Entrevista com Geoff Marcy

(Crédito: NASA)

Geoffrey Marcy, actualmente professor de astronomia na Universidade de Berkeley, é um dos pioneiros no estudo dos exoplanetas e um dos investigadores mais influentes da área. A sua carreira está pontuada por várias contribuições fundamentais em que esteve envolvido, de entre as quais se destacam: primeira medição do efeito de Zeeman (desdobramento das linhas espectrais na presença de campos magnéticos) no espectro de estrelas de tipo solar, a descoberta do “deserto de anãs castanhas” (inexistência de anãs castanhas a menos de 5 u.a das estrelas hospedeiras), o desenvolvimento de métodos capazes de medir a velocidade radial até uma precisão de 3 m/s, a descoberta de 70 dos primeiros 100 exoplanetas conhecidos, descoberta do primeiro sistema múltiplo (upsilon Andromedae), descoberta do primeiro planeta em trânsito (HD209458b), descoberta do primeiro exoplaneta a orbitar para lá de 5 u.a. (55 Cancri d) e descoberta dos primeiros planetas do tamanho de Neptuno (GJ 436b e 55 Cancri e).

O seu contributo para a área é alvo de contínuo reconhecimento como o demonstra o facto de ser recipiente de inúmeros prémios prestigiados como por exemplo: California Scientist of the Year (Abril de 2000), membro da National Academy of Sciences (Abril de 2002), NASA Medal for Exceptional Scientific Achievement (Junho de 2003), Shaw Prize (Setembro de 2005) e Carl Sagan Prize for Science Popularization (Novembro de 2009).

Recentemente o Geoff Marcy concedeu uma entrevista ao AstroPT onde nos falou do seu percurso científico e dos seus projectos e descobertas mais recentes.

[AstroPT] – What is your academic background and when did you first become interested in exoplanets as a research topic ? What was the field like at the time ? What problems did you have to solve in order to finally produce the first discoveries ?

[Marcy] – I received a bachelors degree in both physics and astronomy from UCLA in Los Angeles in 1976. I received my Ph.D. from the University of California at Santa Cruz in 1982. My research was on magnetism of stars. I was offered a position as a “Carnegie Fellow” at the Carnegie Observatories in Pasadena California in 1982, giving me a chance to use the Mount Wilson 100-inch telescope. It was at Mount Wilson Observatory that I realized one could measure Doppler shifts very precisely, enabling the detection of the wobble of a star caused by the gravitational pull by any orbiting brown dwarfs or planets. That’s when I started the planet search.

No one else was searching for brown dwarfs or extrasolar planets at that time (1982), as far as I knew. The biggest problem I faced was finding a technique to measure Doppler shifts with adequate precision. Back then, I worked hard to finally achieve a precision of 230 meters/sec. For comparison, now in 2011 we achieve a Doppler precision of 1 meter/sec.

From 1984 until 1995 I searched for planets and brown dwarfs orbiting other stars, but I failed to find them. Those 11 years of failure were very difficult for me. I felt nervous that I was wasting telescope time and wasting money and my own life.

(O cume do Mauna Kea no Hawaii com os dois telescópios do Observatório Keck utilizado por Geoff Marcy no projecto Eta-Earth e no seguimento de candidatos do Kepler. À esquerda pode ver-se o telescópio Subaru e à direita o IRTF (InfraRed Telescope Facility) da NASA. Crédito: Kelly E. Fast)

[AstroPT] – You are one of the leading investigators for the NASA/UC Eta-Earth project, that focuses on the detection of planets in the lower end of the mass spectrum. Can you describe the research problem you are trying to answer, and the observing and data-processing infrastructure used by the project ?

[Marcy] – The key question is the fraction of stars that harbor an Earth-size planet. Currently we don’t know if Earth-size planets exist around 1%, 10%, or 80% of all stars. The “Eta-Earth project” is designed to measure the Doppler shifts of 230 nearby solar-type stars with extreme Doppler precision, to detect planets of nearly Earth-size orbiting them in close-in orbits.

We use the Keck telescope in Hawaii about 20 nights per year for the Eta-Earth project. Every night we point the telescope at about 120 stars, spending 5 minutes per star to spread the light into all of its wavelengths (colors) and storing that spectrum on the computer. The next day, we analyze those spectra to determine the Doppler shifts, hoping to detect slight changes that would indicate that the star is wobbling due to a small planet pulling on the star. We use 32 Mac (Apple) computers in parallel to analyze all of the data from the previous night, taking all day to finish the calculations. Then we hunt through the Doppler measurements, looking for evidence that the stars are wobbling.

[AstroPT] – Projects like Eta-Earth, AAPS and HARPS-GTO are slowly revealing a large population of Neptunes and Super-Earths. With the available data, what can be said about the frequency of these planets and the architectures of planetary systems ?

[Marcy] – We now know that there are more small planets, the size of Neptune, than large planets the size of Jupiter. This is an amazing discovery about the distribution of sizes of planets. We now know, for the first time, that small planets are more common than large planets.

(A curva de velocidade radial da Super-Terra HD156668b, descoberta em 2010 pelo projecto Eta-Earth. Crédito: projecto Eta-Earth)

[AstroPT] – As a co-investigator for the Kepler mission your job is to do the follow up and confirmation of planet candidates. How is this process organized ? What are the technical difficulties of measuring precise radial velocities for Kepler’s typically faint stars ?

