A small, cool star 40 light-years away has a planet that just might hold on to an atmosphere. Astronomers used the James Webb Space Telescope (JWST) to watch that world, TRAPPIST-1e, pass in front of its star four times and extract the planet’s faint atmospheric fingerprint.
A new paper reports those observations and what they can – and cannot – show about the gases above this rocky surface.
The team can already reject a thick, hydrogen-dominated atmosphere, but the data are not yet strong enough to prove what is there on the planet instead.
Néstor Espinoza from the Space Telescope Science Institute (STScI) led the analysis with the JWST Telescope Scientist Team.
Star activity masks TRAPPIST-1e
TRAPPIST-1 is a red dwarf star, a compact and relatively cool star type that often flares and develops spots in ways that upset precise measurements.
That activity complicates the search for air around its planets because changes from the star can masquerade as signs from a planet’s atmosphere.
TRAPPIST-1e sits in the star’s habitable zone – the region where a planet could support liquid water on its surface if pressure and temperature are suitable.
That alone does not make it hospitable, since the star’s outbursts could strip or scramble any atmosphere over time.
Earlier Webb data on the inner siblings, such as TRAPPIST-1b, found little evidence of a substantial atmosphere.
That result keeps expectations in check and places more weight on careful, repeated measurements for the middle worlds.
NASA shared a technical feature explaining that the first four TRAPPIST-1e observations are not enough to confirm an atmosphere.
The figure shows that models with and without an atmosphere still overlap in the range allowed by today’s data.
What the telescope actually measured
TRAPPIST-1e is an exoplanet, a planet beyond our solar system. When it crosses the star, a tiny bit of starlight filters through any air at the planet’s edge, and that light carries molecular fingerprints.
This method is called transmission spectroscopy, and it spreads the light into wavelengths to see where particular gases absorb.
Webb’s NIRSpec instrument recorded four such transits in 2023, covering wavelengths where gases like water vapor, methane, and carbon dioxide could show up clearly.
Because the star itself has features that can change over minutes to hours, the team built models to separate stellar effects from any possible planetary signals.
They also combined the four events to reduce random noise while being careful not to blur out real, repeatable features.
No primary atmosphere detected
A hydrogen-rich atmosphere on the planet would have produced large, easily seen features in the spectrum.
The team reports that such primary atmospheres – dominated by hydrogen at levels above roughly 80 percent by volume – are inconsistent with the data.
Ana Glidden is a postdoctoral researcher in MIT’s Department of Earth, Atmospheric and Planetary Sciences and the MIT Kavli Institute for Astrophysics and Space Research.
“The idea is: If we assume that the planet is not airless, can we constrain different atmospheric scenarios? Do those scenarios still allow for liquid water at the surface?” she said.
Glidden noted that the results rule out a hydrogen-dominated atmosphere and tighten limits on several secondary-atmosphere scenarios.
A warm, nitrogen-rich mix remains possible, along with other secondary atmospheres that could arise from volcanic outgassing.
Those would appear much subtler in the data than hydrogen and require more than four transits to emerge from the noise.
Stellar flares complicate tests
TRAPPIST-1’s surface has star spots and frequent flares that change its spectrum over time.
That “stellar contamination” can either add or remove features in the planetary spectrum and, if not modeled correctly, can be misleading.
To tackle that, the authors used Gaussian processes, a flexible statistical tool that learns the typical shape and size of the star’s variations and then subtracts them.
That approach does not assume a perfect star model, which is wise when the star is as active as this one.
The goal is to treat any wiggles that change from one visit to the next as coming from the star, and any persistent pattern as likely belonging to the planet. Webb’s stability helps here, but the source star’s behavior still sets the pace.
TRAPPIST-1e air circulation
A nitrogen-rich atmosphere would resemble Earth in broad strokes, though details such as oxygen, water vapor, and methane would matter greatly.
Nitrogen alone does not prove anything about life on the planet, but it suggests a heavier, secondary atmosphere capable of maintaining surface pressure.
“We are seeing two possible explanations,” said Ryan MacDonald, Lecturer in Extrasolar Planets in the School of Physics and Astronomy at the University of St Andrews.
“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.”
Independent of biology, an atmosphere changes surface temperature and how heat moves from day to night. On a tidally locked world, that could be the difference between an ice cap and a stable ocean.
Webb and TRAPPIST-1e
The team plans many more transits to shrink error bars and test for specific gases. More visits let the statistics work, and they also average over the star’s mood swings.
“In the coming years we will go from four JWST observations of TRAPPIST-1e to nearly twenty,” said MacDonald.
“We finally have the telescope and tools to search for habitable conditions in other star systems, which makes today one of the most exciting times for astronomy.”
As the dataset grows, scientists can check for methane and carbon dioxide together and run climate models that test whether liquid water could persist at the surface.
Caution remains the watchword until a clear atmospheric pattern repeats across many passes.
The study is published in The Astrophysical Journal Letters.
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