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ETAEARTH Report Summary

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

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

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 Guaranteed Time Observations with the HARPS-N spectrograph on the Telescopio Nazionale Galileo (TNG) for 5 years to measure dynamical masses of transiting terrestrial planet candidates with accurate radius measurements identified by the Kepler and K2 missions. The unique combination of Kepler/K2 photometric and HARPS-N spectroscopic data has enabled precise measurements of the bulk densities of these objects, allowing us to learn for the first time about the physics of their interiors. With the ETAEARTH project we have determined with high precision (20% or better) the composition of 70% of currently known planets with masses between 1 and 6 times that of the Earth and with a rocky composition similar to that of the Earth. With ETAEARTH we have provided the first-ever constraints on the density of a planet, in a multiple transiting system, similar to Earth in mass and orbiting within the Habitable Zone of the star known to-date to be closest in mass to the 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. With ETAEARTH we have found the two closest transiting rocky planets, orbiting a solar-type star only 21 light years away, thus providing suitable candidates for spectroscopic characterization of their atmospheres with next-generation space observatories. We have improved significantly our comprehension of planet formation scenarios and orbital evolution models. Using insights from ETAEARTH results we have gauged possible scenarios for the formation of ultra-short-period rocky exoplanets, and identified novel ways for discriminating observationally between scenarios in which such planets formed originally as rocky objects or they are instead the stripped cores of planets that, initially, had much more substantial gaseous envelopes. We have critically advanced our capabilities in structural modeling of the interiors of low-mass, small-radius planets. We have systematically compared the mass-radius relation for terrestrial transiting exoplanets observationally determined thanks primarily to ETAEARTH results with the two-component iron-magnesium silicate internal composition models we developed, and found that that the rocky analogues of the Earth with accurately (better than 20% precision) determined masses below 6 M⊕ appear to be are welldescribed by the same fixed ratio of iron to magnesium silicate. We have combined Kepler, HARPSN, and informed extrapolations of Gaia data products (direct distance measurements) for stars in the Kepler field to underpin the occurrence rates of terrestrial planets (η⊕) as a function of stellar properties with unprecedented accuracy. With ETAEARTH we have determined that 1 in 5 solar-like stars host an Earth-like planet, i.e. an object with a size similar to Earth orbiting within the Habitable Zone of its solar-type parent star. ETAEARTH has thus finally provided a new, quantitative answer to an age-old question of mankind: ‘How common are Earth analogues 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.

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