresponds to a deficiency by a factor of 3 in heavy chemical elements when compared to the solar composition. Apparently, for stars deficient in heavy elements, it is very difficult to form gaseous giant planets during the life time of the protoplanetary disk. Finally, the most important result of (5) was the measurement of the mass of CoRoT-7b with HARPS. This is the first detection of a planet with a density about equivalent to Earth’s (a rocky planet with a mass between 5 and 6 Earth-masses).

[AstroPT] – Several of the above mentioned planets were Neptunes and Super-Earths. This is a new low mass planetary population that is being revealed by HARPS-class spectrographs. What have we learned about these planets so far ? What are their orbits like ? Their mass distribution ? Do they prefer high metalicity stars as the gas giants seem to ?

[M. Mayor] – I prefer to wait a few months before answering to that question as we are presently analyzing the full GTO data.

[AstroPT] – The University of Geneva team is currently involved in the development of HARPS-NEF (New Earths Facility) for the follow-up of exoplanet candidates from the Kepler Space Telescope. Can you describe the instrument being developed, how it compares with the original HARPS, and the current status of the project ?

[M. Mayor] – HARPS-NEF is similar to the existing HARPS except for a few improvements. One of them is a new calibration unit based on a lasercomb instead of the Thorium lamp used in the original HARPS. This produces a more precise reference frame for measuring radial velocities. Other improvements include an improved system to increase the stability of the illumination of the spectrograph’s optic, making it independent of guiding, focus and seeing. The instrument is currently under construction.

[AstroPT] – What technologies and projects do you think may revolutionize exoplanet studies in the coming years ?

[M. Mayor] – In the near future I see several projects that will provide us with powerful tools to study exoplanets, for example: improved adaptive optic systems like SPHERE (under development for the VLT); space missions to search for transiting planets around bright stars like the European Space Agency’s PLATO mission; high stability spectrographs like ESPRESSO (under development for the VLT), and; astrometric tools to search for planets like PRIMA, the dual beam interferometer at Paranal, to be commissioned very soon.

Pontos a reter da entrevista:

1 – A descoberta do 51 Pegasi b foi o culminar de muitos anos de investigação (desde os anos 70) no desenvolvimento de espectrógrafos de alta precisão para a medição da velocidade radial das estrelas;

2 – A existência do 51 Pegasi b já era conhecida em 1994, no entanto Mayor e Queloz, como manda a boa prática científica, adiaram a comunicação da descoberta quase um ano para terem certeza absoluta de que o efeito era real e eliminar outras possíveis explicações;

3 – Mayor classifica como notável a enorme variedade de planetas descobertos. Em sua opinião as descobertas de planetas em trânsito, de sistemas com vários planetas e, mais recentemente, de uma população de Super-Terras e Neptunos, são as que mais marcaram o estudo dos exoplanetas desde 1995;

4 – As descobertas fundamentais realizadas pelo programa GTO do espectrógrafo HARPS: numerosas Super-Terras em torno de estrelas de tipo solar; muitos planetas gigantes em órbitas com períodos longos, permitindo caracterizar estatisticamente esta população e perceber os mecanismos que formam estes planetas; a deficiência de planetas gigantes em torno de estrelas de baixa metalicidade; novos planetas em torno de anãs vermelhas, e; a confirmação do primeiro planeta de tipo terrestre em trânsito (CoRoT-7b);

5 – Mayor fala do HARPS-NEF (New Earths Facility) com um poderoso espectrógrafo, baseado em grande parte no HARPS, mas utilizando um “pente laser” para fornecer um espectro de referência de alta precisão e outros avanços tecnológicos que permitirão em princípio atingir a precisão necessária para detectar um planeta análogo à Terra, pelo menos para estrelas com espectros muito estáveis. Este instrumento será fundamental para o seguimento dos candidatos a exoplanetas descobertos pelo Telescópio Kepler;

6 – Em termos tecnológicos, Mayor está confiante que novos instrumentos actualmente em construção como o SPHERE (um sistema de óptica adaptativa para o VLT), o ESPRESSO (um espectrógrafo de muito alta precisão para o VLT) e o PRIMA (um interferómetro para o VLT), revolucionarão o estudo dos exoplanetas. Em termos de missões espaciais, Mayor destaca a missão PLATO, em avaliação pela Agência Espacial Europeia (ESA).