Abstract
Context. Astrometry is less sensitive to stellar activity than the radial velocity technique when attempting to detect Earth mass planets in the habitable zone of solar-type stars. This is due to a smaller number of physical processes affecting the signal, and a larger ratio of the amplitude of the planetary signal to the stellar signal than with radial velocities. A few high-precision astrometric missions have therefore been proposed over the past two decades. Aims. We aim to re-estimate the detection limits in astrometry for the nearby stars which are the main targets proposed for the THEIA astrometric mission, which is the most elaborate mission to search for planets, and to characterise its performance on the fitted parameters. This analysis is performed for the 55 F-G-K stars in the THEIA sample. Methods. We used realistic simulations of stellar activity and selected those that correspond best to each star in terms of spectral type and average activity level. Then, we performed blind tests to estimate the performance. Results. We find worse detection limits compared to those previously obtained for that sample based on a careful analysis of the false positive rate, with values typically in the Earth-mass regime for most stars of the sample. The difference is attributed to the fact that we analysed full time series, adapted to each star in the sample, rather than using the expected solar jitter only. Although these detection limits have a relatively low signal-to-noise ratio, the fitted parameters have small uncertainties. Conclusions. We confirm the low impact of stellar activity on exoplanet detectability for solar-type stars, although it plays a significant role for the closest stars such as α Cen A and B. We identify the best targets to be the stars with a close habitable zone. However, for the few stars in the sample with a habitable zone corresponding to long periods, namely subgiants, the THEIA observational strategy is not well adapted and should prevent the detection of planets in the habitable zone, unless a longer mission can be proposed.
Highlights
The THEIA mission (The Theia Collaboration et al 2017) is a high precision astrometric mission which was proposed to ESA, which aims at reaching several scientific objectives, including the detection of low mass planets around nearby stars
The median on the blind test for a 50% detection rate is below 1 MEarth and it is close to 1 MEarth for a detection rate of 95%, i.e. lower than the Super-Earth regime of the detection limits of The Theia Collaboration et al (2017)
The transit technique is currently biased towards planets close to their host stars and no planet in the habitable zone of solar-type stars have been detected so far: this is a crucial objective of the PLAnetary Transits and Oscillations of stars mission (PLATO) mission, to be launched in 2026, which should allow one to detect such planets by the end of this decade, Detection limit (MEarth) and will provide key targets for future characteristion missions such as the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (Ariel) or the Habitable Exoplanet Observatory (HabEX)
Summary
The THEIA mission (The Theia Collaboration et al 2017) is a high precision astrometric mission which was proposed to ESA, which aims at reaching several scientific objectives, including the detection of low mass planets around nearby stars. It may be proposed again in the future with new technological innovations (Malbet et al 2021). This study of the solar case followed several approaches based on simple estimations of the stellar contribution (Bastian & Hefele 2005; Reffert et al 2005; Eriksson & Lindegren 2007; Catanzarite et al 2008; Lanza et al 2008), and it was in good agreement with the
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