Seven Milky Way planets that could harbour life discovered

The Habitable Exoplanets Catalog (HEC) celebrated its first anniversary by announcing that it had exceeded expectations in its search for possible new Earths.

Abel Mendez, director of the University of Puerto Rico at Arecibo's Planetary Habitability Laboratory and lead researcher, said the team had hoped to add perhaps one or two planets in the project's first year, the 'SPACE.Com' reported.

He said the addition of five possibly habitable planets over the two already known totally exceeded anyone's expectations.

"There are many press releases announcing discoveries of habitable planets ... And that is confusing. So having a catalogue that everyone can check what is available right now is useful," Mendez said.

Mendez also said scientists are getting smarter about finding exoplanets, and the pace of discovery is increasing.

There are 27 candidate planets waiting for inclusion in the habitable portion of the catalog. Meanwhile, the HARPS (High Accuracy Radial Velocity Planet Searcher) instrument in Chile and orbiting Kepler Space Telescope, among others, are quickly finding new exoplanets every month.

Mendez's team principally assesses the potential of life on a planet using three metrics: the variability of energy from the host star that the planet receives, the mass of the planet and the planet's size.

Simplistically, bigger gas giants orbiting variable stars are less likely to host life than smaller, rocky planets near stable stars.

Much of the catalog's data come directly from the research teams involved in an exoplanet's discovery.

Mendez cautions that the information is preliminary. So far, most of what we know about exoplanets comes from a simple physical assessment.

Sometimes a planet is found that can't be confirmed through independent observation. One famous example is Gliese 581g, which was discovered by one team but could not be found by another team using a different instrument.

New datasets have been released, but there still is a debate. Mendez calls these situations "tricky."

"There are two versions of the story, and the two versions can be supported by data. But because we think that there's still the sense that planet could exist, we are including it [as a candidate]," he said.