Activity time series of old stars from late F to early K. V. Effect on exoplanet detectability with high-precision astrometry

TL;DR

This study evaluates how stellar activity impacts the detection of Earth-mass exoplanets via high-precision astrometry, showing it is more effective than radial velocity methods under certain conditions.

Contribution

It provides a comprehensive analysis of stellar activity effects on astrometric exoplanet detection, using synthetic data and blind tests for old main-sequence stars, highlighting astrometry's advantages.

Findings

Detection rates for 1 Earth-mass planets are very high in the habitable zone.

False-positive rates are extremely low for stars within 10 parsecs.

Classical bootstrap methods overestimate false positives, affecting detection reliability.

Abstract

Stellar activity strongly affects and may prevent the detection of Earth-mass planets in the habitable zone of solar-type stars with radial velocity technics. Astrometry is in principle less sensitive to stellar activity because the situation is more favourable: the stellar astrometric signal is expected to be fainter than the planetary astrometric signal compared to radial velocities. We quantify the effect of stellar activity on high-precision astrometry when Earth-mass planets are searched for in the habitable zone around old main-sequence solar-type stars. We used a very large set of magnetic activity synthetic time series to characterise the properties of the stellar astrometric signal. We then studied the detectability of exoplanets based on different approaches: first based on the theoretical level of false positives derived from the synthetic time series, and then with blind tests for old main-sequence F6-K4 stars. The amplitude of the signal can be up to a few times the solar value depending on the assumptions made for activity level, spectral type, and spot contrast. The detection rates for 1 MEarth planets are very good, however, with extremely low false-positive rates in the habitable zone for stars in the F6-K4 range at 10 pc. The standard false-alarm probability using classical bootstrapping on the time series strongly overestimates the false-positive level. This affects the detection rates. We conclude that if technological challenges can be overcome and very high precision is reached, astrometry is much more suitable for detecting Earth-mass planets in the habitable zone around nearby solar-type stars than radial velocity, and detection rates are much higher for this range of planetary masses and periods when astrometric techniques are used than with radial velocity techniques.