Astronomers announced today the discovery of an extraordinary planetary system: seven Earth-sized planets that could all have liquid water on their rocky surfaces. The planets circle a tiny, dim, nearby star in tight orbits all less than 2 weeks long. Although it isn’t possible today to say whether the planets harbor life, astronomers are excited because each planet’s orbit passes in front of—or “transits”—its parent star. What’s more, the system’s proximity to Earth means that answers to questions about whether the system is habitable may come in just a few years’ time with the launch of a powerful new space telescope.
“If we are to find a biosignature, it may be in this kind of system,” says astrophysicist Nikku Madhusudhan of the Institute of Astronomy in Cambridge, U.K., who was not involved in the study. “In terms of transiting planets, this is as close to the holy grail as we’ve ever seen.” Team member Didier Queloz of the University of Cambridge says that the system, known as TRAPPIST-1, will be “a major driver of the question of whether there is life in the universe.” Says Thomas Henning, director of the Max Planck Institute for Astronomy in Heidelberg, Germany: “Imagine a solar system with seven planets like our own, it’s just amazing.”
Many exoplanets searches have focused on sunlike stars in the hopes of finding an analog to our own solar system—unsurprising because it is the one system known to foster life. But the team behind the Belgium-led TRAPPIST project (Transiting Planets and Planetesimals Small Telescope) took a different tack: They looked for planets that transit in front of dim, dwarf stars, by far the most numerous type of star in the Milky Way. Starting in 2010 with a 0.6-meter robotic telescope at the European Southern Observatory’s (ESO’s) La Silla Observatory in Chile, they quickly came across the star that came to be known as TRAPPIST-1.
Transit surveys stare at stars, watching for the telltale dip in brightness that occurs when an orbiting planet passes in front and blots out a tiny bit of light. The duration of the dip determines the planet’s orbit, while the depth of the dip determines the planet’s size. Because dwarf stars are so small and dim, transiting planets block a bigger proportion of the light—making the transits more apparent from Earth.
TRAPPIST-1, which is 39 light-years distant and just 8% the mass of the sun, caught the team’s attention because it was obvious from multiple dips that more than one planet orbited the star. Last May, the team published in Nature the discovery of three Earth-sized planets in orbit around it. Finding that many was “an amazing discovery,” Henning says.
But there were more planets to come. “There was a forest of transits,” Queloz says. “We could not make sense of it.” The team used NASA’s Spitzer Space Telescope along with observations from telescopes on Earth, including ESO’s Very Large Telescope in Chile and others in Morocco, Hawaii, Spain, and South Africa. A final, nearly continuous 20-day observation with Spitzer in September 2016, during which the team saw 34 transits, allowed them to untangle the mess. “Spitzer made all the difference,” team member Emmanuël Jehin of the University of Liège in Belgium told a press conference yesterday.
In a paper published today in Nature, the team describes a tightly packed group of planets with orbits ranging from 1.5 to 12.3 days. The dimness of the star means that, despite the planets’ close orbits, all seven could conceivably harbor liquid water on their surfaces. Three are firmly in the “habitable zone,” with enough starshine to have liquid water oceans, as long as they have Earth-like atmospheres.
Their orbits are not random but appear to be in a so-called chain of resonance, meaning that the orbital period of each planet is related to that of its neighbors by a ratio of small whole numbers. For example, for every eight orbits made by the innermost planet, the next planet orbits five times, while the next one out orbits three times. Planets don’t form in such tidy arrangements, which suggests that the TRAPPIST-1 planets were born in orbits farther out, before migrating inward and becoming trapped in the stable, resonant orbits. Forming in the system’s colder outer regions, where volatile compounds such as water and carbon dioxide freeze out, makes it possible that the planets incorporated those ices and carried them along to a warmer place where they could melt, evaporate, and become oceans and atmospheres.
One question that hangs over these planets is whether they are rocky, like Earth, or gassy, like mini-Neptunes. A measure of their density would answer that question. But for that, astronomers need to know their mass—individual transit studies reveal only size. However, in the case of TRAPPIST-1 the team was able to estimate masses by watching for a subtle gravitational effect on the planets’ orbits. Because the planets are bunched, they exert a small gravitational pull when they pass by each other. This occasional tug causes some transits to occur slightly later or earlier than expected. By measuring these transit timing variations and performing some fearsome modeling of the system, they were able to estimate the planets’ masses—and work out their densities. They all seemed to be rocky.
The next question for astronomers: Do the planets have atmospheres, and—if so—what are they made of? Transits can reveal atmospheres because as a planet passes in front of its star, atmospheric gases can absorb certain frequencies of the light passing through. Such observations are pushing the spectroscopic powers of even the Hubble Space Telescope to its limits. “Hubble is observing [the system], but it’s a little bit on the edge because of the size of the telescope,” Queloz says. So far, the team has confirmed that neither of the two innermost planets has a thick envelope of hydrogen gas, which is what you would expect if they were mini-Neptunes.
Realistically, any detailed study of TRAPPIST-1’s atmospheres will have to wait for the launch of Hubble’s successor, the James Webb Space Telescope (JWST), due late next year. With the frequent transits, “you can just stare with JWST,” Henning says. He thinks the JWST will be able to tease out the composition of the planets’ atmospheres, which has never yet been achieved for an Earth-sized exoplanet. Discerning biomarkers—which could be a particular mixture of methane, ozone, and oxygen—within those atmospheres, however, will be “extremely challenging,” Henning says. “It’s a goal, but may take longer than the next couple of years.” It may also take the muscle of the next generation of extremely large telescopes on Earth, which will debut next decade.
Researchers are prepared to wait a few more years for this, perhaps the greatest prize in astronomy. But the discovery of TRAPPIST-1 certainly gives them more hope that they will get there. The TRAPPIST project was only a forerunner for a more concerted search for exoplanets around dwarfs called SPECULOOS, which will rely on four 1-meter telescopes currently being installed at ESO’s Paranal Observatory in Chile. Over the next few years it will survey a thousand such stars. “Imagine how many similar systems may be out there,” Madhusudhan says. “The universe could be teeming with these things.”
published on sciencemag.org