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Exoplanets in Transit and their Atmosphere

Final Report Summary - EXTRA (Exoplanets in Transit and their Atmosphere)

With the project ExTrA, we aimed at detecting and characterizing transiting planets around very small stars. These stars are easier targets to detect smaller, cooler planets and targeting them offers the prospect of detecting Earth-size planets in their habitable zones. In turn, these planets readily becomes favorable targets for atmospheric characterization with larger telescopes. To realize this ambitious goal, we articulated our project around 3 axes: (i) a novel method and a new facility, (ii) the detection of transiting planets, and (iii) the characterization of planets detected in transit.

Firstly, the project EXTRA proposed a novel method to record more precise photometric light-curves from the ground. The method makes use of multi-object spectrograph to add spectroscopic resolution on traditional photometry and, in addition, it perform its observations in the infrared. The small, red stars are much brighter in the infrared and, with more photons being collected, one can increase the signal-to-noise of photometry. In addition, systematics with chromatic signatures can be resolved with spectroscopy, and corrected a posteriori.

To implement this new approach, we designed, built, installed, and now operate a new facility : the ExTrA Observatory. It is composed of three 60-cm telescopes, equipped with moving fibers to feed a single near-infrared spectrograph. It is fully functional since 2019, it can be controlled remotely from Grenoble, and can even operate robotically providing a list targets is sent during the day. We have been operating the facility in a until La Silla had to shutdown because of the restrictions imposed by the covid-19 pandemic. We were able to record light curves of several transiting planets and our first publications are being prepared.

We started to operate ExTrA only recently but nevertheless managed several valuable observations and recently contributed to the discovery of TOI-269b, a 3-Earth radii planet orbiting a nearby M dwarf with an eccentric orbit. This is a quiet singular discovery at the transition between ice-giant planets (planets with a significant gas envelop) and telluric planets (planets with --at most-- a thin atmosphere). The discovery is lead by our PhD student Marion Cointepas and about to be submitted to Astronomy & Astrophysics.

Waiting for the completion of the ExTrA facility, we devoted some resources of our project to search for transiting planets with our HARPS radial-velocity program. This contributed the detections of transiting planets identified by either MEarth, K2, or TESS surveys; and also measured their mass. Among the many discoveries we contributed, we can highlight GJ1132b, a transiting planet about the size of Earth (1.2x) the mass of Earth (1.5x) and, most importantly, nearby and easy to characterize compared to other planet of its class (Berta-Thomson et al. 2015, Nature 527, 204), LHS1140b, a transiting terrestrial planet in the habitable zone a nearby cool star, Dittmann et al. (2017, Nature 544, 333), a second terrestrial planet orbiting the nearby M dwarf LHS 1140 (Ment et al. 2019, ApJ), a hot terrestrial planet orbiting the bright M dwarf L 168-9 unveiled by TESS (Astudillo et al. 2020, A&A 636, 58), A precise mass for planet GJ1132b and the discovery of a second planet GJ1132c (Bonfils et al. 2018, A&A 618, 142), the characterization of the K2-18 multi-planetary system with HARPS. A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit (Cloutier et al. 2017, A&A 608, 35), and a HARPS view on K2-3 (Almenara et al. 2015, A&A 581L, 7)

Our third axes was devoted to the _characterization_ of transiting planets. We contributed several studies on atmospheric characterization, and notably the detection of the evaporation of a Neptune-mass planet orbiting a M dwarf Ehrenreich et al. (2015, Nature 522, 459).

We also demonstrated that photo-dynamical modeling of transit light curves, combined with radial-velocity follow-up, can measure accurate masses and radii for all bodies of multi-planetary systems (planets + star), independently of stellar models. Almenara et al. (2018, MNRAS 478, 460; 2015, MNRAS, 453, 2644; 2015, A&A 595, 5)

In the course of these studies on _transiting_ planets we also detected several other non-transiting planets. We can especially highlight "A temperate exo-Earth around a quiet M dwarf at 3.4 parsec", Bonfils et al. (2018, A&A 613, 25)