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

TL;DR

This study shows that the supposed habitable-zone planet around Kapteyn's star is actually an artifact caused by stellar activity, not a real planetary signal, emphasizing the importance of activity analysis in exoplanet detection.

Contribution

The paper demonstrates that stellar activity can mimic planetary signals in radial velocity data, leading to false planet detections, and provides a detailed analysis of Kapteyn's star to illustrate this.

Findings

Kapteyn's star has a 143-day rotation period.

Spectral activity indices correlate with the 48-day RV signal.

The 48-day signal is an artifact of stellar activity, not a planet.

Abstract

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.