Is the key to finding life beyond Earth within 40 light-years of us?
NASA’s James Webb Space Telescope is now giving its most detailed look yet at TRAPPIST‑1e, a Earth-sized rock-and-iron exoplanet orbiting within the habitable zone of its small, dim red dwarf star.
This is where surface temperatures would theoretically be able to sustain liquid water if only the planet has an atmosphere capable of trapping and redistributing heat. Determining whether or not such an atmosphere exists has become an astronomical priority, and Webb’s infrared is particularly well suited for the task.
The tool that is underutilized in this endeavor is Webb’s Near-Infrared Spectrograph (NIRSpec). When the planet passes in front of the star during a planetary transit, TRAPPIST‑1e passes in front of its star from our perspective, some of the starlight filters through any atmosphere that may be around the planet. Atmospheric molecules pick up on specific wavelengths, and they deposit characteristic “dips” in the spectrum chemical signatures that can reveal composition, pressure, and even climate hints. Transmission spectroscopy already has been proved out on bigger exoplanets but is pushing the technology with a small, temperate rocky world. Webb so far has captured four of these transits of TRAPPIST-1e, and initial findings are eliminating things.
Astronomers are certain that the world is no longer surrounded by its original hydrogen-helium atmosphere, the kind of light envelope it would have accumulated when it formed. The most plausible culprit is TRAPPIST-1 itself: the star is very active, emitting regular flares and bursts of high-level ultraviolet radiation capable of stripping lightweight gases into space. Planetary atmospheric loss is an established risk for worlds that orbit close to red dwarfs. The absence of a primary atmosphere does not preclude habitability.
All rocky planets, including our own planet, lose their initial gases and subsequently gain density secondary atmosphere by volcanic outgassing, comet impacts, or other geological processes. Webb observations cannot yet confirm whether TRAPPIST-1e has such a secondary atmosphere, but one possibility is a nitrogen-rich mixture, perhaps with remnants of methane or carbon dioxide. As University of St Andrews’ Ryan MacDonald explained, The most exciting possibility is that TRAPPIST‑1e could have a so‑called secondary atmosphere containing heavy gases like nitrogen. But our initial observations cannot yet rule out a bare rock with no atmosphere. But making the hunt more difficult is the star’s variability. Flares and starspots might produce spectral signatures that mimic or mask atmospheric activity. To overcome this, the Webb team is employing a comparative technique: observing TRAPPIST‑1b, a inner planet within the same system and one that is also known to be a rocky planet. Catching both planets in transit, one after the other, permits astronomers to subtract and isolate the spectral effect of the star, leaving only residual signals from TRAPPIST‑1e’s atmosphere.
There are high stakes here since the presence of an atmosphere would result in the likelihood of liquid water.
Depending on its composition and depth, TRAPPIST-1e could hold a world ocean, local seas at the permanent “day” side, or icy regions on its permanent night side an effect of tidal locking, in which one hemisphere is constantly facing the star. Even a mild greenhouse effect from gases like CO₂ would be enough to stabilize temperatures for water to persist. A little greenhouse effect goes a long way, said Nikole Lewis of Cornell University, noting that small carbon dioxide would be enough to keep surface water around. Infrared spectroscopy’s detection ability for these kinds of gases is based on spectral resolution and signal-to-noise ratio. Good resolution allows researchers to distinguish exoplanetary absorption lines from overlapping telluric (Earth atmospheric) ones, and multiple transits provide added statistical certainty. For TRAPPIST‑1e, this campaign will expand to nearly 20 observed transits from four, greatly increasing sensitivity to subtle atmospheric details.
Webb team technique draws on lessons of earlier triumph, such as the detection of carbon dioxide in WASP‑39b’s atmosphere, a hot gas world. That success demonstrated Webb’s ability to see faint spectral features across the 3–5.5 micron range, a capability now being pushed to cooler, smaller worlds. If TRAPPIST‑1e has nitrogen, methane, or other heavy molecules, their absorption bands should manifest as the dataset grows.
For now, the world is a secret either a desolate rock battered by stellar winds, or a world with an armor of atmosphere and hidden oceans.
As MIT’s Kavli Institute’s Ana Glidden put it, “It’s incredible to measure the details of starlight around Earth-sized planets 40 light-years away and learn what it might be like there, if life could be possible there. We’re in a new age of exploration that’s very exciting to be a part of.”
