The race to look closer at them begins now.
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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.
There was only one problem: Before TRAPPIST launched, the preponderance of astronomical opinion held that such planets couldn’t exist. “The prediction was that the protoplanetary disk around such small stars would be too thin to make planets,” says TRAPPIST team-member Julien de Wit, now completing a post-doc at MIT. Gillon was unbothered by such claims. “The theories for exoplanets are based on very few observations,” he says. “I didn’t believe the theorists. I decided to follow my intuition.”
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For the TRAPPIST group, that meant coming up with an instrument that was powerful enough to make useful exposures on a very faint population of stars, and cheap enough that a funding agency might be willing to take a flyer on work that could very well fail. The design Gillon and his colleagues came up with—a telescope that could be operated by remote control, with a sixty-centimeter mirror (tiny by professional standards) came in at a total cost of around $400,000, which would be a rounding error in the budget for any of today’s major telescope projects.
TRAPPIST went into operation in 2010 at the European Southern Observatory site in La Silla, Chile. The survey of several dozen candidate stars began in 2015, with observations of the star now known as TRAPPIST-1 ending in January, when it disappeared behind the sun. When the team processed the data from this first run, they found five observations of one planet – TRAPPIST-1b—crossing the face of its star, three more of TRAPPIST 1c and two of a third, labeled 1d. With those data, and follow-up observations from other observatories, the team was able to replicate the transit observations and to confirm that all three were roughly earth-sized objects.