Stellar Activity Manifesting at a One Year Alias Explains Barnard b as a False Positive

TL;DR

This study demonstrates that the proposed planet around Barnard's star is likely a false positive caused by stellar activity aliasing, emphasizing the importance of careful analysis of long-term activity signals in RV data.

Contribution

The paper shows that the 233-day signal attributed to a planet is actually an alias of stellar activity, challenging previous planet detection claims.

Findings

The 233-day signal is transitory and linked to stellar activity.

New data do not confirm the planetary candidate.

Stellar activity can mimic planetary signals in RV measurements.

Abstract

Barnard's star is among the most studied stars given its proximity to the Sun. It is often considered $the$ Radial Velocity (RV) standard for fully convective stars due to its RV stability and equatorial declination. Recently, an $M \sin i = 3.3 M_{\oplus}$ super-Earth planet candidate with a 233 day orbital period was announced by Ribas et al. (2018). New observations from the near-infrared Habitable-zone Planet Finder (HPF) Doppler spectrometer do not show this planetary signal. We ran a suite of experiments on both the original data and a combined original + HPF data set. These experiments include model comparisons, periodogram analyses, and sampling sensitivity, all of which show the signal at the proposed period of 233 days is transitory in nature. The power in the signal is largely contained within 211 RVs that were taken within a 1000 day span of observing. Our preferred model of the system is one which features stellar activity without a planet. We propose that the candidate planetary signal is an alias of the 145 day rotation period. This result highlights the challenge of analyzing long-term, quasi-periodic activity signals over multi-year and multi-instrument observing campaigns.