Using the Sun to estimate Earth-like planets detection capabilities . III. Impact of spots and plages on astrometric detection
Résumé
Aims: Stellar activity is a potentially important limitation to the detection of low-mass extrasolar planets with indirect methods (radial velocity, photometry, astrometry). In previous papers, using the Sun as a proxy, we investigated the impact of stellar activity (spots, plages, convection) on the detectability of an Earth-mass planet in the habitable zone (HZ) of solar-type stars with radial velocity techniques. We here extend the detectability study to astrometry.
Methods: We used the sunspot and plages properties recorded over one solar cycle to infer the astrometric variations that a Sun-like star seen edge-on, 10 pc away, would exhibit, if covered by such spots/bright structures. We compare the signal to the one expected from the astrometric wobble (0.3 μas) of such a star surrounded by a one Earth-mass planet in the HZ. We also briefly investigate higher levels of activity.
Results: The activity-induced astrometric signal along the equatorial plane has an amplitude of typically less than 0.2 μas (rms = 0.07 μas), lower than the one expected from an Earth-mass planet at 1 AU. Hence, for this level of activity, the detectability is governed by the instrumental precision rather than the activity. We show that for instance a one Earth-mass planet at 1 AU would be detected with a monthly visit during less than five years and an instrumental precision of 0.8 μas. A level of activity five times higher would still allow this detection with a precision of 0.35 μas. We conclude that astrometry is an attractive approach to search for such planets around solar type stars with most levels of stellar activity.
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