Planet Detectability in the Alpha Centauri System

Abstract

We use more than a decade of radial velocity measurements for $\alpha$ Cen A, B, and Proxima Centauri from HARPS, CHIRON, and UVES to identify the $M\sin i$ and orbital periods of planets that could have been detected if they existed. At each point in a mass-period grid, we sample a simulated, Keplerian signal with the precision and cadence of existing data and assess the probability that the signal could have been produced by noise alone. Existing data places detection thresholds in the classically defined habitable zones at about $M\sin i$ of 53 M${}_{\oplus }$ for $\alpha$ Cen A, 8.4 M${}_{\oplus }$ for $\alpha$ Cen B, and 0.47 M${}_{\oplus }$ for Proxima Centauri. Additionally, we examine the impact of systematic errors, or "red noise" in the data. A comparison of white- and red-noise simulations highlights quasi-periodic variability in the radial velocities that may be caused by systematic errors, photospheric velocity signals, or planetary signals. For example, the red-noise simulations show a peak above white-noise simulations at the period of Proxima Centauri b. We also carry out a spectroscopic analysis of the chemical composition of the $\alpha$ Centauri stars. The stars have super-solar metallicity with ratios of C/O and Mg/Si that are similar to the Sun, suggesting that any small planets in the $\alpha$ Cen system may be compositionally similar to our terrestrial planets. Although the small projected separation of $\alpha$ Cen A and B currently hampers extreme-precision radial velocity measurements, the angular separation is now increasing. By 2019, $\alpha$ Cen A and B will be ideal targets for renewed Doppler planet surveys.

16 pages, 7 figures, data provided in appendix. Accepted for publication in ApJ

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• Orbital Stability of Circumstellar Planets in Binary Systems
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• Precision Millimeter Astrometry of the $\alpha$ Centauri AB System
• The test case of HD26965: difficulties disentangling weak Doppler signals from stellar activity
• Implications of Captured Interstellar Objects for Panspermia and Extraterrestrial Life
• Long-Term Stability of Tightly Packed Multi-Planet Systems in Prograde, Coplanar, Circumstellar Orbits within the $\alpha$ Centauri AB System
• The chemical composition of $\alpha$ Cen AB revisited
• Extending the FIP bias sample to magnetically active stars. Challenging the FIP bias paradigm?
• RefPlanets: Search for reflected light from extra-solar planets with SPHERE/ZIMPOL
• PEXO: a global modeling framework for nanosecond timing, microsecond astrometry, and $\mu$m/s radial velocities
• Activity and telluric contamination in HARPS observations of Alpha Centauri B
• Europium as a lodestar: diagnosis of radiogenic heat production in terrestrial exoplanets
• Exomoons in Systems with a Strong Perturber: Applications to $\alpha$ Cen AB
• An Early Catalog of Planet Hosting Multiple Star Systems of Order Three and Higher
• Evolution of $\alpha$ Centauri B's protoplanetary disc
• Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of $\alpha$ Cen A. I. Observations, Orbital and Physical Properties, and Exozodi Upper Limits
• Simulation of the Earth's radio leakage from mobile towers as seen from selected nearby stellar systems
• Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of $\alpha$ Cen A. II. Binary Star Modeling, Planet and Exozodi Search, and Sensitivity Analysis
• SPORES-HWO. II. Companion Mass Limits and Updated Planet Properties for 120 Future Exoplanet Imaging Targets from 35 yr of Precise Doppler Monitoring