Color difference makes a difference: four planet candidates around tau Ceti

TL;DR

This study improves radial velocity planet detection by modeling wavelength-dependent noise, leading to the discovery of four planet candidates around tau Ceti, including two new signals, with implications for detecting Earth-like planets.

Contribution

The paper introduces a novel noise modeling approach using differential radial velocities, enhancing the detection of low-amplitude planetary signals in radial velocity data.

Findings

Four planet candidates identified around tau Ceti.

Two new signals at 20.0 and 49.3 days confirmed.

Detection of signals with semi-amplitudes as low as 0.3 m/s.

Abstract

The removal of noise typically correlated in time and wavelength is one of the main challenges for using the radial velocity method to detect Earth analogues. We analyze radial velocity data of tau Ceti and find robust evidence for wavelength dependent noise. We find this noise can be modeled by a combination of moving average models and "differential radial velocities". We apply this noise model to various radial velocity data sets for tau Ceti, and find four periodic signals at 20.0, 49.3, 160 and 642 d which we interpret as planets. We identify two new signals with orbital periods of 20.0 and 49.3 d while the other two previously suspected signals around 160 and 600 d are quantified to a higher precision. The 20.0 d candidate is independently detected in KECK data. All planets detected in this work have minimum masses less than 4$M_\oplus$ with the two long period ones located around the inner and outer edges of the habitable zone, respectively. We find that the instrumental noise gives rise to a precision limit of the HARPS around 0.2 m/s. We also find correlation between the HARPS data and the central moments of the spectral line profile at around 0.5 m/s level, although these central moments may contain both noise and signals. The signals detected in this work have semi-amplitudes as low as 0.3 m/s, demonstrating the ability of the radial velocity technique to detect relatively weak signals.