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Just 20 years ago, astronomers didn’t know if there were any planets at all outside our own solar system—whether other places like Earth, which is brimming with life, are common, exceedingly rare or even non-existent. Two years ago, NASA scientists announced that, using the powerful Kepler space telescope, they’d found well over 1,000 new planets, more than doubling the number they’d previously known about. It was a stunning revelation, but few people realized, even then, that this was just the beginning.

Astronomers now believe our galaxy alone is filled with literally billions of planets—maybe even more planets than stars. There are at least 100 billion planets in the Milky Way, and some think that estimate is conservative. Some are more bizarre than anything dreamed up in science fiction: diamond worlds and double-sunned worlds, and worlds where another planet hangs in the sky like our moon. Others are eerily similar to Earth. A few of them, like a newly found planet orbiting Alpha Centauri, just 4.3 light years away, are tantalizingly close. That planet is nearer to its host star than Mercury is to our sun, and would be blisteringly hot—far too hot for life as we know it. But where there’s one planet, there are often several, and astronomers are scouring the skies around Alpha Centauri for more worlds in our own cosmic backyard. Continue…