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ETAEARTH Report Summary

Project ID: 313014
Funded under: FP7-SPACE
Country: Italy

Periodic Report Summary 3 - ETAEARTH (Measuring Eta_Earth: Characterization of Terrestrial Planetary Systems with Kepler, HARPS-N, and Gaia)

Project Context and Objectives:
The ETAEARTH project is a transnational collaboration between European countries and the US setup to optimize the synergy between space- and ground-based data whose scientific potential for the characterization of extrasolar planets can only be fully exploited when analyzed together.

We have used the HARPS-N spectrograph for 5 years to measure dynamical masses of terrestrial planet candidates identified by the Kepler mission. With the unique combination of Kepler photometric and HARPS-N spectroscopic data we have learned for the first time about the physics of their interiors. Some of these planets have characteristics (masses, radii, densitie) similar to Earth. One multiple system of small-radius planets characterized by ETAEARTH has a companion orbiting within the habitable zone of a star with just about 60% the mass of our Sun. We have carried out selected experiments in the original Kepler and K2 fields (mass measurements of multiple-planet systems and circumbinary planets) to probe models of planet formation, orbital migration, and long-term dynamical evolution. We have searched for planets similar to Earth orbiting a carefully selected sample of nearby bright solar-type stars, and provided suitable candidates for spectroscopic characterization of their atmospheres with next-generation space observatories. We have combined Kepler, HARPS-N, and extrapolations of Gaia data products for stars in the Kepler field to underpin the occurrence rates of terrestrial planets (h_earth) as a function of stellar properties with unprecedented accuracy. ETAEARTH has finally provided a new, quantitative answer to an age-old question of mankind: ‘How common are Earth analogs in our Galaxy?’.

Our unique team expertise in observations and modelling of exoplanetary systems has allowed us to fully exploit the potential for breakthrough science intrinsic to this cutting-edge, multi-techniques, interdisciplinary project, making the best use of data of the highest quality gathered from NASA and ESA space missions and ground-based instrumentation.
Project Results:
The progress of the work over the period in question fully met our expectations. The GTO programme observations were executed throughout the period with the aim of determining the spectroscopic masses of short- and longer-period transiting candidates uncovered by the Kepler and K2 missions, with radii similar to Earth’s and which had already been identified during Phase 1 of the Project. Several interesting systems were characterized. We also continued searching for terrestrial-type planets around a well-defined sample of chromospherically quiet solar-type stars in the immediate solar neighbourhood. Finally, we made provisions for the lack of Gaia Data Release 2 products towards fulfilment of the last year’s project objectives, and successfully carried out the two elements of the global statistical analysis in the Kepler field utilizing informed simulations of Gaia performance (precision in parallax determination). The main results achieved by the project over the period are:

1) The Kepler-454 System: A small, not-rocky inner planet, a Jovian world, and a distant companion (Gettel et al. 2016, ApJ).

2) A 1.9 Earth radius rocky planet and the discovery of a non-transiting planet in the Kepler-20 system (Buchhave et al. 2016, AJ).

3) The orbit and mass of the third planet in the Kepler-56 system (Otor et al. 2016, AJ).

4) Kepler-21b: A rocky planet around a V = 8.25 magnitude star (Lopez-Morales et al. 2016, AJ).

5) The Kepler-19 system: A thick-envelope Super-Earth with two Neptune-mass companions characterized using radial velocities and transit timing variations (Malavolta et al. 2017, AJ).

6) Three's company: an additional non-transiting planet in the HD 3167 System, and masses for all three planets (Christiansen et al. 2017, AJ).

7) Precise masses in the WASP-47 multiple-planet system (Vanderburg et al. 2017, AJ).

8) Identification of an ultra-short period rocky Super Earth with a secondary eclipse and a Neptune-like companion around K2-141 (Malavolta et al. 2018, AJ accepted).

9) An accurate mass determination for KOI-280b, a moderately-irradiated world with a significant volatile envelope (Haywood et al. 2018, AJ submitted).

10) Characterization of the K2-3 planetary system: A court of three likely sub-Neptunes characterized with HARPS-N and HARPS (Damasso et al. 2018, A&A submitted).

11) Stellar and planetary parameters in the Kepler field and a new derivation of η⊕, the fraction of solar-type stars with Earth-like planets.

The successful determination of the masses of intermediate-period (HD 3167d, Kepler-20g) non-transiting planets in multiple transiting systems, the early investigations of the composition of new rocky terrestrial planets (e.g., Kepler-20b, Kepler-21b) all the way in the regime of ultra-short period (USP) companions (K2-141b), and the identification of a peculiar non-rocky object among the ultra-short period transiting sample (WASP-47e) are very significant results for the ETAEARTH project during the period, and they testify of the project’s ability to set new standards beyond the state-of-the-art of the field. The most spectacular achievement of the project comes from the determination of the first constraints on the bulk composition of the three small-radius companions in the K2-3 system, and in particular for K2-3d, that straddles the habitable zone of what is to-date the brightest host closest in mass to that of our Sun. This most important result fulfilled Science Objective 1 of the Description of Work document. Overall, the HARPS-N GTO observations alone executed within ETAEARTH have contributed to populate 55% of the Mass-Radius diagram of USP terrestrial planets, and overall 70% of the mass radius diagram of small-radius, low-mass planets (in single and multiple systems) with precisely determined masses (< 30% relative error). This has brought Phase 2 to a successful conclusion and has allowed achieving Milestone 3 of the project. The global statistical analysis in the Kepler field (including the reassessment of the value of η⊕= 20.8±7.1%) executed over the last year of the project was completed in due time, despite the lack of direct distance measurements from Gaia due to the delay in the publication of DR2 data products. Phase 3 of the ETAEARTH project was thus concluded successfully, allowing us to achieve the last milestone of the project, Milestone 4. Throughout the reporting period, cooperative and coordination efforts between members of our team and researchers at other European institutes and in the US have led to a number of additional publications in relation to the activities of the project’s work-packages.
Potential Impact:
ETAEARTH has made the best use of across-techniques data of the highest quality gathered from NASA and ESA space missions and ground-based instrumentation:

• Kepler photometry
• HARPS-N spectroscopy
• Gaia astrometry

Within the 5 years of the project, ETAEARTH has determined physical properties and occurrence rates of terrestrial planets (h_Earth) as a function of stellar characteristics with unprecedented accuracy. The ETAEARTH project has, for the first time, combine the highest-precision datasets available to the scientific community working in the fast-developing, highly interdisciplinary feld of exoplanets, gathered with three different ground-based as well as space-borne techniques. ETAEARTH has ultimately brought us close to being able to answer age-old questions concerning the uniqueness of Earth as a habitat for the complex biology we call life.

The ETAEARTH project has helped placing the European community in the future of planetary science, and its spectacular results will provide inspiration for younger scientists in cross-disciplinary fields. The ETAEARTH project and its legacy will allow to transfer the scientist’s curiosity towards some of the most fundamental questions of modern astronomy (is Earth unique? How varied is the class of potentially habitable terrestrial planets?) across the European society.
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