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Centro de Astrofísica da Universidade do Porto

CAUP Researchers: Xavier Dumusque, Nuno C. Santos
Team at CAUP: Origin and Evolution of Stars and Planets
Other Researchers: Francesco Pepe (CH), Christophe Lovis (CH), Damien Ségransan (CH), Johannes Sahlmann (CH), Willy Benz (CH), François Bouchy (CH/FR), Michel Mayor (CH), Didier Queloz (CH), Stéphane Udry (CH)

An Earth-mass planet orbiting α Centauri B,
Nature, Volume 491, pp. 207 (2012)

Exoplanets down to the size of Earth have been found, but not in the habitable zone—that is, at a distance from the parent star at which water, if present, would be liquid. There are planets in the habitable zone of stars cooler than our Sun, but for reasons such as tidal locking and strong stellar activity, they are unlikely to harbour water–carbon life as we know it. The detection of a habitable Earth-mass planet orbiting a star similar to our Sun is extremely difficult, because such a signal is overwhelmed by stellar perturbations. Here we report the detection of an Earth-mass planet orbiting our neighbour star α Centauri B, a member of the closest stellar system to the Sun. The planet has an orbital period of 3.236 days and is about 0.04 astronomical units from the star.

Figure 1 | Periodograms of the radial-velocity residuals after removing the non-planetary signals. a, The periodogram of the velocities after correction for stellar, imprecise coordinates and binary effects, with continuous, dashed and dotted lines indicating the 0.1%, 1% and 10% false alarm probability (FAP), respectively. The highest peak, at 3.236 d inside the shaded region, has a FAP of 0.02%. b, A small part of the periodogram around the planet signal is represented. The periodogram for all seasons is shown in black, and the yearly periodograms for each observational period (2008, 2009, 2010 and 2011) are shown in different colours. The amplitudes of the yearly periodograms are normalized so that the 10% FAP of each matches the 10% FAP of the periodogram for all seasons. The phase of the most important peaks is shown (arrows); the direction of the arrow gives the phase between 0º and 360º. For each year of observation, the peak at 3.236 d conserves the same phase, which is expected for a planetary signal. On the contrary, the peak at 2.8 d and its alias at 3.35 d do not keep the same phase and are therefore associated with noise (these peaks appear only in 2009 and their FAPs are higher than 10%).

The search for exoplanets has already yielded several hundred confirmed planets and a few thousand additional candidates. However, most of these planets are too massive and too close to their stars in order to be able to maintain liquid water. Much of the current effort in the exoplanets search is thus focused in finding earth-like planets, i. e., exoplanets with a mass similar to that of the Earth, orbiting its star’s habitable zone.

The star α Centauri B is among those most well-suited for the search of such planets, due to its proximity to the Earth and also to its physical properties. Besides displaying a relatively low stellar activity, it is somewhat less massive and cooler than the Sun, thus placing the habitable zone closer to the star. As the radial velocities method most easily detects exoplanets which are closer to their star, this increases the probability of detecting a planet within the habitable zone. Furthermore, the less massive is the harboring star, the larger is the effect in its radial velocity imprinted by a given planet and, therefore, the relative low mass of α Centauri B also increases the probability of detection.

However, Earth-like planets are typically expected to imprint a radial velocity signal with amplitudes of less than 0.5 m/s. These amplitudes lie at the technical limits of the most stable spectrographs currently available, besides being an order of magnitude smaller than the radial velocity signal imprinted by a number of stellar phenomena like stellar oscillation modes, granulation at the surface of the star, stellar activity, and, for this particular star, also the orbital motion of the binary composed of α Centauri A and B and light contamination from α Centauri A.

Thus, in order to evaluate the presence of an exoplanet around α Centauri B, using the radial velocity method, all these sources of noise were considered and its effects accounted for using adequate modeling and observational strategies. One of the most severe, the noise introduced by stellar oscillation modes, was efficiently averaged out using an appropriate observational strategy, as determined by the team with previous work [1]. Overall, the data used was obtained with the HARPS Spectrograph, installed in the 3.6 m telescope in La Silla, from February 2008 to July 2011.

After applying to the data all the corrections needed, the periodogram revealed the existence of two peaks with a false alarm probability lower than 1% (Fig. 1). Of these, the peak with a period of 3.236 days was proven to be the true peak, with the other being an alias for this one. With this, the team determined that a planet with a minimum mass of 1.13±0.09 Earth masses and an orbital period of 3.2357±0.0008 days orbits α Centauri B with an eccentricity compatible with zero with a false alarm probability under 0.02%.

Although this planet orbits α Centauri B at a distance of just 0.04 astronomical units – so close to the star that it’s located outside of the habitable zone – and, therefore, is not yet a true Earth-like planet, its detection proves that HARPS is capable of detecting habitable super-Earths around solar-type stars, in spite of requiring a large amount of observation time.

The CAUP members had an important contribution in this work. The lead author, Xavier Dumusque, was responsible for part of the data analysis and Nuno C. Santos obtained part of the observational data used. Both CAUP members contributed to the discussion of the results and to the manuscript.

[1] X. Dumusque et al., “Planetary detection limits taking into account stellar noise II. Effect of stellar spot groups on radial-velocities”, Astronomy and Astrophysics, Volume 527, A82, March 2011.

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