Less than a year ago, astronomers announced an astounding discovery: They'd found three Earth-size planets orbiting a small, dim red star.
But as it turns out, their first observations missed a whole bunch of planets.
The astronomers now think seven rocky planets similar to our own may circle the star, called TRAPPIST-1 — an "ultracool" red dwarf located about 39 light-years away in the constellation Aquarius. Thirty researchers from around the world announced the new discovery on Wednesday in the journal Nature.
The team's surprise didn't end with an increased planet count, though.
Each planet orbits TRAPPIST-1 in the star's Goldilocks-like habitable zone — a region where there's enough starlight to permit the existence of liquid water. Three or four of the seven planets may also be candidates for hosting alien life, team members said on Wednesday during a press teleconference call.
"This is really the first time we have seven planets that we can say are in the terrestrial zone, and it's really, really surprising," said Michaël Gillon, one of the study's authors and an astronomer at the Université de Liège in Belgium.
The discovery of the new planets, each about 80% or 90% of the size of Earth, may have profound implications for the search for extraterrestrial life beyond the solar system.
"We've made a giant, accelerated leap forward in the search for habitable worlds," Sara Seager, a planetary scientist at MIT who wasn't involved in the research, said during a NASA press conference on Wednesday. "In this system, it's like Goldilocks has many sisters."
An alien — yet possibly common — solar system
The dwarf star is roughly the size of Jupiter, and its system of seven known planets — called TRAPPIST-1b, 1c, 1d, 1e, 1f, 1g, and 1h — is relatively small compared with our own.
In fact, one "year" on the innermost planet, 1b, lasts only about 1.5 days. The outermost planet, 1h — which scientists have caught only a fleeting glimpse of — may take about 20 days to orbit the star.
"If we put the TRAPPIST-1 star [in] the place of the sun, we'd have all seven planets inside the orbit of Mercury," Emmanuël Jehin, another coauthor at the Université de Liège, said on the call.
But that isn't a deal-breaker for the possibility of life.
TRAPPIST-1's surface temperature is about half that of the sun's, making it ultracool as stars go. However, if a planet orbits closely enough, it can receive the same amount of solar energy as the Earth receives from the sun at 93 million miles away. All of TRAPPIST-1's planets appear to be "temperate" and not close enough to get roasted.
The team said it's "very hard to know" if it just got lucky with its unprecedented discovery of seven Earth-size planets around a single star, or if such an abundance of planets is common, said Amaury Triaud, a fellow coauthor and an astronomer at the Institute of Astronomy at the University of Cambridge.
Either way, the bounty of planets may vastly improve the chances that humans aren't alone in the Milky Way galaxy, let alone in the universe at large.
That's because red dwarf stars constitute 30% to 50% of our galaxy's 100 billion to 400 billion stars, Triaud said, which makes them the most abundant and long-lived type of star around. (Sunlike solar systems represent only about 10% of those discovered thus far, the researchers said.)
"I think we've made a crucial step toward finding life out there," Triaud told Business Insider during the call.
"I don't think we've had the right planets to look at," he said. "If life managed to thrive and release gases similar to those here on Earth, then we will know. We have the right targets."
Another target could also be Proxima Centauri, a red dwarf 4.24 light-years away — many times closer than TRAPPIST-1. In 2016, astronomers learned that this star system may harbor an Earth-like planet called Proxima b.
NASA's upcoming James Webb Space Telescope might be able to study the planet's atmosphere for signs of life. A Russian billionaire is also backing an ambitious mission called Starshot to laser-propel a fleet of tiny spacecraft toward Proxima Centauri and its close neighbor Alpha Centauri to directly study and photograph any planets around the stars.
Why there could be life around TRAPPIST-1
Since astronomers verified the discovery of the first exoplanet 25 years ago, scientists have gradually come around to the idea that life is probably common in the universe. (Whether it's intelligent, angry, friendly, or indifferent, however, is a different matter.)
The transit method, in which a planet passes in front of a star and dims its light ever so slightly, showed humanity that perhaps trillions of planets may exist in our galaxy alone.
Gillon and his colleagues used the same method to discover the seven planets. They found the first three in previous years using a ground-based telescope named TRAPPIST. They detected the four additional planets featured in the new study by staring down TRAPPIST-1 for 20 consecutive days with NASA's Spitzer Space Telescope.
And abundant stars that are much cooler than the sun can warm up rocky planets enough to melt water — that is, if they have enough insulating gases in their atmospheres.
"It's possible their atmospheres are very similar to the Earth, or Venus, or something completely different," Gillon said.
The study suggests that the diameters and masses of the planets, estimated based on the amount of starlight dimming they cause, match up with that of rocky planets and possibly of those that harbor water as ice or liquid oceans.
That's not to say life is guaranteed around TRAPPIST-1.
Dangerous space weather, including solar flares and coronal mass ejections, could endanger any life by blasting it with dangerous high-energy particles. And red dwarf stars — more than most other types of stars — are known for such temper tantrums.
The planets also seem to orbit in "resonance," which could mean they are tidally locked and always facing the star with the same side, much like Jupiter's largest moons.
But neither of these necessarily means doom for any life that may exist there, the researchers said.
For one, Gillon said, TRAPPIST-1 "is a very quiet star," as far as giant balls of fusing plasma go.
Gillon also says orbital resonances and tidal locking may be an advantage.
"This could lead to a huge tidal heating in the cores of the planets," he said, by kneading them with gravitational strain.
Such warmth could melt ice into liquid, belch insulating gases into the planet's atmosphere through volcanism, and generally stir up the ingredients for life.
What's next for the TRAPPIST-1 system
Gillon said the team's discovery of TRAPPIST-1's seven planets "is just the beginning."
Direct photographs of the planets may not be possible, the researchers said. But within the next five years, they hope to use telescopes like JWST — scheduled to launch in 2018 — to peek at starlight that passes through the atmospheres of the planets.
The technique could help them measure how much oxygen, ozone, and other gases the worlds might contain.
"For instance, oxygen can be produced by photolysis of water on a water-rich planet," Gillon said. "It's really the molecules and their relative abundances that enable us to give the plausible or restricted extent of life. We'll see."
The Search for Extraterrestrial Intelligence, or SETI, has already tuned in to listen to the system and found no unusual signals, Gillon said during NASA's press conference on Wednesday.
And even if TRAPPIST-1 seems to be a dead or sterile planetary system now, that doesn't mean it's an unlikely place to seek aliens in the future.
"Could any of the planets harbor life? We simply do not know,"Ignas Snellen, a researcher at Leiden University who wasn't part of the research team but reviewed the study for Nature, wrote in an accompanying editorial.
"But one thing is certain: In a few billion years, when the sun has run out of fuel and the solar system has ceased to exist, TRAPPIST-1 will still be only an infant star. It burns hydrogen so slowly that it will live for another 10 trillion years," Snellen wrote, adding that that's "more than 700 times longer than the universe has existed so far, which is arguably enough time for life to evolve."