[Marcy] – The Kepler team has a committee called the “TCERT” which stands for, “Transit Crossing Event Review Team”. We judge the quality of the planet candidates that come from the photometry pipeline. We then determine what types of follow-up observations are needed to verify the planet candidates. For example, the TCERT may recommend that the Keck telescope take spectra, to determine if the Doppler shift is consistent with the planet found by Kepler. The TCERT may also request that we obtain enough Doppler measurements with the Keck telescope to determine the mass and density of the planet.

The greatest technical difficulty is determining the point-spread-function (PSF) of the spectrometer during the observation. Any asymmetries in the PSF of the spectrometer will cause false Doppler shifts. We guide the telescope very precisely on the star to avoid this problem.

We also use a long slit, to allow us to subtract the moonlit sky from our spectra. These stars are so faint, typically 13th or 14th magnitude, that the moonlit sky contaminates our spectra. We subtract the moonlight, to yield a pure spectrum of the Kepler star.

[AstroPT] – Currently, the best precision obtained with the radial velocity technique is about 1 m/s. How then will you be able to measure the fainter signals (around 0.1 m/s) required to confirm the planetary status of Earth-size candidates in the habitable zone ?

[Marcy] – Achieving a precision of 0.1 m/s is not possible. The stars have convection on their surfaces preventing a firm Doppler measurement to a precision better than 1 m/s. That is the best anyone can achieve for solar-type stars. We will not be able to confirm Earth-size planets in the habitable zone [using the radial velocity technique].

(A localização dos 1235 candidatos do Kepler no campo de visão da câmara CCD. Crédito: missão Kepler)

[AstroPT] – The Kepler team has recently announced the discovery of 1235 planet candidates in their data. Is there some estimate of how many of these are expected to be real planets ? Most of the candidates are Neptune- to Earth-size, with gas giants being comparatively rare. What is the data telling us about the frequency of each of these different kinds of planets around other stars ? Is this in agreement with the findings of the Eta-Earth, AAPS, and HARPS-GTO projects ?

[Marcy] – Yes [there is an estimate]. Tim Morton and John Johnson at Caltech calculated the probability that an eclipsing binary located behind the bright Kepler star could cause repeated dimmings, thereby giving the appearance of a transiting planet around the Kepler star. The probability of such alignments is only 5 to 10%. [In other words] 90 to 95% must be real planets.

Yes [it is in agreement with the data from those other projects]. The Kepler results show that nature makes 8 times more planets of 2 Earth-radii than planets of 10 Earth-radii. In other words, there are 8 times more super-Earths than Jupiter-size planets. We just published this result in a paper by Andrew Howard, myself, and the Kepler team. That paper is available on astro-ph web server. [Na mesma linha, num artigo recente na revista Scientific American, Marcy dizia: “If you take a sample of G-type, main-sequence stars, 8 percent of them will have 2- to 2.8-Earth-radii planets with orbital periods of less than 50 days.”]

(Histograma mostrando a distribuição dos 1235 candidatos do Kepler por tipo de planeta. O número de planetas de tamanho igual ou inferior a Neptuno supera em muito o número de planetas gigantes. O aparente decréscimo no número de planetas com tamanho inferior a Neptuno pode ser devido, de acordo com Bill Borucki, a um efeito real ou resultar apenas de uma maior dificuldade na detecção dos planetas mais pequenos, mais facilmente perdidos no ruído das observações. Estes números referem-se aos primeiros 120 dias da missão. A acumulação de mais observações e o aperfeiçoamento da pipeline de processamento dos dados irá certamente clarificar qual dos cenários corresponde à realidade. Crédito: missão Kepler)

[AstroPT] – Kepler also discovered many multiple systems, with up to six transiting planets (Kepler-11). These systems seem to have a distinct architecture from the Solar system, with the planets tightly packed together close to the host star. Is this simply an observational bias, given the 4 month baseline for the data, or are these systems typical and the Solar System an odd ball ?

[Marcy] – Yes, this is probably an observational bias. Kepler has only released data from its first 120 days, and so we have detected planets that have short orbital periods of less than 1/2 year. As time proceeds, Kepler will probably find planets of longer and longer orbital periods, including longer than 1 year. The Solar System is somewhat rare because its planets remain in the same plane and in nearly circular orbits. About 90% of other planetary systems have more inclined orbits and more eccentric orbits, as our Doppler surveys showed.

[AstroPT] – A few years ago a correlation was found between the metallicity of a star and the probability that it hosts giant planets. Does the Kepler data imply that this correlation is valid for planets in the Neptune- to Earth-size range ? What are the implications for the theories of planetary formation ?

[Marcy] – We currently do not have good metallicity measurements for the Kepler host stars. We are working hard on this, by obtaining spectra of all of the host stars of the Kepler planets. We will see if the metallicity correlation is still true for the small, Earth-size planets!

[AstroPT] – Besides the above mentioned projects, what other research collaborations are you involved in ?

[Marcy] – That’s it! Finding the first Earth-like planets is enough for me! But I am also trying to detect the laser transmission from advanced, technological civilizations.

Podem ver a apresentação do Prof. Marcy no 218º encontro da American Astronomical Society, descrevendo os resultados preliminares do Kepler sobre a frequência dos diferentes tipos de planetas, aqui.