Is Kepler getting close to finding another Earth?

Size comparisons of Kepler planetary candidates. Credit: NASA Ames/Wendy Stenzel

The longer the Kepler mission keeps searching for exoplanets, the more amazing discoveries it makes. The Kepler mission team just released their third catalogue of new planetary candidates – 1,091 to be exact. This bring the total number of planetary candidates found by Kepler so far to 2,321, which orbit 1,790 stars. Confirmed planets so far now number 760, from Kepler and other telescopes.

These are the same candidates which had been previously discussed at the Kepler Science Conference last December, but now the official catalogue is available to the public.

A primary goal of Kepler is to find out how many planets among the stars being studied are Earth-sized and how many orbit within a star’s “habitable zone” where it would be possible for liquid water to exist. Ideally, planets which fit both criteria would be the “holy grail” of the mission.

Over 200 Earth-sized planets have now been added to the catalogue, and over 900 planets that are less than twice the size of Earth. There are also 46 planets in the habitable zone, and of these, ten are Earth-sized.

With the latest results, the number of planets found that are less than twice the size of Earth increased by 197%. Planets larger than twice the size of Earth, however, increased by 52%. This confirms the earlier trends seen that smaller rocky planets outnumber larger gas or ice giants.

There are also more multi-planet systems being discovered; 20% of the stars studied compared to 17% last year.

One of the Earth-sized planetary candidates found would seem to be of particular interest – KOI-494.01. It is still awaiting final confirmation, but the data so far support it being a real planet. It is estimated to have a mass 15% greater than Earth and a radius only 5% greater. The mean surface temperature is now estimated to be only about 1 ºC (33.8 ºF) cooler than on Earth. The estimated mean surface temperature (Ts) of KOI-494.01 is listed at 287 K (14 ºC / 56.93 ºF). Not bad!

According to the Earth Similarity Index (ESI), an estimated measure of how similar a planet is to Earth, KOI-494.01 now has an ESI rating of 99%. In the previous catalogue, it was listed at 97%. The confirmed planet with the highest rating so far is Gliese 667Cc, at 85%.

There is also a growing list of other Earth-sized candidates with ESI ratings close to that of KOI-494.01. See the summary lists here, updated March 3, 2012.

According to Natalie Batalha, Kepler deputy science team leader at San Jose State University in California, “With each new catalog release a clear progression toward smaller planets at longer orbital periods is emerging. This suggests that Earth-size planets in the habitable zone are forthcoming if, indeed, such planets are abundant.”

The updated findings continue the trend of exoplanets which are smaller like Earth, some of which orbit in their stars’ habitable zones. While these are likely to be of a wide variety, depending on other factors like composition, star type, etc., the discovery of planets like KOI-494.01 is an encouraging sign that there may indeed be other worlds out there similar to our own.

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