An Earth-mass planet orbiting α Centauri B

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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 (one astronomical unit is the Earth–Sun distance).

At a glance


  1. Radial velocities of [agr] Centauri B and fitting the long-timescale stellar signals.
    Figure 1: Radial velocities of α Centauri B and fitting the long-timescale stellar signals.

    a, Raw radial velocities (RV) and the fit of the binary’s signature (α Centauri B orbiting α Centauri A). In the residuals (c), signals at long period are visible. These signals, highlighted by grey arrows (‘long-term activity’) in periodogram b, correspond to the effect of the magnetic cycle. The grey curve in c shows the variation of the low-frequency part of the activity index scaled to the radial-velocity variation. When these low-frequency perturbations are removed, signals induced by rotational activity, pointed out by grey arrows in periodogram d, can be seen at the rotation period of the star and its harmonics. JD, Julian date. Error bars in c, 1σ.

  2. Magnetic cycle of [agr] Centauri B.
    Figure 2: Magnetic cycle of α Centauri B.

    a, The grey curve represents a low pass filter applied to the activity index measurements (data points). b, The observations done in 2010 are zoomed in to show the variation induced by rotational activity, which highlights the HARPS precision in determining activity indexes. Error bars (1σ) are smaller than the data points (that is, smaller than 0.015 dex).

  3. Fit of the rotational activity.
    Figure 3: Fit of the rotational activity.

    ad, Radial velocities (RV) of α Centauri B after correction of the binary’s signature (of α Centauri B orbiting α Centauri A), magnetic-cycle and coordinates effects, for the years 2008 (a), 2009 (b), 2010 (c) and 2011 (d), are shown as black dots with 1σ error bars. The grey curve represents for each plot the fit of the rotational activity signal, adjusting sinusoids at the stellar rotational period and the corresponding harmonics. The rotational period estimated from the stellar activity model decreases from the second season of observation to the last, with estimated periods (in days) of 39.76 (b), 37.80 (c) and 36.71 (d) (Supplementary Information section 6). This can be explained if the star exhibits differential rotation. Indeed, it has been shown for the Sun that spots appear at a latitude of +30° or −30° at the start of a magnetic cycle (like in 2008) and then migrate towards the equator during the cycle. Owing to differential rotation, observed for the Sun, the rotational period estimated by activity modelling should decrease from the start to the end of a magnetic cycle. A similar effect is seen here for α Centauri B. We therefore believe that differential rotational has been detected here for this slow rotator41 (X.D. et al., manuscript in preparation).

  4. Periodograms of the radial-velocity residuals after removing the non-planetary signals.
    Figure 4: 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% FAP, respectively. The highest peak, at 3.236d inside the shaded region, has an 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 0° and 360°. For each year of observation, the peak at 3.236d conserves the same phase, which is expected for a planetary signal. On the contrary, the peak at 2.8d and its alias at 3.35d 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%).

  5. Phase-folded radial-velocity curve with a period of 3.2357[thinsp]d.
    Figure 5: Phase-folded radial-velocity curve with a period of 3.2357d.

    Green dots, radial velocities after correction of the stellar, binary and coordinates effects. Red dots, the same radial velocities binned in phase, with a bin size of 0.05. The error bar of a given bin is estimated using the weighted r.m.s. of the global fit residuals (including the planetary fit) that make this bin, divided by the square root of the number of measurements included in this bin. This estimation of the bin error bars assumes Gaussian noise, which is justified by the binning in phase, which regroups points that are uncorrelated in time. The r.m.s. around the planetary solution is 1.20ms−1 for the raw points (green dots) and 0.21ms−1 for the binned points (red dots). The red curve represents the global fit solution of the planet, with a semi-amplitude of 0.51ms−1.


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Author information


  1. Observatoire de Genève, Université de Genève, 51 chemin des Maillettes, CH-1290 Sauverny, Switzerland

    • Xavier Dumusque,
    • Francesco Pepe,
    • Christophe Lovis,
    • Damien Ségransan,
    • Johannes Sahlmann,
    • François Bouchy,
    • Michel Mayor,
    • Didier Queloz &
    • Stéphane Udry
  2. Centro de Astrofìsica da Universidade do Porto, Rua das Estrelas, P-4150-762 Porto, Portugal

    • Xavier Dumusque &
    • Nuno Santos
  3. Physikalisches Institut Universitat Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland

    • Willy Benz
  4. Institut d’Astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis Boulevard Arago, F-75014 Paris, France

    • François Bouchy
  5. Departamento de Fìsica e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre s/n, P-4169-007 Porto, Portugal

    • Nuno Santos


F.P., C.L., W.B., F.B., M.M., D.Q., N.S. and S.U. obtained data under the ESO programme ‘Searching for Earth-analogs around nearby stars with HARPS’. The HARPS spectrograph was designed and built by F.P., C.L., W.B., F.B., M.M., D.Q. and S.U. C.L. and D.S. performed the reduction of the data. Data analysis was carried out by X.D., J.S., F.P., D.S. and C.L. All the work was supervised by S.U. All authors discussed the results and contributed to the manuscript.

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Supplementary information

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  1. Supplementary Information (4.7M)

    This file contains Supplementary Text and Data 1-9, Supplementary Figures 1-13, Supplementary Tables 1-2 and additional references.

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  1. Supplementary Data (56K)

    This text file contains all data used in our analysis, in tab-separated format.

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