Stellar activity mimics a habitable-zone planet around Kapteyn's star

2; 2); (2) Center for Exoplanets & Habitable Worlds; 3); 3) ((1) Dept. of Astronomy; (3) The Penn State Astrobiology Research Center); Arpita Roy (1; Astrophysics; Paul Robertson (1; Penn State University

Stellar activity mimics a habitable-zone planet around Kapteyn's star

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arxiv 1505.02778 v2 pith:N7BHRV53 submitted 2015-05-11 astro-ph.EP astro-ph.SR

Paul Robertson (1 , 2) , Arpita Roy (1 , 2 , 3) , Suvrath Mahadevan (1 , 3) ((1) Dept. of Astronomy , Astrophysics

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Penn State University (2) Center for Exoplanets & Habitable Worlds (3) The Penn State Astrobiology Research Center)

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keywords activitystellarkapteynstarperiodplanetrotationaround

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Kapteyn's star is an old M subdwarf believed to be a member of the Galactic halo population of stars. A recent study has claimed the existence of two super-Earth planets around the star based on radial velocity (RV) observations. The innermost of these candidate planets--Kapteyn b (P = 48 days)--resides within the circumstellar habitable zone. Given recent progress in understanding the impact of stellar activity in detecting planetary signals, we have analyzed the observed HARPS data for signatures of stellar activity. We find that while Kapteyn's star is photometrically very stable, a suite of spectral activity indices reveals a large-amplitude rotation signal, and we determine the stellar rotation period to be 143 days. The spectral activity tracers are strongly correlated with the purported RV signal of "planet b," and the 48-day period is an integer fraction (1/3) of the stellar rotation period. We conclude that Kapteyn b is not a planet in the Habitable Zone, but an artifact of stellar activity.

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