The galaxy is getting very crowded. There may be 300 billion stars in the Milky Way, but until just over 20 years ago, we knew of only one of them that was orbited by planets. In the years since, the galactic census has exploded, with more than 4,700 confirmed or candidate planets discovered so far and astronomers concluding that every star in the galaxy is parent to at least one world.
What has always been harder to spot are Earthlike planets — relatively small ones with a rocky surface, orbiting their sun at the not-too-close, not-too-far distance that would allow liquid water to exist. Today, however, that changed in a big way, as NASA announced that a single star relatively close to Earth is home to no fewer than seven Earthlike planets. If you're looking for extraterrestrial life, there may be no place better.
The new findings, published in the current issue of Nature, are the result of more than six years of study of the small star Trappist-1, located just over 39 light years from Earth — barely one town over in a galaxy that measures 100,000 light years across. The star got its name from a rough acronym of the telescope in the Chilean desert that has studied it the most: the Transiting Planets and Planetesimals Small Telescope. As the name suggests, the Trappist telescope looks for planets by watching for the portion of their orbit in which they transit — or pass in front of — their star, causing a tiny but regular dimming in starlight.
Three Earthlike planets were discovered around Trappist-1 early in 2016 using this method. That prompted the astronomers who made the find — led by Michaël Gillon of the University of Liège in Belgium — to bring in some bigger guns. Conducting more surveys with ground-based telescopes in Morocco, Hawaii, South Africa, Spain and Liverpool, as well as with NASA's orbiting Spitzer Space Telescope, the investigators found four more planets. All seven except the outermost one are closely grouped, and all orbit Trappist-1 at the right, cozy distance to sustain biology, at least theoretically.
"The planets form a very compact system," said Gillon during a teleconference prior to the paper's release. "They are very close to their star and are reminiscent of the system of moons that orbit Jupiter. They could have liquid water and life."
In a solar system like ours, very close to the sun is not the best place to live if you're looking to harbor life. Consider Mercury, our innermost planet, where surface temperatures reach 800º F (430º C). Never mind water surviving; at that heat, lead melts.
Trappist-1, however, is nothing like the sun. It's what's known as a red dwarf, a very small, comparatively cool star, barely 11% of the radius of our sun and less than half its 10,000º F (5,500º C) surface temperature. Historically, astronomers ignored red dwarfs in their search for habitable planets. If the only star you know of that has given rise to life is a larger, yellow, so-called M class sun like ours, why look at ones that are so much smaller and cooler? But if you huddle up close to an M dwarf you can soak up all the light and warmth you need. What's more, there are at least three time more red dwarfs in the galaxy than all other classes of stars combined.
“If you draw a 30 light-year bubble around our sun,” said Harvard University astronomer David Charbonneau in a conversation with TIME, “you’d take in about 20 sun-like stars and 250 red dwarfs.”
Even better, a planet around a red dwarf is often easier to spot than one around a bigger star, since it is larger relative to its smaller parent. "These planets are 80 times easier to study in front of a red star than they are in front of a yellow one," says Gillon.
By no means is life anything like a sure thing in the Trappist-1 system. For one thing, the planets are so close to their sun that they are almost certainly tidally locked, which means that they keep one side forever facing toward the solar fires and one side away, the way the moon does with the Earth. That creates a stark temperature differential in the two hemispheres of the world, with one perhaps too hot for life to thrive and one too cold. Still, if any of the planets has an atmosphere — a big if — the heat and the cold could mix and moderate, at least in the regions that are forever fixed in dawn or dusk.
Another problem could be in the nature of red dwarfs themselves. They tend to be more volatile than M class stars, producing more solar flares, which blast out heat and radiation — an especially dangerous state of affairs for a planet in a tight orbit. But not all red dwarfs are equally turbulent, says Gillon, and Trappist-1 is "a very quiet star."
If there is life on any of the planets, it could be discovered relatively soon. Sending a spacecraft to visit is out of the question, of course. Even traveling at the speed of light, which is nearly 671 million miles per hour (1.1 billion k/h), the ship would take 39 years to make the journey. The fastest spacecraft ever built, the New Horizons probe to Pluto and beyond, is creeping along at about 36,000 mph (59,000 k/h).
Instead, the hunt for life on the newly discovered planets will be conducted by both orbiting and Earth-based telescopes, which will study the spectrum of Trappist-1's starlight as it streams through the atmospheres of any of the planets during their transits. Different chemicals absorb different wavelengths of light and if you know what you're looking for, you can pick out not just the presence but the concentrations of organic gasses like oxygen, carbon dioxide, carbon monoxide and especially methane. The closer that chemical fingerprint comes to matching that of Earth's atmosphere, the likelier it is something's living on one of the other worlds.
So epochal a discovery could be made within the decade, the Trappist-1 team believes, especially when the James Webb Space Telescope — the much more powerful follow-on to the Hubble Space Telescope — goes into service in 2018. And if the seven planets don't harbor life yet, they still have plenty of time. Trappist-1 is very young, just 500 million years old, compared to our 4.5 billion-year-old sun. That makes the sun middle-aged with only another 5 billion or so years left to it. Red dwarfs, however, burn through their hydrogen fuel much more slowly.
"Trappist-1 will live for one thousand billion years," says Gillon. If life is going to emerge in the system, it has all the time it needs.