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Exploring Shortcuts for the Characterization of the Atmospheres of Planets similar to Earth

Periodic Reporting for period 1 - ESCAPE (Exploring Shortcuts for the Characterization of the Atmospheres of Planets similar to Earth)

Okres sprawozdawczy: 2019-04-01 do 2021-03-31

The question of whether or not there is life beyond our solar system has recently taken a giant leap forward with the detection of several nearby Earth-sized, temperate exoplanets (Proxima b, TRAPPIST-1 planets, etc.). Finding exoplanets with atmospheres and identifying their atmospheric composition is a crucial step in pinpointing places with signs of life. Future ground and space-based telescopes such as the European-Extremely Large Telescope, the James Webb Space Telescope and LUVOIR will theoretically be able to perform the first characterization of the atmosphere of these potentially habitable planets. Yet, the implementation of these telescopes is either risky, far in the future, or both.

The ESCAPE project aims to investigate possible shortcuts for the characterization of the atmospheres of Earth-like exoplanets with existing ground and space-based telescopes, thanks to innovative combinations of observing techniques and instruments.

The general strategy of the ESCAPE project is to use a suite of sophisticated 1D and 3D numerical climate models to assess the possibility to make the first atmospheric characterization observations of potentially habitable planets with existing telescopes.
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.
The ESCAPE project led to the formulation of an observational test – based on the result of numerical climate model simulations – to characterize the water content of temperate rocky planets, which was successfully implemented for the first time on the planets of the TRAPPIST-1 system. These results obtained in the framework of the first research axis of the ESCAPE project led us to set up, for the first time, 3D numerical simulations of water-dominated planetary atmospheres that we further used to obtain major results about the conditions of formation of oceans on Venus, Earth and rocky exoplanets. This work is currently under revision in a high-impact scientific journal. These simulations will also allow to revise the mass-radius relationships of water-rich planets (because 3D simulations calculate the thermal profile and the position of clouds more precisely than 1D models) with implications as large as (1) the evaluation of the water content of small planets or (2) the characterization of the true nature of mini-Neptunes (hydrogen or water dominated atmospheres?).

In addition, the numerical pipeline set up as part of the second research axis of the ESCAPE project, which will be the subject of a scientific publication in a specialized astronomical journal, will allow not only to contribute to the preparation, but also and especially to the interpretation of the HCHR (high contrast high resolution) observations of Proxima b and other nearby rocky exoplanets, which will be carried out by the RISTRETTO instrument and then eventually the HIRES instrument.
Revised mass-radius relationships of water-rich planets and application to the TRAPPIST-1 planets