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ExTrA Report Summary

Project ID: 337591
Funded under: FP7-IDEAS-ERC
Country: France

Periodic Report Summary 2 - EXTRA (Exoplanets in Transit and their Atmosphere)

The project EXTRA aims to detect and to characterize transiting planets around cool stars, with the appealing prospect of detecting Earth-size planets in their habitable zones. Owing to the small size of their parent star, such planets would be the most favorable for further characterization of their atmospheres. To realize this ambitious goal, we proposed a novel method to record more precise photometric light curves from the ground: we add the spectroscopic resolution to the traditional photometry and, also, we choose to observe in the infrared. This shall enable the fine correction of atmospheric variations that would otherwise hinder ground-based observations, and the infrared enables more efficient observations of cool stars.

To implement this new approach, we proposed to design, built, install and operate a new facility: the ExTrA Observatory. It is composed of three 60-cm telescopes linked to a multi-object spectrograph with optical fibers. It shall be controlled remotely and eventually be robotic. After we learned the selection of the project (July 2013) we worked with in-house resources toward the final design and specifications. The project formally started with the ERC in July 2014, which enabled us to procure the equipment (telescopes, cameras, domes, computers...) as well as to validate several aspects of the experiment with laboratory measurements.

Today, most of the elements of EXTRA are being assembled and tested at IPAG (Grenoble, France). The construction just started on-site at La Silla Observatory, in Chile, and we foresee to be operational before the end of this year (2016).

Once achieved, we hope the EXTRA facility will be a workhorse to detect small planets around small stars. Yet, our search for Earth-size planets orbiting small stars has already started: we combine the radial-velocity data we are collecting with the HARPS spectrograph to the existing MEarth photometric survey (PI=David Charbonneau at Harvard). One major recent achievement of this collaboration has been the detection of GJ1132b. This transiting planet has about the size of Earth (1.2x), the mass of Earth (1.5x) and, most importantly, is the most nearby and the easiest to characterize compared to other planet of its class (Berta-Thomson et al. 2015). Our team also contributed to the fine characterization of several planets transiting M dwarfs. We performed the follow-up of K2-3, a M dwarf with 3 transiting super-Earths detected by the K2 mission, including one possibly in the habitable zone. We collected radial velocities of K2-3 and we were able to report the mass of the inner planet. We also made progress in measuring the masses of the 2 other planets, including the super-Earth in the habitable zone (Almenara et al. 2015a). Beside K2-3, we contributed to the detection of the evaporation of a Neptune-mass planet orbiting a M dwarf (Ehrenreich et al. 2015). Finally, we 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. 2015b).

- ExTrA : Exoplanets in Transit and their Atmospheres, X. Bonfils et al. (2015, SPIE 9605, 1)
- A rocky planet transiting a nearby low-mass star, Berta-Thomson et al. (2015, Nature 527, 204)
- A HARPS view on K2-3, Almenara, Astudillo, Bonfils et al. (2015, A&A 581L, 7)
- A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b, Ehrenreich et al. (2015, Nature 522, 459)
- Absolute masses and radii determination in multiplanetary systems without stellar models, Almenara et al. (2015, MNRAS 453, 2644)

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