TRAPPIST-1 e, an Earth-sized exoplanet 40 light years away, may have an atmosphere that could support having liquid water on the planet's surface in the form of a global ocean or icy surface, according to new research by an international collaboration including Cornell astronomers.
"TRAPPIST-1 is a very different star from our Sun, and so the planetary system around it is also very different, which challenges both our observational and theoretical assumptions," said Nikole Lewis, associate professor of astronomy in the College of Arts and Sciences, who is PI of the program.
The team's initial results indicating the possibility of an atmosphere on TRAPPIST-1 e, and a paper describing the theory behind the findings, published Sept. 11 in the Astrophysical Journal Letters. Elijah Mullens, a doctoral candidate in astronomy, and Ryan Challener, postdoctoral researcher at the Cornell Center for Astrophysics and Planetary Science (A&S), are co-authors.
Seven Earth-sized worlds orbit the red dwarf star TRAPPIST-1. Of those, planet e is of particular interest because it orbits the star in the habitable zone, a distance from the star where liquid surface water is theoretically possible. Researchers aimed NASA's James Webb Space Telescope's powerful near-infrared spectrograph (NIRSpec) instrument at the system as planet e transited, or passed in front of, TRAPPIST-1.
The team analyzed four transits, and found evidence that planet e no longer has its primary, or original, atmosphere. That indicates planet e it could in theory have developed a secondary atmosphere that in turn could create the conditions for liquid water on the surface of the planet.
Their analysis showed that starlight passing through the planet's atmosphere, if there is one, would have been partially absorbed by the atmosphere and the corresponding dips in the light spectrum that reaches the telescope would tell astronomers what chemicals are found there. With each additional transit, the atmospheric contents become clearer. For now, multiple possibilities remain open for planet e's atmosphere.
TRAPPIST-1 is a very active star, with frequent flares, so researchers say it is not surprising that any hydrogen-helium atmosphere with which the planet may have formed would have been stripped off by stellar radiation.
"But after losing their primary atmosphere, many planets - including Earth - build up a heavier secondary atmosphere. Given our preliminary observations, we feel there's a chance that planet e was able to do this," said Lewis.
That's important, she noted, because a secondary atmosphere is heavier than a primary atmosphere and so would be more likely to keep water on the surface.
If there is liquid water on TRAPPIST-1 e, the researchers say it would be accompanied by a greenhouse effect like the one on Earth, in which various gases, particularly carbon dioxide, keep the atmosphere stable and the planet warm.
"A little greenhouse effect goes a long way," said Lewis. "Although the measurements do not provide a detection of carbon dioxide in TRAPPIST-1 e's atmosphere, they do not entirely rule out adequate carbon dioxide to sustain some amount of liquid water on the surface."
According to the team's analysis, the liquid water could take the form of a global ocean, or cover a smaller area of the planet where the star is at perpetual noon, surrounded by ice. This would be possible because, due to TRAPPIST-1 planets' sizes and close orbits to their star, they all are tidally locked, with one side always facing the star and one side always in darkness.
Webb is currently making 15 additional observations of planet e, timed so that the telescope catches both planets b and e transiting the star one right after the other. Abby Boehm, a doctoral candidate in the field of astronomy, is on the team processing this data.
"We're fairly confident that planet b is a bare rock without an atmosphere," said Lewis, "which means any elements detected during planet b's transit can be attributed to the star only. This gives us a way to compare data from both planets and determine what in planet e's spectrum can be attributed to its atmosphere. It will be exciting to see the results of these new observations."
Linda B. Glaser is news and media relations manager for the College of Arts and Sciences.
