“This is our first Goldilocks planet – just the right size and the right distance from its sun,” said astronomer and “planet-hunter” Paul Butler with the Carnegie Institution of Washington. “A threshold has been crossed.”

The planet, called Gliese 581G, is quite close at 20 light years from Earth’s solar system. It is considered to be in the habitable zone because of its distance from its sun and its size.

Together, those two measurements tell scientists that any water on the planet will be in liquid form, and that the planet is large enough to have the gravitational pull to hold an atmosphere around it.