NASA Kepler science team announced a discovery of a planet (Kepler 62 f) of only 1.4 times Earth size in the habitable zone, on April 18th, 2013. "Habitable zone" means the range of orbital radius at which an equilibrium temperature is mild and a solid (rock or ice) planet can have ocean on its surface. Life could live on the planet.

The orbital radius of Kepler 62 f is 0.72 astronomical unit (au), where 1au is the distance of the Earth from the Sun. Although it is similar to the orbital radius of Venus, it is in the habitable zone due to a less bright host star (K dwarf star).

Astronomers have been expecting that Earth-size planets may ubiquitously exist in habitable zones in exo-planetary systems.

Kepler space telescope detects exoplanets by the transit method that observes periodic impulsive decrease of star light due to eclipse by an unseen planet. For the eclipse to occur, the orbital plane of a planet must be almost aligned. Since the probability of the alignment is inversely proportional to the orbital radius, the detection of exoplanets by Kepler is highly biased to close-in (small orbital radius) planets.

Kepler has shown so far that more than a few tens of percents of solar-type stars would harbor close-in (say, less than 0.3 au) relatively small-size planets, which are terrestrial (rocky) planets or icy planets (Gaseous planets like Jupiter usually are much larger). The equilibrium temperature of these close-in planets is too high for the planets to be "habitable." Nevertheless, the ubiquity of close-in planets allows us to expect the ubiquity of terrestrial planets in habitable zone as well.

However, there is one point we are worried about. In our Solar system, the terrestrial planets (Mercury, Venus, Earth, and Mars) are distributed near the habitable zone centered at 1 au. There is no planet inside of Mercury's orbit (0.39 au). So, we can't rule out the possibility of the inverse trend that planets do not exist in habitable zones in the systems with close-in planets.

Kepler is now starting detecting possible terrestrial planets as far from their host stars as habitable zones. Five planets have been discovered in Kepler 62 system. Their sizes are 0.5-1.6 times of Earth's size. The relatively small sizes suggest that they are terrestrial planets or icy planets. Kepler 62 f is an outermost planet. Other discovered planets have radii of 0.055, 0.93, 0.12, and 0.43 au. Inner three planets, Kepler 62 b, c, and d, are one of "typical" close-in planetary system that Kepler has discovered.

The discovery of Kepler 62 f is very important not only because it is an Earth-like size planet but also because it exists in a system with close-in multiple small planets; Kepler has discovered many systems with close-in multiple small planets. Although this is a first discovery of the system in which close-in multiple small planets and a planet in habitable zone coexist, it is expected that many similar systems will be discovered. The ubiquity of close-in terrestrial planets may imply that of terrestrial planets in habitable zone as well.

Shigeru Ida (ELSI deputy director, PI)