Is a gossamer shroud of gas, 40 light-years from our planet, the key to life outside our planet? Researchers examining TRAPPIST-1e, an Earth-sized planet around the midpoint of its star’s habitable zone, now suspect that it has a nitrogen-rich atmosphere a characteristic shared by only our planet and Saturn’s moon Titan in our own cosmic backyard.
The finding comes from four meticulously planned observations with NASA’s James Webb Space Telescope (JWST), which used transit spectroscopy to probe the planet’s atmospheric makeup. In this technique, starlight passing through a planet’s atmosphere during a transit is split into its component wavelengths, revealing the “fingerprints” of specific gases. As Ryan MacDonald of the University of St Andrews explained, “If we see no variation in color, then the planet is probably just a bare rock.” In TRAPPIST-1e’s case, the data ruled out a hydrogen-rich envelope and made a carbon dioxide–dominated atmosphere unlikely, leaving nitrogen as a compelling possibility.
It is not an easy task to detect nitrogen. Unlike molecules like methane or carbon dioxide, the spectral fingerprint of nitrogen is weak, and high signal-to-noise ratio and sophisticated calibration are needed to extract planetary signals from stellar “noise.” TRAPPIST-1, a short-period, flare and starspot active red dwarf, adds to the challenge with frequent starspots and flares that can simulate or mask atmospheric signals. Scientists took more than a year scrubbing away at these contaminating effects, which took the form of making comparisons among many transits and apportioning up signals consistent between observations. MIT’s Ana Glidden put it this way: “any signal you can see varying visit-to-visit is most likely from the star, while anything that’s consistent between the visits is most likely the planet.”
The identification of nitrogen has significant implications for habitability. In the terrestrial environment, nitrogen composes most of the atmosphere, contributing pressure balance and serving as a buffer gas controlling greenhouse effects. Titan’s methane-enriched, frigid nitrogen-encrusted cloak, postulated to be draped over a hidden ocean. The uniqueness of nitrogen-rich atmospheres in the solar system, limited to these two worlds, implies their genesis to require a subtle interplay between volcanic outgassing, atmosphere preservation, and surface chemistry. Plate tectonics and the carbon cycle removed the carbon dioxide from Earth, but photochemical degradation of ammonia or delivery by comets might have been in the way on Titan.
For TRAPPIST-1e, astronomers speculate a “secondary atmosphere” one formed after the planet, through volcanic degassing or comet impacts with heavier molecules such as nitrogen. This type of environment would be able to retain enough heat to maintain liquid water, even around the reduced light of a red dwarf. Interior modeling of TRAPPIST-1e, in combination with its well-constrained mass and radius, indicates that it may harbor enormous stores of water, either on the surface or in the form of subsurface oceans sustained by tidal and radiogenic heating.
NIRSpec/PRISM on the JWST, with its wide wavelength range and high resolution, has played a crucial role in further narrowing possibilities. By seeing how various colors of light are being absorbed during transits, astronomers are able to rule out some atmospheric compositions with greater confidence. Future measurements 15 more are already on the schedule will further limit these boundaries, conceivably even finding trace greenhouse gases like methane or carbon dioxide, which with nitrogen might indicate a steady, temperate climate.
The technology challenge is huge. Transit spectroscopy of small rocky planets requires exquisite timing and sensitivity, since their atmospheres absorb less than 1 percent of light from their star. High-resolution spectroscopy, with the ability to see individual molecular lines, can further enhance detection by separating planetary signals from telluric and stellar features. Methods developed on TRAPPIST-1e will underpin studies of other temperate exoplanets, particularly those orbiting the near-by M dwarfs, where planet-starry contrast is enhanced.
If nitrogen is confirmed, TRAPPIST-1e would become the first known rocky planet in another star’s habitable zone with a detected atmosphere a milestone in exoplanet science. As Caroline Piaulet-Ghorayeb of the University of Chicago observed, “This is an exciting step and it really helps us narrow down the possibilities of an atmosphere that is perhaps more Earth-like.” The next few years, as JWST and future observatories continue their watch, may reveal whether this distant world’s thin blue line is just the beginning of a deeper story about life in the cosmos.
