Astronomers Have Found Planets in the Habitable Zone of a Nearby Star

The newly discovered worlds are now the most promising targets in the search for life among the stars—and the race to take a closer look at them has begun.

Artist's impression of the TRAPPIST-1 system.  (ESO / M. Kornmesser / N. Risinger)

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.

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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.

This is a stellar system very different from our own. 1b, nearest to its star, completes an orbit every 1.5 earth days, and 1c is almost as quick:  Its “year” is just 2.4 days long. 1d lies further out, with an orbital period of no less than 4.5 days, and perhaps as long as almost 79 days. (The team needs more transit data to squeeze that uncertainty.) The two inner planets are likely to be tidally locked. As our own moon does to Earth, they would always present the same side to their sun.

The sun that warms these exotic worlds puts out much less energy than our own. That’s why it’s described as “ultracool.” But because these planets travel in tight orbits around the star, numbering only a few days, it can heat them to an equilibrium temperature of roughly 400 degrees Kelvin, or 260 degrees Fahrenheit. That’s a lot warmer than the earth, and above the boiling point for water, but life could still lurk in the shadows.

“The day side should be very hot,” Gillon says, but if these planets do possess atmospheres, they will also experience weather.  Then, “the heat will be distributed by winds to the night side. “Weather modeling for such systems has shown that such heat transfer could produce conditions near the western edge of the planet’s dark side that would fall into that goldilocks situation exobiologists believe suitable for life: not too hot, nor too cold, but just right for liquid water.  That would be, MIT’s DeWit says “a pretty sweet spot—a nice temperature, protected from irradiation from its sun.” 1d, the outermost of the three planets, is the wild card. It might be far enough to have liquid water everywhere, or it could be a too-distant ice world.

All of this is, of course, speculative. Were life to have gained a foothold in a dark pocket on one of these planets, it would almost certainly seem very strange to us.  “It would receive no light in the optical range,” Gillon says, and couldn’t rely on photosynthesis. “It would have to develop techniques to get energy from the infrared.” There are a few microbes on earth that do this, “but on this planet it would be the rule, not the exception.” And, the dreamt-of habitable pockets may very well not exist at all. The planets could have lost whatever atmosphere they may have had to erosion by ultraviolet light early in the system’s history. Stellar flares could roast the planets at regular intervals. Tidal heating could produce outrageous amounts of volcanic activity, just as Jupiter’s pull has driven volcanism on its moon, Io.

“The surface could really be a hell,” Gillon says.  “We don’t know. That’s what’s exciting.” The campaign to explore the planets is already under way. It will begin with observations of the TRAPPIST-1 system across the range of wavelengths from X-rays to radio. “We can even search for communication”—from intelligent life—“if we are optimistic enough,” says Gillon.  “I’m not very optimistic, but it’s basically free to try.”

Astronomers will get an even closer look when the next generation of mega-telescopes comes online, especially those optimized for infrared spectroscopy, a technique that can detect the chemical contents of the planets’ atmospheres. It takes a fair amount of light to make a detailed spectrographic analysis of the light that passes through the planets' atmospheres—which is why this observation can only be made on large telescopes. The payoff is that such observations could reveal whether or not a planet or planets within the TRAPPIST-1 system have produced the chemical signatures of life, like the presence of abundant oxygen and other bio-marker molecules in the Earth’s atmosphere.

The James Webb Space Telescope—the successor to the Hubble—is scheduled to launch in 2018, and it will be equipped to tease spectrographic results out of the TRAPPIST-1 system.  The next generation of giant, ground-based telescopes will come online in the 2020s, and will be able to extend the Webb’s capacities into detailed spectroscopy.  “It would be very surprising to detect life on our first opportunity.” Gillon says. There are so many ways to get false positives, he adds, so many ways to misunderstand the processes being observed.  But if the new telescopes do see something suggestive or better, it could, Gillon says, “mean life is everywhere.”

And what a discovery that would be. The possibility of open-ended exploration has diminished on Earth’s surface. Most of our planet’s frontiers have been reached, save for a few in the deep ocean. But during the last few decades, the night sky has opened up, as a front for exploration, to an extent that still may not be fully appreciated. Every time someone figures out a way to look out into the universe in a new way, we see new phenomena—not simply variations on well-characterized systems or events. And now, circling a nearby star, a member of a class long derided for its dimness, there are potentially life-bearing planets.

These discoveries occur, says Robert Kirshner, research professor of astronomy at Harvard because “our collective imagination is insufficient.” The tendency to think that you know how the universe works based on your current state of knowledge is hard to shake. But it turns out, says Kirshner, that “nature has done this experiment many, many times, with different outcomes.”

Gillon echoed Kirshner’s point, at the end of our conversation. “I don’t like the idea of incremental science where you just add up a few details of knowledge in theories that are well established,” he told me.  “I want to detect new things, new worlds, and to be amazed at what we find.”