Earth-sized

Kepler has found that smaller planets (Earth-sized rather than Jupiter-sized) are likely to be the most common in the Milky Way galaxy.

Even if a planet is rocky, Earth-sized, and just the right distance from its star, it also must have an atmosphere that contains the correct ingredients to foster and sustain life.

Basic physics tells us that smaller planets must be rocky and larger ones gaseous, but for planets ranging from Earth-sized to about twice that radius, astronomers can't tell a large rocky planet from a small gaseous planet.

Scientists at the Harvard-Smithsonian Center for Astrophysics, after analyzing data from NASA's Kepler space telescope, calculated that six percent of the galaxy's 75 billion red dwarf stars have potentially habitable, Earth-sized planets. Red dwarfs — which are smaller, cooler and much dimmer than our own Sun — are the most common stars in the galaxy, so the closest Earth-like planet could be just 13 light years away, the astronomers conclude in their new study.