Using the Sun to estimate Earth-like planets detection capabilities

I. Impact of cold spots

Laboratoire d'Astrophysique de l'Observatoire de Grenoble, Université Joseph Fourier, BP 53, 38041 Grenoble, France e-mail: Lagrange@obs.ujf-grenoble.fr

Received: 5 August 2009
Accepted: 28 October 2009

Abstract

Aims. It is known that stellar spots may in some cases produce radial velocity (RV) signatures similar to those of exoplanets. To date, the most extensive set of data on spots, active regions, and activity in general for any star is that obtained for the Sun. To investigate the impact of these spots, we aim to study the detectability of Earth-mass planets in the habitable zone (HZ) of solar-type stars, if covered by spots similar to sunspots.

Methods. We used the sunspot properties recorded over one solar cycle between 1993 and 2003 to infer the RV curve that a solar-type star seen edge-on would exhibit, if covered by these spots. We also derive interesting parameters such as bisector velocity span (BVS) and photometric curves, commonly used in the analysis of RV data. We compare the obtained data with archival solar data available for the same epoch (e.g., irradiance, Ca index). We also simulate the RV of such a spotted star surrounded by an Earth-mass planet located in the HZ.

Results. The RV of the spotted star appears to be very variable, in a complex way, depending on the activity level, with amplitudes from a few tens cm/s up to 5 m/s (assuming ΔT_s = - T_spot = 550 K). A correlation between the BVS and the RV data is observed even when several spots are present with a slope so small that only data of very high precision (better than 5 cm/s) can enable its detection. Photometric variations of up to 0.5% are predicted, depending on the level of activity, in agreement with measured solar photometric variations. 
Based on present assumptions, the detection of a 1 M_Earth planet located between 0.8 and 1.2 AU requires intensive monitoring (weekly or more frequent), over several years. The temporal sampling is more crucial than the data precision (assuming precisions in the range [1-10]  cm/s). Cooler spots may become a problem for these detections. We also anticipate that plages, not considered in this paper, could further complicate or even compromise such detections.