Abstract
Context. 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. Aims. 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. Methods. 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. Results. 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. Conclusions. 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.
Highlights
Stellar activity affects all indirect techniques that are used to detect exoplanets, that is, radial velocity (RV), photometric transits, and astrometry
The low impact of stellar activity on the astrometric signal compared to radial velocity is one of the reasons why high-precision astrometric space missions have been proposed to detect low-mass planets around stars in our neighbourhood (Léger et al 2015; Janson et al 2018), such as the Space Interferometry Mission (SIM; e.g. Shao et al 1995; Svensson & Ludwig 2005; Catanzarite et al 2008; Makarov et al 2009), the Nearby Earth Astrometric Telescope (NEAT; Malbet et al 2012; Crouzier et al 2016), or Telescope for
A first simple approach we considered was to define a criterion based on the signal to noise (S/N) ratio (Casertano & Sozzetti 1999; Sozzetti et al 2002; Sozzetti 2005; Eriksson & Lindegren 2007; Traub et al 2010), where the signal S is equal to α, the amplitude of the planetary signal, and the noise N is the of the signal due to activity and instrumental noise, which can be computed for each simulation
Summary
Stellar activity affects all indirect techniques (except for those that are based on microlensing) that are used to detect exoplanets, that is, radial velocity (RV), photometric transits, and astrometry. Because of the nature of the photometric transits and their typical timescales, stellar activity mostly affects the transit characterisation of the radius and atmosphere (e.g. Silva 2003; Pont et al 2008; Chiavassa et al 2017) and not detectability It is difficult, to reach long orbital periods, which would allow detecting Earth-like planet in the habitable zone (HZ) of solar-type stars, for example, with transits: this is one of the main goals of the PLAnetary Transits and Oscillations of stars (PLATO) mission (Rauer et al 2014), which will be launched in 2026. Transiting planets will be detected by PLATO, that is, a very small fraction of existing planetary systems Their mass will have to be estimated from RV follow-ups, which is expected to be difficult given the stellar activity impact. The low impact of stellar activity on the astrometric signal compared to radial velocity is one of the reasons why high-precision astrometric space missions have been proposed to detect low-mass planets around stars in our neighbourhood (Léger et al 2015; Janson et al 2018), such as the Space Interferometry Mission (SIM; e.g. Shao et al 1995; Svensson & Ludwig 2005; Catanzarite et al 2008; Makarov et al 2009), the Nearby Earth Astrometric Telescope (NEAT; Malbet et al 2012; Crouzier et al 2016), or Telescope for
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