At last! An Earth-sized planet in a star's “habitable zoneâ€. Hurrah for Kepler!
The habitable zone is the region around a star where surface water could be liquid. This doesn't mean the planet does have life, but it's a good start. Planets previously found in a star's habitable zone have been notably bigger than Earth. But
Kepler-186f is actually close to Earth's size and therefore probably rocky. There are also at least four other planets in its system. They orbit very close to the star and would be too hot to sustain life.
Kepler-186f may be similar in size to Earth, but it's no twin sister. Its star is cooler than ours so even though the planet is close enough to its Sun to orbit in 130 days, it still doesn't get as much light as we do. High noon there would be more like an hour before sunset on Earth.