The effect of M dwarf starspot activity on low-mass planet detection thresholds

TL;DR

This study assesses how starspots affect the detection of low-mass planets around M dwarf stars using radial velocity methods, highlighting the benefits of infrared observations and the impact of stellar activity on detection limits.

Contribution

It introduces a model for simulating starspot effects on M dwarfs and evaluates detection thresholds for Earth-mass planets considering various stellar activity levels.

Findings

Infrared observations reduce starspot jitter by up to an order of magnitude.

<= 50 epochs suffice to detect 1-2 Earth-mass planets around quiet M dwarfs.

Active stars with high rotation velocities pose significant challenges for detecting small planets.

Abstract

In light of the growing interest in searching for low mass, rocky planets, we investigate the impact of starspots on radial velocity searches for earth-mass planets in orbit about M dwarf stars. Since new surveys targeting M dwarfs will likely be carried out at infrared wavelengths, a comparison between V and Y band starspot induced jitter is made, indicating a reduction of up to an order of magnitude when observing in the Y band. The exact reduction in jitter is dependent on the photosphere to spot contrast ratio, with greater improvements at smaller contrasts. We extrapolate a model used to describe solar spot distributions to simulate the spot patterns that we expect to find on M dwarfs. Under the assumption that M dwarfs are near or fully convective, we randomly place starspots on the stellar surface, simulating different levels of spot coverage. Line profiles, distorted by spots are derived and are used to investigate the starspot induced jitter. By making assumptions about the degree of spot activity, detection limits for earth-mass planets in habitable zones are simulated for between 10 and 500 observation epochs. We find that <= 50 epochs are required to detect 1 - 2 MEarth planets (with < 1 per cent false alarm probability) orbiting slowly rotating 0.1 and 0.2 MSun stars. This sensitivity decreases when typical rotation velocities and activity levels for each stellar mass/spectral type are considered. No detections of below 20 MEarth planets are expected for <= 500 observations for the most active stars with vsini >= 20 km/s and dark spots.