Research ArticlePLANETARY SCIENCE

A low-mass planet candidate orbiting Proxima Centauri at a distance of 1.5 AU

See allHide authors and affiliations

Science Advances  15 Jan 2020:
Vol. 6, no. 3, eaax7467
DOI: 10.1126/sciadv.aax7467
  • Fig. 1 Frequency analysis of the RV residuals.

    Top: RV residuals after subtracting from the original dataset the spectroscopic signal induced by Proxima b, the instrumental offsets, and a secular acceleration term, as fitted by a global model including a GP quasi-periodic term and only the eccentric orbital equation for Proxima b. The residuals still include a stellar activity term. The red line corresponds to the best-fit sinusoid as derived with GLS (P = 1907 days). Middle and bottom: GLS and BGLS periodograms of the residuals. For the GLS periodogram, we calculated the false alarm probability thresholds, indicated by the dashed horizontal lines, through a bootstrap analysis. For clarity, the inset plot shows a zoom-in view of the low-frequency region, with the highest peak at P = 1907 days marked by a vertical dotted line. The second highest peak in both periodograms occurs at P ~ 307 days, which is the 1-year alias of the candidate planetary signal. Bottom: Window function of the RV time series. The inset plot shows a zoom-in view of the low-frequency region, with the period P = 1907 days marked by a dotted vertical line.

  • Fig. 2 Stability and coherence of the long-period signal in the RVs.

    Top: SBGLS periodograms of the RV residuals (same data as in Fig. 1). Middle: Zoomed-in view of the top plot, starting from the 150th RV measurement. The vertical dashed line marks the orbital period P ~ 1900 days of the candidate planet to guide the eye. Bottom: Evolution of the orbital period, semi-amplitude, and phase of the candidate planet signal with increasing number of RV points, as calculated by GLS through a least squares fit.

  • Fig. 3 Phase-folded spectroscopic orbits for Proxima b and c.

    Top: RV curves of Proxima b and of the candidate planet Proxima c, phase-folded to the orbital periods listed in Table 1. The red curves represent the best-fit orbital solutions, and the red points are phase-binned RV values. Bottom: Distributions of the number of measurements along the planets orbits.

  • Fig. 4 Analysis of the light curve of Proxima Centauri.

    Top: ASAS light curve of Proxima (gray dots). The red curve represents our best-fit model of the photometric data, which includes two sinusoids (for the rotational and activity cycle modulations) and a quadratic term to take into account the rise in brightness particularly evident after the epoch HJD 2,457,500. Bottom: ASAS light curve, after removing the 83-day rotational signal and the quadratic long-term trend, folded at the best-fit period of the activity cycle. The best-fit sinusoid modeling the activity cycle is represented by the red curve. The epoch corresponding to phase 0 is HJD 2,458,049.79

  • Fig. 5 Outcomes of the combined analysis of the astrometric and RV datasets.

    Left: True mass of Proxima c versus the sine of the orbital inclination, as obtained from the astrometric simulations. The black line is the simulated exact solution, the blue dots represent the values derived from the Gaia astrometry alone, while the red dots are the values derived by combining the Gaia astrometry with the radial velocities. Right: Fractional deviation of the true mass (defined as the difference between the simulated and retrieved masses for Proxima c divided by the simulated value) versus sine of the orbital inclination.

  • Table 1 Results of the GP regression analysis applied to RVs, including two circular orbital equations, and to ASAS photometry.

    Jump parameterPriorBest-fit value
    GP hyperparameters
      h (m s−1)U(0,4)1.70.2+0.3
      λ (days)U(0,1000)398279+122
      wU(0,1)0.300.05+0.06
      θ (days)U(80,95)87.80.8+0.6
    Planet parameters
      Kb (m s−1)U(0,3)1.2 ± 0.1
      Pb (days)U(10.5,12)11.185 ± 0.001
      Tb, conj (BJD-2,450,000)U(7895,7910)7897.90.2+0.3
      eb0 (fixed)
      Kc (m s−1)U(0,3)1.2 ± 0.4
      Pc (days)U(1600,2200)190082+96
      Tc, conj (BJD-2,450,000)U(5000,7300)5892102+101
      ec0 (fixed)
    Other parameters
      dVr/dt (m s−1 day−1)U(−0.001,+0.001)0.0002 ± 0.0003
      σjit, HARPSpre − 2016 (m s−1)U(0,3)1.0 ± 0.3
      σjit, HARPSpost − 2016 (m s−1)U(0,3)0.50.3+0.2
      σjit, UVES (m s−1)U(0,3)1.0 ± 0.2
      γHARPSpre − 2016 (m s−1)U(−5,5)0.7 ± 0.5
      γHARPSpost − 2016 (m s−1)U(−5,5)−1.2 ± 1.1
      γUVES (m s−1)U(−5,5)0.4 ± 0.6
    Derived parameters
      Minimum mass, mb sin ib (M)1.0 ± 0.1
      Orbital semi-major axis, ab (AU)0.048 ± 0.002
      Equilibrium temperature, Teq, b (K]21699+91
      Minimum mass, mc sin ic (M)5.8 ± 1.9
      Orbital semi-major axis, ac (AU)1.48 ± 0.08
      Equilibrium temperature, Teq, c (K)3918+16
      Minimum astrometric semi-amplitude of Proxima c,
    α sin ic (μas)
    16650+54
      Maximum angular separation of Proxima c (arc sec)1.14 ± 0.06
      Bayesian evidence ln Z−580.51 ± 0.06
    Photometry (ASAS-3–ASAS-4) GP hyperparameters
      λphoto (days)U(0,10,000)367.677.3+94.6
      wphotoU(0,10)0.740.12+0.17
      θphoto (days)U(1,100)85.10.7+1.0

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/6/3/eaax7467/DC1

    Fig. S1. Stacked BGLS periodograms for subsets of RV residuals.

    Fig. S2. Posterior distributions of parameters and hyperparameters relative to an MC exploratory run where the global model is composed of a GP quasi-periodic kernel and two planetary orbital equations, with eccentricities treated as free parameters.

    Fig. S3. Posterior distributions for all the free parameters of the model adopted in this work, which includes a GP quasi-periodic kernel to fit the stellar activity term in the RV, and two circular orbital equations (see Table 1).

    Fig. S4. Stellar activity signal as found in the RV through a GP regression using a quasi-periodic kernel.

    Fig. S5. Analysis of Ha activity index.

    Fig. S6. Minimum mass versus orbital semi-major axis of confirmed planets orbiting low-mass stars (M < 0:6 M) discovered with the RV technique.

    Table S1. Other models used for fitting the RVs, as discussed in the text.

    References (4051)

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Stacked BGLS periodograms for subsets of RV residuals.
    • Fig. S2. Posterior distributions of parameters and hyperparameters relative to an MC exploratory run where the global model is composed of a GP quasi-periodic kernel and two planetary orbital equations, with eccentricities treated as free parameters.
    • Fig. S3. Posterior distributions for all the free parameters of the model adopted in this work, which includes a GP quasi-periodic kernel to fit the stellar activity term in the RV, and two circular orbital equations (see Table 1).
    • Fig. S4. Stellar activity signal as found in the RV through a GP regression using a quasi-periodic kernel.
    • Fig. S5. Analysis of Ha activity index.
    • Fig. S6. Minimum mass versus orbital semi-major axis of confirmed planets orbiting low-mass stars (M < 0:6 M) discovered with the RV technique.
    • Table S1. Other models used for fitting the RVs, as discussed in the text.
    • References (4051)

    Download PDF

    Files in this Data Supplement:

Stay Connected to Science Advances

Navigate This Article