Yin or yang : confronting stellar activity in radial-velocity searches for exo-Earths

Abstract

Stellar activity has a radial velocity (RV) signature which complicates the detection of planetary signals using RV method. Because magnetic activity of the host star occurs on identical timescales to Earth-like small planets, it can entirely hide or mimic their RV signals. This is stopping us from harnessing the full potential of the state-of-the-art instruments to definitively enter the realm of terrestrial planet discovery. I formulated a workflow called TWEAKS to obtain reliable detections of planets whose RV signals were previously below the detection threshold. This workflow integrates stellar activity mitigation techniques in wavelength and time domains, utilising SCALPELS for spectral line-shape decorrelation and KIMA Gaussian Process to counteract any shift-like activity signal leakage.

I used this statistically rigorous approach to search for planets around stars with different levels of intrinsic variability. I chose CoRoT-7 to represent the high-activity star case. I applied TWEAKS to the archival HARPS data and refined the mass of the transiting Super-Earth CoRoT- 7b to a precision of 10%, while confidently confirming the existence of an additional non- transiting Neptune, CoRoT-7d. My work definitively demonstrated how stellar variability and limited knowledge about additional planets in the system, complicate the detection, mass determination, and characterisation of exoplanetary systems near the detection threshold.

To represent the moderate and low activity cases, I extended the search to two HARPS-N RPS stars, HD144579 and HD166620. Despite finding no planets in the 10 years of data of either system, TWEAKS enabled me to achieve a sub-ms⁻¹ detection limit (0.54 ms⁻¹), close to the long-term instrumental calibration precision of the HARPS-N spectrograph (0.50 ms⁻¹). TWEAKS have also facilitated independent robust detections and precise mass and bulk-density determinations for various Super-Earths and mini-Neptunes across multiple TESS, K2, and RPS planetary systems, such as TOI1778, TOI1730, TOI5398, HD99492, and HD48948. In 78% of these cases, TWEAKS provided mass measurements that are either more precise or comparable to those obtained using conventional time-domain alone approaches.