Artistic representation of all the five known potential habitable worlds including now Gliese 581g, the best candidate for an Earth-like exoplanet so far. All of these planets are superterrans (aka Super-Earths) with masses estimated between two and ten Earth masses. Numbers below the planet names correspond to their similarity with Earth as measured in a scale from zero to one with the Earth Similarity Index, one being identical to Earth. Credit: The Habitable Exoplanets Catalog, PHL @ UPR Arecibo