We really don't know what sort of composition a planet the size of Kepler-22b would have.

BBC: Viewpoint: The obsession with a 'twin Earth'

To be Earthlike, you also need a sun like ours and Kepler-22b has that.

BBC: Viewpoint: The obsession with a 'twin Earth'

Kepler-22b was thought to have a temperature similar to that of Earth, according to modeling by Borucki and colleagues.

CNN: 3 new planets could host life

You may recall planet Kepler-22b, which was announced in December 2011 and also was hailed as a potential candidate for hosting life.

CNN: 3 new planets could host life

There is also a planet called Gliese-581g, discovered in September 2010, which is thought to be even more like Earth than Kepler-22b in terms of its suitability for plants and animals.

CNN: 3 new planets could host life

Kepler-22b is quite a bit larger than the biggest rocky planet in this Solar System (which happens to be Earth), but much smaller than the smallest of our local gaseous worlds (Neptune).

BBC: Viewpoint: The obsession with a 'twin Earth'

This past December, for example, NASA's Kepler space telescope confirmed its first planet in the habitable zone, a "super Earth" known as Kepler-22b that's thought to be 2.4 times as wide as our planet.

MSN: Does 'potentially habitable' alien world exist? - Technology & science - Space - Space.com | NBC News

Kepler-22b satisfies on that score, too.

BBC: Viewpoint: The obsession with a 'twin Earth'

The newfound world, dubbed Kepler-22b, is much bigger than our planet - but, even so, it's also the smallest world yet discovered outside our Solar System, and finding it does bode well for the eventual detection of a true twin, a planet that really is a second Earth.

BBC: Viewpoint: The obsession with a 'twin Earth'