The first Earth-like planet to be confirmed around another star was Kepler 22b. Located about 600 light years away in the constellation Cygnus, and was discovered by the Kepler space telescope in 2011.
Kepler 22b is unlike Earth in one major way, it's size. In fact, Kepler 22b is belongs to a newly discovered class of planets known as super-earths, which are smaller in size than Neptune but are larger than the Earth. Kepler 22b has a radius roughly 2.4 times the size of Earths. It's mass and composition remain unknown. There's still much debate about Kepler 22b, mostly its composition is debated. Some think that it may not be a super-earth, but rather, it may be a mini Neptune. This was first proposed because astronomers could not detect the planet using the radial velocity method, meaning that it's a very "light" planet. However, just because it's "light" doesn't mean it's primarily composed of gas. One hypothesis is that Kepler 22b is actually a water world.
So is Kepler 22b habitable? This still remains unknown. Although we do know that it's the right size and the right distance from its star to have water as a liquid on its surface, it's atmospheric and surface composition remain a mystery. It's orbit is also unknown. For all we know Kepler 22b could have an extremely elongated elliptical orbit, rendering it uninhabitable to life as we know it. Astronomers have predicted that if Kepler 22b has an atmosphere similar to Earth, it would have an average surface temperature of 20°C. Without an atmosphere it would likely have an average temperature of -11°C. Kepler 22b also orbits a G-type star very similar to our own.
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Image credit: NASA/JPL/Caltech
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