By Amina Khan
A team of astronomers says they’ve caught wind of an atmosphere around a super-Earth known as GJ 1132b, just 39 light-years away. The findings, described in the Astronomical Journal, mark another step on the road to determining whether alien worlds near and far might be capable of hosting life.
“It’s a great proof of concept that we can observe atmospheres on these small, rocky planets even from the ground,” said Laura Schaefer, a planetary scientist at Arizona State University who was not involved in the paper. “That’s really exciting and that means that we’ll be able to do it with more planets down the line as we find more planets of this size.”
The planet, Gliese 1132b, was first spotted in 2015 orbiting a small, dim M dwarf (a type of red dwarf) star about one-quarter of the Sun’s radius. At the time, the discovery excited scientists even though — with a surface temperature of about 620 degrees Fahrenheit — the planet wasn’t considered habitable.
“Receiving 19 times more stellar radiation than the Earth, the planet is too hot to be habitable but is cool enough to support a substantial atmosphere,” the authors of the 2015 study in Nature wrote. Because the host star was so close by, they added, “existing and upcoming telescopes will be able to observe the composition and dynamics of the planetary atmosphere.”
Astronomers often try to find exoplanets using the transit method: They wait for a planet to cross in front of its star and measure how much starlight is blocked. It’s much easier to pick out gas giants akin to Jupiter than it is to pick out small, rocky planets because oversized gas giants cause more dramatic dips in the amount of starlight. Those few other exoplanets whose atmospheres have been studied are gas-giant planets or very large super-Earths, with something like eight times our planet’s mass.
But because GJ 1132b is so close, and because its star is so small, the rocky planet blocks proportionally more light, making it easier to study.
Gliese 1132b is interesting not just because it’s the first low-mass super-Earth to have a detectable atmosphere. It’s near Earth, putting it in the sights of existing telescopes; and it was also thought to be about 1.2 times the Earth’s radius, making it very close in size — and potentially composition — to our planetary home.
For this paper, a team of European astronomers used the 2.2-meter ESO/MPG telescope in Chile to track nine of the planet’s transits in front of its star. They studied the starlight in seven different bands of light across optical and infrared wavelengths.
As a planet passes in front of the star, it blocks a certain amount of starlight across all seven bands. But the small amount of starlight that passes through the atmosphere will be selectively filtered: Certain chemicals in the atmosphere will absorb (and thus, block) certain wavelengths while allowing others to pass straight through. The missing wavelengths can tell scientists which atoms and molecules are present.
The astronomers found that, based on the amount of light blocked, the silhouetted planet has a radius about 1.4 times that of Earth, making it a little bigger than previously thought.
GJ 1132b also appears larger in one of the infrared wavelength bands than it does in the rest — signaling the presence of an atmosphere that is transparent to some wavelengths but opaque to others. The researchers then modeled different scenarios, finding that the atmosphere could potentially be rich in water and methane.
If that’s true, then it means the planet could have a steamy atmosphere and perhaps a magma ocean, said Schaefer, who cautioned that many more follow-up observations need to be done to ensure the vapor-filled atmosphere really is there.
“It’s not confirmed that it’s water,” she pointed out. “So it’s very exciting but we definitely need more data on it.”
Because M dwarfs are so abundant throughout the galaxy, and habitable-zone planets circling them are easier to find, many scientists think these dim stars may offer one of the best chances for finding a life-friendly world. But these red dwarfs are also thought to be much more active than main-sequence stars like the sun — with flares, eruptions and other activity that could blow a planet’s atmosphere away.
Keep in mind, Gliese 1132b circles its star so closely that its “year” lasts just 1.6 Earth days. So if the planet does turn out to have an atmosphere in spite of that proximity, it would be good news for astronomers — because it would mean that M dwarfs are more stable places for habitable planets than thought.
For now, researchers are somewhat limited in what they can probe with existing ground and space telescopes. But with the launch of NASA’s Transiting Exoplanet Survey Satellite (TESS) later this year and the James Webb Space Telescope in 2018, researchers may soon be able to find more interesting nearby targets and then probe their atmospheric composition with precision.