TRAPPIST-1 e has an atmosphere like Venus, is just 40 light years away, and may be home to life

The James Webb Space Telescope has started to peel back the layers of a small, rocky planet 40 light years away, and while the case isn’t closed, the clues are piling up.

TRAPPIST-1 e, one of seven Earth-sized planets circling a cool red dwarf, sits in that just-right orbital zone where liquid water could exist – if the planet has the right kind of atmosphere.

Early Webb data suggest what it doesn’t have, hints at what it might have, and lays out a clever plan to find out for sure.

Why this planet matters

Among the TRAPPIST-1 septet, planet e has always been the headliner. It receives roughly Earth-like starlight and, on paper, could keep surface water stable.

But “could” does a lot of work there. Without an atmosphere to trap heat, a world at this distance would be a frozen cue ball.

With the wrong atmosphere – too thick, too thin, or made of the wrong stuff – it could be hostile in other ways. That’s where Webb comes in.

Faint signals from TRAPPIST-1 e

Using the telescope’s Near-Infrared Spectrograph (NIRSpec), researchers watch TRAPPIST-1 e as it passes in front of its star.

If the planet has air, starlight filters through that thin shell and certain wavelengths are absorbed by atmospheric molecules.

Those missing wavelengths show up as telltale dips in the spectrum – chemical fingerprints of whatever’s floating above the surface. Each transit adds another layer of signal, gradually turning a faint hint into a readable signature.

“Webb’s infrared instruments are giving us more detail than we’ve ever had access to before,” said Néstor Espinoza from the Space Telescope Science Institute.

“The initial four observations we’ve been able to make of planet e are showing us what we will have to work with when the rest of the information comes in.”

An atmosphere on TRAPPIST-1 e

Even with just four transits in hand, scientists feel confident ruling out a bloated, hydrogen-helium “primary atmosphere,” the kind many planets are born with.

TRAPPIST-1 is a feisty star; frequent flares and high-energy radiation would have stripped that lightweight envelope long ago.

That leaves two broad possibilities: TRAPPIST-1 e either rebuilt a heavier “secondary” atmosphere – like Earth’s – or it never managed to, and the planet is a bare rock.

Hints of liquid water persist

What about a carbon dioxide-dominated blanket like Venus’s thick haze or Mars’s thin veil? The team’s initial analysis suggests that’s unlikely, at least in the simple, solar-system-style versions of those atmospheres.

“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, a astronomer at Cornell University. In other words, don’t expect one-to-one analogies – but do expect surprises.

One of those surprises could be water. If TRAPPIST-1 e retains some carbon dioxide along with a mix of other greenhouse gases, even modest amounts might keep parts of the surface warm enough for liquid water.

“A little greenhouse effect goes a long way,” Lewis noted. TRAPPIST-1 e almost certainly keeps one face perpetually turned toward its star through tidal locking. In that setup, water could collect in a permanent dayside ocean.

It might also form a smaller band of liquid around the substellar point, with ice covering the rest. For now, all of those scenarios remain on the table with the current data.

Webb cancels stellar noise

Red dwarfs are notorious for their “mood swings,” and stellar variability can masquerade as planetary signals.

To cut through that noise, Espinoza and co-principal investigator Natalie Allen devised an observing sequence that grabs back-to-back transits of two planets: b and e.

Planet b, the innermost world, looks like a bare rock without an atmosphere based on earlier Webb results.

That makes b an excellent “stellar baseline” – any spectral wiggles seen during b’s transit are chalked up to the star, not the planet.

With e’s transit following immediately after, the team can subtract the star’s characteristics in nearly real time. What’s left over – features that appear only during e’s transit – would be the long-sought fingerprints of e’s air.

The ongoing program will collect 15 more observations using this tandem approach, dramatically boosting sensitivity to faint atmospheric signals.

What we can (and can’t) say yet

With four transits analyzed, the team can exclude some scenarios and keep others in contention. A light, primordial hydrogen-helium envelope is out. A simple, Venus- or Mars-style carbon dioxide-dominated atmosphere looks unlikely.

Several flavors of secondary atmospheres – richer, heavier, and potentially capable of supporting surface water – remain viable. The next wave of data will be crucial for distinguishing between them.

“We are really still in the early stages of learning what kind of amazing science we can do with Webb,” said Ana Glidden of MIT’s Kavli Institute, who led the analysis of potential atmospheres for planet e.

“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.”

TRAPPIST-1 e lessons learned

TRAPPIST-1 e is a proving ground for studying small, rocky planets around active red dwarfs – by far the most common type of star in the galaxy.

The techniques honed here, especially the back-to-back transit method, will carry over to other systems.

And every atmosphere (or lack thereof) we find reshapes our understanding of where habitable environments can survive in the face of stellar tantrums.

The study is published in the Astrophysics Journal Letters.

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