Twenty-Two New Candidates in the Habitable Exoplanets Catalog

Post date: Mar 01, 2012 3:36:30 PM

The Habitable Exoplanets Catalog lists now four confirmed and forty-five unconfirmed exoplanets that are high priority targets for habitable worlds.

The NASA Kepler Mission data added twenty-two new exoplanets candidates to the twenty-three already listed in the Habitable Exoplanet Catalog (HEC). There are now 45 unconfirmed potential habitable worlds including Kepler-22b, which was announced last December 2011. Kepler-22b is also listed in the HEC list of the only four potential habitable exoplanets confirmed so far, together with Gliese 581d, HD 85512b, and Gliese 667Cc.

The new 45 candidates are 200 to 3,000 light years away from Earth, which is much farther than most of the previous confirmed ones. Gliese 667Cc is the best candidate now for a habitable planet but the new Kepler data shows that many better candidates are just waiting to be confirmed.

Five particular candidates are especially interesting because they have an Earth Similarity Index (ESI), a measure of Earth-likeness, above 0.9 and close to Earth 1.0 value. Most of the others have ESI values between 0.7 and 0.9, a necessary condition to be considered habitable worlds, at least by microbial life.

The Kepler team added 1091 new exoplanet candidates last February 2012 to their previous 1,235 list of candidates from April 2011. NASA Kepler candidates now total 2,321 from which only 63 have been confirmed by further observations.

Most of Kepler candidates are superterran worlds, formally defined in HEC as planets with masses between 2 to 10 Earth masses, but equivalently interpreted as those with radius between 1.25 to 2.6 Earth radii for Kepler purposes (we do not know the mass for most of these objects). About 80% percent of Kepler candidates are in a single planet star system. The rest is divided between stellar systems of two to six planets per star. HEC estimates that 24% are rocky-iron planets, like Earth, and 40% a rock-water mix. Nearly 11% are probably too close to their parent star to have an atmosphere.

There are 760 confirmed exoplanets as of today. Most of them are hot jovians, planets like Jupiter but orbiting very close to their star. Of these, only four match the basic physical requirements for habitable planets. Unfortunately, their habitability depend on many physical and chemical parameters that scientist can not measure today due to the great distances to these worlds. Future better ground and space observatories will be needed to observe these interesting targets.

Confirmed exoplanets are distributed all around the night sky, including most of the constellations. Kepler observes a fixed patch of the sky and all its 2,321 candidates are in the constellations of Cygnus, Lyra, and Draco. Kepler probably could detect about that many exoplanets if pointed elsewhere, but its detection ability depends on looking to a fixed region of the sky for many years.

Kepler had identified more planetary candidates than it is now possible to observe by other ground or space observatories in the world. It will take many years to confirm all the new revealing hints of Kepler. Astronomers will be very busy.

HEC uses new habitability metrics and classifications to identify and compare potential habitable exoplanets. It combines information from four different exoplanet catalogs to automatically extract those best candidates for habitable worlds including moons.

HEC is a project of the Planetary Habitability Laboratory (PHL) of the University of Puerto Rico at Arecibo with the help of international partners. More details of these discoveries are available in the PHL website (


Image Caption: This table shows all NASA Kepler candidates from February 2012 classified as function of their size relative to Earth and orbital location with respect to the habitable zone (warm zone). Those in the warm zone and with the right size (subterran, terran, or superterran) are good candidates for habitable planets, 45 in total now. The number at the center of each box corresponds to the current number of planets in each category. The lower left number to the previous Kepler numbers from April 2011 and the lower right to those that are already confirmed. Credit: PHL @ UPR Arecibo.

HEC Science Contact: Abel Mendez, PHL @ UPR Arecibo.

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