-
Nobody in NASA is claiming that. Some astronomically illiterate reporters are, but that's another story.
What NASA has found is evidence of a planet (Kepler-22b) that lies in the "habitable zone" of it star. That's the zone where liquid water may exist on the surface. This does *not* mean that we actually know the planet has liquid water on it. In our own solar system, Venus and Mars are both technically inside the habitable zone, but Venus's thick atmosphere makes that planet much too hot to have liquid water, while Mars's super-thin atmosphere makes it too cold. While Kepler-22b has a suitable size and suitable distance from its star to have liquid water, we know nothing about its atmospheric conditions (or even whether it has an atmosphere at all). There are any number of factors which might prevent Kepler-22b from having liquid water on its surface.
BTW, the Kepler observatory was designed specifically for the purpose of finding extrasolar planets like this. It measures the reduction in starlight reaching us when an extrasolar planet passes between its star and us. Kepler has now detected three dips in starlight around the Kepler-22 system, spaced 290 days apart. Three identical, evenly spaced dips in starlight pretty much prove that there's a planet around that star that orbits once every 290 days. The 290-day time tells us that the planet is in the habitable zone, and the size of the dip gives us the planet's size. We're a long ways off from being able to actually determine the atmospheric composition of extrasolar planets, though.
For further details, see the link below.
Hope that helps!
What NASA has found is evidence of a planet (Kepler-22b) that lies in the "habitable zone" of it star. That's the zone where liquid water may exist on the surface. This does *not* mean that we actually know the planet has liquid water on it. In our own solar system, Venus and Mars are both technically inside the habitable zone, but Venus's thick atmosphere makes that planet much too hot to have liquid water, while Mars's super-thin atmosphere makes it too cold. While Kepler-22b has a suitable size and suitable distance from its star to have liquid water, we know nothing about its atmospheric conditions (or even whether it has an atmosphere at all). There are any number of factors which might prevent Kepler-22b from having liquid water on its surface.
BTW, the Kepler observatory was designed specifically for the purpose of finding extrasolar planets like this. It measures the reduction in starlight reaching us when an extrasolar planet passes between its star and us. Kepler has now detected three dips in starlight around the Kepler-22 system, spaced 290 days apart. Three identical, evenly spaced dips in starlight pretty much prove that there's a planet around that star that orbits once every 290 days. The 290-day time tells us that the planet is in the habitable zone, and the size of the dip gives us the planet's size. We're a long ways off from being able to actually determine the atmospheric composition of extrasolar planets, though.
For further details, see the link below.
Hope that helps!