Stellar Activity Masquerading as Planets in the Habitable Zone of the M dwarf Gliese 581

TL;DR

This study shows that stellar activity in Gliese 581 can mimic planetary signals, leading to false detections, and emphasizes the importance of accounting for stellar activity in exoplanet searches.

Contribution

It demonstrates that stellar activity can produce false planetary signals and provides a method to correct for this, clarifying the true planetary system of Gliese 581.

Findings

GJ 581d is a false positive caused by stellar activity.

Correcting for stellar activity reduces the significance of GJ 581d.

The purported planet GJ 581g does not exist.

Abstract

The M dwarf Gliese 581 is believed to host four planets, including one (GJ 581d) near the habitable zone that could possibly support liquid water on its surface if it is a rocky planet. The detection of another habitable-zone planet--GJ 581g--is disputed, as its significance depends on the eccentricity assumed for d. Analyzing stellar activity using the H-alpha line, we measure a stellar rotation period of 130+/-2 days and a correlation for H-alpha modulation with radial velocity. Correcting for activity greatly diminishes the signal of GJ 581d (to 1.5 sigma), while significantly boosting the signals of the other known super-Earth planets. GJ 581d does not exist, but is an artifact of stellar activity which, when incompletely corrected, causes the false detection of planet g.