The robot telescope settles on its target, a star that sits closer than all but a tiny fraction of the tens of billions of stellar systems that make up the Milky Way. Its mirror grabs light for 55 seconds, again and again. The robot telescope—called TRAPPIST—will observe the star for 245 hours across sixty-two nights, making 12,295 measurements. Eleven times, it will see the star dim, ever so slightly. This dip in luminosity, called a transit, has a straightforward astronomical explanation: It’s a planet passing in front of the star, blocking just a bit of its light. In this case, the transits tell us that 3 planets orbit the star.
“So what?” you might think.
Astronomers have been spotting planets around distant stars for years now, using the transit method, among others. Not a month goes by without a headline, touting the discovery of new “exoplanets.” But these planets are different, and not only because they’re near. Like the Earth these planets could potentially permit liquid water to persist on their surfaces—which is thought to be a key pre-condition for the emergence of life. Today, when their discovery is published in Nature, they will instantly become the most promising planets yet found in the search for life among the stars.
The race to look closer at them begins now.
Michaël Gillon, the first author on the Nature paper, says he’s always wanted to know whether humankind—and all the biology with which we share our planet—have company in the galaxy. “I’ve always been focused on extraterrestrial life,” he says. Gillon, an astronomer at the University of Liège, says he hit upon the idea behind the TRAPPIST survey by reasoning backwards from that goal: to find life one must find not just planets (easy enough now) but those that could be explored in depth from Earth.
But earth-like planets orbiting sun-like stars aren’t the best targets for such a search. From a close distance, an earth-like planet’s fine details will be lost in the glare of a relatively bright, hot star like our sun. Such fairly bright stars have been the most common targets of large exoplanet surveys, but there is a class of stars that looked more promising to Gillon: tiny, cold (in stellar terms), and utterly unexciting, a group termed, with just a hint of dismissal, “ultracool dwarfs.”
The ultracool dwarf at the center of the newly discovered planetary system—dubbed TRAPPIST-1—is just 80 times the mass of Jupiter, barely above the minimum threshold required to fuse hydrogen into helium. For Gillon, such insignificance is the glory of his ultracool targets: because they are small and dim, the best of current and coming telescopes could, in principle, peer into the atmosphere of planets orbiting these unassuming stars.