The number of inhabitable planets within the Milky Way galaxy could be as high as 10 billion. Of course, the majority of these planets are further away than current technology could transport us. The best candidates for planets that can support life are those within a star's "habitable zone," meaning that water on the planet can maintain a fairly stable liquid form. A planet too far away from a star is too cold, leaving water on it in the form of ice; while planets closer than the habitable zone are too hot, meaning any water evaporates.
- So far, at least nine other planets have been discovered in our galaxy with masses similar to that of Earth, and two of them are in their star's habitable zone.
- There are over 100 billion stars in our galaxy, which means that 10% of stars might support an inhabitable planet in its solar system.
- Future developments in telescope technology should let astronomers directly view light from distant planets to determine if the chemicals necessary to support life are present in their atmosphere.