Seven planets in the Milky Way outside our solar system could potentially harbour life, researchers from an ambitious project to catalogue all habitable worlds have announced.
The Habitable Exoplanets Catalog (HEC) celebrated its first anniversary with the announcement that it had exceeded expectations in its search for possible new Earths.
Lead researcher Abel Mendez, director of the University of Puerto Rico at Arecibo's Planetary Habitability Laboratory, said the team had hoped to add perhaps one or two planets in the project's first year.
The addition of five possibly habitable planets over the two already known totally exceeded anyone's expectations, he said.
'There are many press releases announcing discoveries of habitable planets ... and that is confusing,' Professor Mendez told SPACE.com. 'So having a catalog that everyone can check what is available right now is useful.'
With scientists improving their techniques for finding planets outside the solar system, the pace of discovery is increasing, he added. The High Accuracy Radial Velocity Planet Searcher (HARPS) instrument in Chile and the orbiting Kepler Space Telescope are just two of the tools helping researchers find new exoplanets every month.
Professor Mendez's team at the Planetary Habitability Laboratory launched the HEC in December last year. It was conceived out of the need to measure the suitability for life of these emerging worlds and as a way to organise them for the general public.
There are nearly 80 confirmed exoplanets with a similar size to Earth but only a few of those have the right distance from their star to support liquid water on the surface, the researchers say.
The catalogue uses habitability assessments like the Earth Similarity Index (ESI), the Habitable Zones Distance (HZD), the Global Primary Habitability (GPH), classification systems, and comparisons with Earth past and present.
However the exoplanet classifications use the word terran instead of ‘Earth-size’ to avoid a strict comparison with Earth since many of these worlds might have some physical similarities with Earth but might not necessary be Earth-like.
Seven potentially habitable exoplanets are now listed by HEC, including the disputed Gliese 581g, plus some 27 more from NASA Kepler candidates waiting for confirmation.
But the organisation cautions that while these are objects of interest for further astronomical observations, there is no guarantee on the habitability or even the existence of many of these planets.
It will take many new observations, and more ground and space observatories to verify and characterize these objects, the HEC team says.
The HEC was launched, December 5th, 2011, the same day that the NASA Kepler team announced the discovery of Kepler 22b during the First Kepler Science Conference in NASA Ames, California.
It started with just two planets, Gliese 581d and HD 85512b. Later, Kepler 22b, Gliese 667Cc, Gliese 581g, Gliese 163c, and HD 40307g were added, in chronological order.
All these exoplanets are superterrans, aka Super-Earths, and quite larger than Earth but still considered potentially habitable. Scientists have not yet found a true Earth analogue.
The HEC is now a very complex computer program that incorporates data from many exoplanet databases, such as the Extrasolar Planet Encyclopedia and NASA Exoplanet Archive, plus many updates from other research groups.
It receives most of the data of new relevant discoveries directly from the research teams, which is then analysed and added to the catalog at the moment of their public announcement.
In the coming year, the HEC will be expanded with new models and analysis, which is expected to affect many of the objects listed and offer new visualisations and habitability assessments.
However, the biggest impact will come not from changes in the HEC's maths, but from new discoveries, the team says.
'A true Earth analogue or a potentially habitable exomoon would be big discoveries,' they said. 'Certainly, this was the right time to start mapping the habitable universe around us.'
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