The effects of granulation and supergranulation on Earth-mass planet detectability in the habitable zone around F6-K4 stars

Abstract

Context. The detectability of exoplanets and the determination of their projected mass in radial velocity are affected by stellar magnetic activity and photospheric dynamics. Among those processes, the effect of granulation, and even more so of supergranulation, has been shown to be significant in the solar case. The impact for other spectral types has not yet been characterised. Aims. Our study is aimed at quantifying the impact of these flows for other stars and estimating how such contributions affect their performance. Methods. We analysed a broad array of extended synthetic time series that model these processes to characterise the impact of these flows on exoplanet detection for main sequence stars with spectral types from F6 to K4. We focussed on Earth-mass planets orbiting within the habitable zone around those stars. We estimated the expected detection rates and detection limits, tested the tools that are typically applied to such observations, and performed blind tests. Results. We find that both granulation and supergranulation on these stars significantly affect planet mass characterisation in radial velocity when performing a follow-up of a transit detection: the uncertainties on these masses are sometimes below 20% for a 1 MEarth (for granulation alone or for low-mass stars), but they are much larger in other configurations (supergranulation, high-mass stars). For granulation and low levels of supergranulation, the detection rates are good for K and late G stars (if the number of points is large enough), but poor for more massive stars. The highest level of supergranulation leads to a very poor performance, even for K stars; this is both due to low detection rates and to high levels of false positives, even for a very dense temporal sampling over 10 yr. False positive levels estimated from standard false alarm probabilities sometimes significantly overestimate or underestimate the true level, depending on the number of points: it is, therefore, crucial to take this effect into account when analysing observations. Conclusions. We conclude that granulation and supergranulation significantly affect the performance of exoplanet detectability. Future works will focus on improving the following three aspects: decreasing the number of false positives, increasing detection rates, and improving the false alarm probability estimations from observations.

Spatially resolved spectroscopy across stellar surfaces

“…The full solar disk is greater in effective area (recognizing the limb darkening) by a factor of ∼50 000, and when we assume random fluctuations and neglect center-to-limb variations, for the full disk these amplitudes would decrease by a factor 200 to about 0.5 m s −1 per 50 ppm in irradiance. These values are consistent with values deduced by Meunier et al (2015) and Meunier & Lagrange (2020) for the granulationinduced signal in apparent solar full-disk radial velocity although further contributions may come from larger-scale structures such as supergranulation (Meunier & Lagrange 2019), which are not yet accessible to detailed 3D modeling. A further modeling caution is that brightness variations are also driven by p-mode oscillations which are incompletely treated in the small simulation volumes.…”

Spatially resolved spectroscopy across stellar surfaces. V. Observational prospects: Toward Earth-like exoplanet detection

“…sume random fluctuations and neglect center-to-limb variations, for the full disk these amplitudes would decrease by a factor 200 to about 0.5 m s −1 per 50 ppm in irradiance. These values are consistent with values deduced by Meunier et al (2015) and Meunier & Lagrange (2020) for the granulation-induced signal in apparent solar full-disk radial velocity although further contributions may come from larger-scale structures such as supergranulation (Meunier & Lagrange 2019), which are not yet accessible to detailed 3D modeling. A further modeling caution is that brightness variations are also driven by p-mode oscillations which are incompletely treated in the small simulation volumes.…”

Connecting photometric and spectroscopic granulation signals with CHEOPS and ESPRESSO

“…The variability induced by granulation constitutes a significant noise source hampering the detection of smaller stellar signals, like low-amplitude acoustic or gravity modes (see e.g., García & Ballot 2019 for solar-like oscillations, Rodríguez et al 2016 for M-dwarfs pulsations, and Appourchaux et al 2010 for g-modes in the Sun) and planetary signals (Dumusque et al 2011;Meunier et al 2015;Meunier & Lagrange 2020). From solar observations, we learn that granulation can generate variability with RMS around 40 parts-per-million (ppm) in photometry (Dravins 1988, Fröhlich et al 1997Aigrain et al 2004;Sulis et al 2020a) and around 30-46 cm/s in RVs (Elsworth et al 1994;Pallé et al 1999;Garcia et al 2005;Appourchaux et al 2018;Collier Cameron et al 2019;Dumusque et al 2021).…”