From Scientific American:
As shocking as it sounds, we find exoplanets, or planets orbiting stars other than our Sun, all the time now. We know of nearly 3,000 such planets plus another 2,500 planet candidates. So what makes this recent discovery, dubbed Proxima b, so particularly exciting? Not only is this newfound planet very similar to the Earth in mass at 1.3 times the mass of the Earth, but it also orbits its host star in that star’s habitable zone.
As we’ve previously discussed, the habitable zone describes the range of distances from a star with the right temperature for liquid water to exist. You can think of the habitable zone as a band or torus around the host star. Planets too close to the star will have temperatures that are too high, causing any liquid water to boil while planets too far from the star will be too cold leading to frozen worlds. Planets within the habitable zone—also called the “Goldilocks zone”—have temperatures that are “just right” and thus at least have the potential to host liquid water.