TRAPPIST-1 is a very faint, very cool red dwarf star only 40 light-years from Earth. It’s so faint it wasn’t even discovered until 1999. It’s a tiny star, hardly bigger than Jupiter!

Despite this it hosts at least seven planets, all very roughly Earth-sized (and they’re named in order outwards from the star, so TRAPPIST-1b is closest in, c is next farther out, and so on). They huddle close in to the star; the entire system could easily fit inside Mercury’s orbit around the Sun with a lot of room to spare.

But the star is so cool that only three of the planets orbit in the habitable zone, where temperatures are in the right range to allow liquid water to exist on the surface. This depends on how you define that zone, and it also depends on things like how reflective a given planet is; a dark surface will absorb more light and be warmer. It also depends on the planet’s atmosphere; the greenhouse effect can be strong enough to take a planet that would otherwise be frozen solid and warm it into clemency. Earth is like that — without the carbon dioxide in our air the temperature would be well below freezing everywhere.

It’s hard to know if a planet has an atmosphere or not. But nature has lent us a hand here: the planets we know of around TRAPPIST-1 orbit it in such a way that they all transit the star, passing directly in front of it from our point of view once per orbit (think of it as seeing their orbits edge-on). That is how they were discovered in the first place — when they are in front of the star they block a little bit of its light, and that dip can be detected. We can determine the sizes of the planets that way, as well as their orbital periods (their years).

But it gets better: if the planet has an atmosphere, different atoms and molecules in the air will absorb very specific colors of the star’s light. If we use a spectrograph during a planetary transit to break up the incoming starlight into its component wavelengths (that is, colors), we can look for dips in the spectrum where it gets darker, measure those wavelengths, and identify the corresponding molecules in the planetary atmosphere. This has been done many times; it’s extremely difficult and delicate work, but possible. It was done with JWST for TRAPPIST1-b, for example, a hot world that astronomers unsurprisingly found to be airless.

Astronomers did just this with TRAPPIST-1d, the third planet out from the star [link to journal paper] and for TRAPPIST-1e, the fourth planet out [link to journal paper 1 and paper 2].

First let’s look at the inner planet, 1d. It’s about 3.3 million kilometers away from the star, on the inside edge of the habitable zone, kinda (again, depending on how you define that zone; it’s a bit on the warm side). It’s about 80% the diameter of Earth but less dense, so its surface gravity is only about 60% of what we feel here (in other words, you’d weigh 60% of your Earth weight on planet d).

That’s close enough to our own world’s numbers that we’d like to know if it has any air! The team observed planet d in transit with JWST, which sees infrared light, and which is where a lot of interesting molecules absorb that light — like water, carbon monoxide and dioxide, methane, and a few others.

What they found is… nothing. And that’s interesting! The spectrum was pretty much flat, with no dips they could see. It could be the planet has a thin enough atmosphere (like Mars) that they just couldn’t detect it, or it could simply be airless. That latter strikes me as the most likely possibility. But they can’t rule out other scenarios; it’s possible the planet is covered in water and has thin clouds at high altitude that could obscure any spectral features.

If it is airless that’s not too surprising; it’s a small world after all. Because the surface gravity is lower than Earth’s it could lose its atmosphere more easily. Red dwarfs like TRAPPIST-1 blast out lots of stellar flares and such, which can, over billions of years, strip a planet of its air.

What about the planet 1e? It’s about 4.4 million km out from the star, and is 0.92 times Earth’s diameter, and a surface gravity about 80% of Earth’s — more Earth-like than 1d. Also, its temperature is more likely to be like Earth’s as well, so we’d love to know more about it.

Unfortunately, what the astronomers found looking at it with JWST was… ambiguous. They’re pretty sure that it doesn’t have an atmosphere made of hydrogen and helium — what’s called the primary atmosphere, the one that formed originally after the planet did. Smaller planets can lose that atmosphere pretty easily, since they don’t have enough gravity to hold on to those lighter elements, unlike, say Jupiter.

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