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Of course, science fiction fans had known all along, but the existence of planets in solar systems other than our own was only confirmed a few decades ago. And it took even longer for exoplanet discovery to yield the first other world resembling Earth in size within the habitable zone of a Sun-like star, as recently as 2011.
In 2013, the EU-funded project ETAEARTH set out to establish how many Earth-like exoplanets there might be in our galaxy. To do so, it generated new knowledge on such rocky worlds beyond our solar system.
‘ETAEARTH determined with unprecedented accuracy the physical properties of terrestrial exoplanets orbiting stars that are similar in size to our Sun, or smaller,’ says project coordinator Alessandro Sozzetti of Italy’s National Institute for Astrophysics.
Stellar data
Recapping on the project’s achievements in February 2019, he attributes the success of this five-year endeavour to the combination of outstanding data from three sources. One of these is located down here on Earth: HARPS-N, the ‘High Accuracy Radial velocity Planet Searcher for the Northern hemisphere’, is firmly grounded at an observatory in the Canary Islands. The other two – NASA’s Kepler telescope and the Gaia observatory at the European Space Agency – are acquiring data in space.
The opportunity to draw on data from both ground-based and space-borne observation held the key, Sozzetti underlines. It notably enabled ETAEARTH to determine the composition of 70 % of currently known rocky planets with high precision, he adds. The partners concluded that the ratio of iron to magnesium silicate, of which terrestrial planets are mainly composed, is common to all with masses not exceeding six times that of Earth.
Other worlds
ETAEARTH, which ended in December 2017, discovered 10 rocky exoplanets in the process, including the first-known exoplanet of a size, density and internal composition approximating those of Earth, Sozzetti reports.
The project team reached the conclusion that more than 20 % of the many billions of Sun-like stars in our galaxy are likely to host a terrestrial planet within their ‘Goldilocks’ zones – the stretch that would be just right in temperature for water to potentially exist in liquid form.
Not that these Earth analogues would necessarily be hospitable. Nonetheless, given how many of them this ratio works out to, some of them might well be. The term ‘eta-Earth’ – the average number of rocky planets in the habitable zone of a star that has planets – is one of the elements of the Drake equation, which aligns parameters to consider when estimating the potential number of extraterrestrial civilisations whose signals we might be able to detect.
News of such a detection would certainly rock our world, but there are many other mysteries to unravel. For Sozzetti and his colleagues, the journey into the unknown continues, currently with work focusing on exoplanet candidates identified by TESS, NASA’s Transiting Exoplanet Survey Satellite.