Using the Sun to estimate Earth-like planets detection capabilities

Abstract

Aims. Stellar activity produced by spots and plages affects the radial velocity (RV) signatures. Because even low activity stars would produce such a signal, it is crucial to determine how it influences our ability to detect small planetary signals such as those produced by Earth-mass planets in the habitable zone (HZ). In a recent paper, we investigated the impact of sunlike spots. We aim here to investigate the additional impact of plages. Methods. We used the spot and plage properties over a solar cycle to derive the RV that would be observed if the Sun was seen edgeon. The RV signal comes from the photometric contribution of spots and plages and from the attenuation of the convective blueshift in plages. We analyzed the properties of the RV signal at different activity levels and compared it with commonly used activity indicators such as photometry and the Ca index. We also compared it with the signal that would be produced by an Earth-mass planet in the HZ. Results. We find that the photometric contributions of spots and plages to the RV signal partially balance each other out, so that the residual signal is comparable to the spot signal. However, the plage contribution due to the convective blueshift attenuation dominates the total signal, with an amplitude over the solar cycle of about 8-10 m/s. Short-term variations are also significantly greater than the spot and plage photometric contribution. This contribution is very strongly correlated with the Ca index on the long term, which may be a way to distinguish between stellar activity and a planet. Conclusions. Providing a very good temporal sampling and signal-to-noise ratio, the photometric contribution of plages and spots should not prevent detection of Earth-mass planets in the HZ. However, the convection contribution makes such a direct detection impossible, unless its effect can be corrected for by methods that still need to be found. We show that it is possible to identify the convection contribution if the sensitivity is good enough, for example, by using activity indicators.

Planetary detection limits taking into account stellar noise

“…These convective phenomena can be found all over the stellar surface, except in active regions where convection is significantly lower (e.g. Dravins 1982;Livingston 1982;Brandt & Solanki 1990;Gray 1992;Meunier et al 2010). The amplitudes A&A 525, A140 (2011) of granulation phenomena are similar to the ones observed for pressure modes (e.g.…”

Stellar noise and planet detection. II. Radial-velocity noise induced by magnetic cycles

“…This cycle can also be seen looking at the total number of sunspots or the Sun luminosity variation. In a recent paper, Meunier et al (2010) show a correlation between the Sun RV variation and its magnetic cycle. Amplitudes of tenths of meter per second could be induced by such cycles, hiding signals of long period small-mass planets.…”

“…Since upward flows of convection (granules) have a total surface more important than downward flows (intergranules), the stellar spectrum will be blushifted (Dravins 1982;Gray 2009). When the activity level increases, the number of magnetic features (spots and plages) raise up, and since these regions are known to inhibit the convection due to strong magnetic fields, the stellar spectrum will be shifted to the red.…”

ACCESS I. AN OPTICAL TRANSMISSION SPECTRUM OF GJ 1214b REVEALS A HETEROGENEOUS STELLAR PHOTOSPHERE

“…On the Sun, spots are accompanied by bright plage, which include faculae with temperature contrasts of 300-500K with their surroundings (Topka et al 1997). Brightening from these faculae overpowers spot darkening, causing total solar irradiance to increase during solar maxima (Fröhlich & Lean 1998;Meunier et al 2010). Faculae cover roughly 0.36% of the sky-projected solar disk during periods of low solar activity and 3% during high activity (Shapiro et al 2014).…”