A Gaussian Process Regression Reveals No Evidence for Planets Orbiting Kapteyn's Star

Abstract

Radial-velocity (RV) planet searches are often polluted by signals caused by gas motion at the star's surface. Stellar activity can mimic or mask changes in the RVs caused by orbiting planets, resulting in false positives or missed detections. Here we use Gaussian process (GP) regression to disentangle the contradictory reports of planets vs. rotation artifacts in Kapteyn's star (Anglada-Escude et al. 2014, Robertson et al. 2015, Anglada-Escude et al. 2016). To model rotation, we use joint quasi-periodic kernels for the RV and H-alpha signals, requiring that their periods and correlation timescales be the same. We find that the rotation period of Kapteyn's star is 125 days, while the characteristic active-region lifetime is 694 days. Adding a planet to the RV model produces a best-fit orbital period of 100~years, or 10 times the observing time baseline, indicating that the observed RVs are best explained by star rotation only. We also find no significant periodic signals in residual RV data sets constructed by subtracting off realizations of the best-fit rotation model and conclude that both previously reported "planets" are artifacts of the star's rotation and activity. Our results highlight the pitfalls of using sinusoids to model quasi-periodic rotation signals.

15 pages, including 10 figures. Accepted for publication in Astronomical Journal

References in corpus (22)

• The generalised Lomb-Scargle periodogram. A new formalism for the floating-mean and Keplerian periodograms
• Planets and Stellar Activity: Hide and Seek in the CoRoT-7 system
• Exoplanet population inference and the abundance of Earth analogs from noisy, incomplete catalogs
• EVEREST: Pixel Level Decorrelation of K2 Light curves
• Inferring probabilistic stellar rotation periods using Gaussian processes
• Revisiting Proxima with ESPRESSO
• Two planets around Kapteyn's star : a cold and a temperate super-Earth orbiting the nearest halo red-dwarf
• Stellar activity mimics a habitable-zone planet around Kapteyn's star
• Measuring precise radial velocities on individual spectral lines : II. Dependance of stellar activity signal on line depth
• Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit
• Radial Velocity Observations and Light Curve Noise Modeling Confirm That Kepler-91b is a Giant Planet Orbiting a Giant Star
• The discovery and mass measurement of a new ultra-short-period planet: EPIC~228732031b
• On Sunspot and Starspot Lifetimes
• Stellar Activity and its Implications for Exoplanet Detection on GJ 176
• Exoplanet Imitators: A test of stellar activity behavior in radial velocity signals
• The CARMENES search for exoplanets around M dwarfs: Convective shift and starspot constraints from chromatic radial velocities
• Proxima Centauri reloaded: Unravelling the stellar noise in radial velocities
• A vigorous activity cycle mimicking a planetary system in HD200466
• HADES RV program with HARPS-N at TNG. IX. A super-Earth around the M dwarf Gl686
• K2-291 b: A rocky super-Earth in a 2.2 day orbit
• EXPRES. II. Searching for Planets Around Active Stars: A Case Study of HD 101501
• Hunt for Starspots in HARPS Spectra of G and K Stars

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