On June 19, 2017, NASA’s Kepler space telescope team has released a mission catalog of planet candidates. The final catalog introduces 219 new exoplanet candidates, and 10 of them are near-Earth size and orbiting in circumstellar habitable zone.Notes 1
This the eighth release of the Kepler candidate catalog. With this release, there are now 4,034 planet candidates (as of June 21, 2017) identified by Kepler space telescope, which launched by NASA on March 7, 2009 to discover Earth-size planets orbiting other stars. Of those, 2,335 have been verified as exoplanets. Of roughly 50 near-Earth size habitable zone candidates detected by Kepler, more than 30 have been verified. All the Kepler’s catalog is publicly available on NASA Exoplanet Archive.
In February, NASA has announced that seven Earth-sized planets discovered around TRAPPIST-1, and three of these planets were firmly in the habitable zone.
Small planets come in two sizes
There’s also an interesting gap in the distribution of the planet sizes: researchers using data from the W. M. Keck Observatory and NASA’s Kepler mission have noticed that most planets discovered by Kepler so far fall into two distinct size classes: the rocky Earths and super-Earths (similar to Kepler-452b – sometimes nicknamed Earth 2.0 or Earth’s Cousin, the first potentially rocky super-Earth planet discovered orbiting within the habitable zone of a star very similar to the Sun), and the mini-Neptunes (similar to Kepler-22b, which discovered in December 2011 and was the first known transiting planet to orbit within the habitable zone of a Sun-like star). This histogram shows the number of planets per 100 stars as a function of planet size relative to Earth. Image credit: NASA/Ames/Caltech/University of Hawaii (B. J. Fulton)
1. Circumstellar habitable zone
The circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.