Although no clear definition exists, an exo-Earth is generally understood to be a planet of which the size and mass are almost equal to the Earth's. Usually, the requirement that it needs to be at the right distance from itshoststar is also added, so that it hasliquid water and is capable of supporting life. This implies that the planet is not necessarily around a Sun-type star. It could be around a calm red dwarf, orbiting in less than an Earth year. Inhabitable exomoons have also been considered such as in James Cameron's film Avatar.
In February 2011, NASA announced that Kepler may have discovered several exo-Earths. It should be remembered that the fact that a rocky planet is at the right distance from a star for liquid water to exist does not necessarily mean that there actually is liquid water. The example of Venus shows that the composition of the atmosphere must be taken into account.