Ghost in the time series: no planet for Alpha Cen B

TL;DR

This paper critically re-examines the claimed detection of an Earth-mass planet around Alpha Cen B, revealing that the supposed planetary signal is likely a spurious artifact caused by data sampling and analysis methods.

Contribution

The study demonstrates that the 3.24-day signal attributed to the planet is actually a result of the window function and stellar activity removal, challenging previous claims of planet detection.

Findings

The 3.24-day signal is linked to the data sampling pattern, not a planet.

Removing stellar activity signals can artificially suppress or create signals.

Synthetic data analysis shows the original detection method can produce false positives.

Abstract

We re-analyse the publicly available radial velocity (RV) measurements for Alpha Cen B, a star hosting an Earth-mass planet candidate, Alpha Cen Bb, with 3.24 day orbital period. We demonstrate that the 3.24 d signal observed in the Alpha Cen B data almost certainly arises from the window function (time sampling) of the original data. We show that when stellar activity signals are removed from the RV variations, other significant peaks in the power spectrum of the window function are coincidentally suppressed, leaving behind a spurious yet apparently-significant 'ghost' of a signal that was present in the window function's power spectrum to begin with. Even when fitting synthetic data with time sampling identical to the original data, but devoid of any genuine periodicities close to that of the planet candidate, the original model used to infer the presence of Alpha Cen Bb leads to identical conclusions: viz., the 3$\sigma$ detection of a half-a-metre-per-second signal with 3.236 day period. Our analysis underscores the difficulty of detecting weak planetary signals in RV data, and the importance of understanding in detail how every component of an RV data set, including its time sampling, influences final statistical inference.