Roughly 39 light-years from Earth, the dim red star TRAPPIST-1 possesses a record-breaking seven Earth-size planets. Now astronomers using the Hubble Space Telescope have discovered hints that some of these planets could possess water, suggesting they may be suitable for life as we know it.
TRAPPIST-1, located in the constellation of Aquarius, is 2,000 times dimmer than our Sun, a bit less than half as warm, and only slightly larger in diameter than Jupiter. It is an “ultracool M dwarf,” a kind of star that makes up about 15 percent of the stars neighboring our solar system.
Using TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope), a telescope in Chile, in February 2017 astronomers discovered TRAPPIST-1 possessed the largest number of Earth-size worlds found so far in a single planetary system. At least three of these planets lie within the star’s habitable zone, the so-called “Goldilocks zone” in which temperatures could be just right for liquid water to flow in rivers and pool in lakes, seas and oceans. Since there is life virtually wherever there is water on Earth, these findings raised the possibility that TRAPPIST-1’s Earth-size planets could support life.
However, just because an Earth-size planet lies within a star’s habitable zone does not mean it is actually habitable. A key factor in whether such worlds can support life is how much ultraviolet light they receive, since ultraviolet rays can destroy water and organic molecules.
Scientists used the Hubble Space Telescope to measure the amount of ultraviolet light emitted by TRAPPIST-1 and to estimate the amount of ultraviolet light that each planet in the system received. When they were first discovered, each of the seven worlds was assigned a letter in order of their discovery as a placeholder for other possible names, starting with “b” for the innermost and going to “h” for the outermost. Planets e, f and g all lie within TRAPPIST-1’s putative habitable zone.
Lower-energy ultraviolet rays can break up water molecules into their atomic constituents, hydrogen and oxygen, while higher-energy ultraviolet light can heat up the upper atmosphere of a planet to allow that hydrogen and oxygen to escape into outer space. Moreover, planets especially close to their stars can suffer the same fate as Venus — intense starlight can turn all their water into steam, which is a potent heat-trapping greenhouse gas, leading to a steam-filled atmosphere and a runaway greenhouse effect that can bake a planet over hundreds of millions or billions of years until it is dry as a bone.
No one yet knows the exact age of TRAPPIST-1 and its planets—the current best estimate is anywhere between five and a half to nearly 10 billion years. Hubble’s measurements of TRAPPIST-1’s ultraviolet radiation suggest that, in that time, some of the star’s planets could have lost vast amounts of water. For instance, planets TRAPPIST-1b to TRAPPIST-1d might now be in a runaway greenhouse phase, and planets within the orbit of TRAPPIST-1g may have each lost more than 20 Earth-ocean’s worth of water.
Still, the researchers’ calculations hint that not all hope is lost for the worlds of TRAPPIST-1: under certain conditions the star’s outermost worlds, including e, f and g may have each lost less than 3 Earth-ocean’s worth of water during their lifetimes. Consequently, they may still harbor substantial amounts of moisture.
“It’s very promising that these outer planets could have retained their water,” says Mark Marley, a planetary scientist at NASA Ames Research Center in Mountain View, California, who did not take part in the research. “They did not say that water could be there, but if you imagine the planets were water towers, they could say how much of a leak each had.”
Moreover, when planets form, they can trap some of their water in their deep interiors, and “this water can then be released into the atmosphere over time through, for example, volcanism,” says study lead author Vincent Bourrier, an astrophysicist at the Geneva Observatory in Switzerland. “We found that the outer planets would, as well, have been the ones likely to inject the most water into their atmospheres.” The researchers detailed their findings online August 31 in The Astronomical Journal.
The researchers also looked for a giant hydrogen cloud surrounding TRAPPIST-1c that could have originated from water molecules in the planet’s atmosphere being broken apart by ultraviolet light. Although they did not detect such a cloud, they plan to use Hubble again to search for such clouds around all of TRAPPIST-1’s planets, Bourrier says.
As much as ultraviolet light could render some planets too dry for life around TRAPPIST-1 and other M dwarfs, another new study found a dearth of it may also make these red stars uninhabitable for life as we know it. Using computer models and the known properties of red dwarfs, researchers found that such stars might not emit enough of the long-wavelength, medium-energy ultraviolet light that likely helped trigger chemical processes key to the rise of biology on Earth.
“This illustrates the dangers of treating ultraviolet radiation as a monolith, when in fact different wavelengths of ultraviolet interact very differently with planets and putative life,” says planetary scientist Sukrit Ranjan at the Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts, the lead author of that study. He and his colleagues detailed these findings online July 10 in The Astrophysical Journal.
It remains uncertain whether the best hosts for habitable planets are ultracool M dwarfs such as TRAPPIST-1 or much warmer stars such as the sun, says Jeffrey Linsky, an astrophysicist at the University of Colorado at Boulder, who did not participate in either study. “The argument has gone back and forth like a pendulum several times and will likely continue to do so as we learn more about physical processes in planet atmospheres and the emission of host stars at all wavelengths,” Linsky says. “It would not surprise me if the best host stars are intermediate in temperature.”