So where should we go? That's a tough question to answer, says Billings. Scientists haven't yet spotted the perfect home-away-from-home for Earthlings. And in the past, we would have answered the question wrong. At various points in time, three different exoplanets orbiting the dwarf star Gliese 581—Gliese 581c, 581d, and 581g—were considered the most potentially habitable planets out there. But it turned out that 581c is probably too hot and gassy to be habitable, and 581d and 581g don't even exist. If we had launched explorers to those planets, Billings points out, they would travel for hundreds of years and find nothing at the other end.