The work performed within the framework of the ESCAPE project has been carried out in two main research axes.
In the first research axis, we looked in the possibility of characterizing the nature of temperate rocky exoplanets with already existing astronomical facilities.
In a review article (Turbet et al. 2020, Space Science Reviews, volume 216), we first showed that the only known transiting Earth-sized temperate planets amenable to such characterization – namely, the “TRAPPIST-1” planets – do not have hydrogen/helium-dominated atmospheres (with or without clouds/hazes). This result was obtained by combining HST and Spitzer transit spectroscopy measurements along with unprecedented bulk density measurements (Agol et al. 2021, The Planetary Science Journal, volume 2). Given the current lack of Earth-sized targets with potentially H2/He-rich atmospheres, we then focused our research on the detectability of water-rich atmospheres, which are the most likely types of atmospheres but also the most accessible after the hydrogen/helium-rich atmospheres.
We made the important discovery, using the results of 1-D numerical climate models, that the size a water-rich rocky planet can drastically increase when the insolation it receives exceeds a certain limit, known as the runaway greenhouse insolation threshold (Turbet et al. 2019, Astronomy & Astrophysics, volume 628). We then showed that this so-called “runaway greenhouse radius inflation effect” plays a first-order role in the mass-radius relationships of rocky planets (Turbet et al. 2020, Astronomy & Astrophysics, volume 638 ; see the Image). We then used this new theoretical result to constrain – with unprecedented accuracy – the water content of the seven temperate rocky planets in the TRAPPIST-1 system (Agol et al. 2021, The Planetary Science Journal, volume 2). This result, which was obtained by combining the precise mass and radius observations obtained with existing ground-based (in particular SPECULOOS) and space-based telescopes (Kepler, Hubble, Spitzer) with the theoretical work performed in the ESCAPE research project, is now the strongest constraint obtained to date on the possible nature of temperate rocky planets outside the solar system. The results of these predictions will be further tested in the future with observations from the James Webb Space Telescope (Fauchez et al. 2019, The Astrophysical Journal, volume 887 ; Turbet et al. 2020, Space Science Reviews, volume 216).
These results have been presented in numerous international conferences (e.g. ExoClimes 2019, EPSC/DPS 2019 Joint Meeting, Exoplanet III) and disseminated to the general public (radio, public events) through several press releases (1 UNIGE press release, 1 CNRS press release, 2 NASA press releases).
In the second research axis, we looked in the possibility of characterizing the nature of the most nearby exoplanet Proxima b, using reflected light observations. This work is carried out in particular within the framework of the development of the RISTRETTO instrument (Chazelas et al. 2020, Proceedings of the SPIE, volume 11448) currently under construction, which will be mounted within a few years on the 8-m ESO Very Large Telescope (VLT). For this, we have set up a first version of a pipeline (Turbet et al., in preparation) capable of generating synthetic observations (high-resolution albedo spectra of Proxima b) using a library of 3-D Global Climate Model simulations. This work is performed in order to prepare the flagship observations of the VLT@RISTRETTO instrument, pathfinder for the future E-ELT@HIRES instrument. In the framework of this research axis, we also obtained observing time (PI: Martin Turbet ; training by the supervisor David Ehrenreich and collaborator Christophe Lovis) on the VLT@RISTRETTO instrument (observations originally planned in October 2020, but postponed to July 2021 due to COVID-19) to perform preparatory reflected light observations using Titan, a satellite of Saturn.
This work is currently being used for the instrumental development of RISTRETTO, and will then be useful for the development of the future HIRES spectrograph.
