Reading the Air of Other Worlds

Imagine holding a glass of wine up to a candle and reading the label on the bottle behind it. That’s roughly what astronomers are doing when they study exoplanet atmospheres. They hold a distant world up against the blinding glare of its star, and from the faintest flicker of light, they read what that world is made of.

For decades, this technique worked only on giant planets — Jupiter-sized worlds with thick, gaseous envelopes that block enough starlight to be detectable. Rocky planets, the ones small enough to be interesting from a life-and-habitability standpoint, were simply too faint, too thin-atmosphered, too impossibly subtle.

The James Webb Space Telescope changed that.

In the past two years, JWST has done something that was genuinely not possible before: it has measured the thermal emission — essentially, the heat signature — of individual rocky exoplanets. It has begun to tell us whether those worlds have air. The results have been electrifying. Some worlds appear to be bare rocks, stripped naked by their stars. And then there’s one world — close, hot, and covered in magma — that appears to have held onto a real atmosphere after all.

We are, for the first time, reading the air of other worlds.

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The Most Studied Star System You’ve Never Heard Of

In 2017, a Belgian astronomer named Michaël Gillon and a team of colleagues made an announcement that briefly broke the internet. Around a dim, cold red dwarf star called TRAPPIST-1, lying 40 light-years away in the constellation Aquarius, they had found seven rocky planets. Not one or two — seven. All of them similar in size and mass to Earth. Three of them orbiting in the star’s habitable zone, where liquid water might pool on a surface.

TRAPPIST-1 immediately became the most scrutinized planetary system in history outside our own. The discovery paper in Nature has been cited more than 1,500 times. Space agencies scheduled follow-up observations. Theorists ran thousands of computer simulations. A single question obsessed everyone who looked at this system: do any of those seven worlds have an atmosphere?

The reason this matters is simple. An atmosphere is not just air to breathe. It’s a heat blanket, a radiation shield, a chemical factory, a prerequisite — as far as we know — for any liquid water to persist at the surface. No atmosphere means no weather, no oceans, no life as we understand it. Everything interesting about a planet, astrobiologically speaking, runs through the presence or absence of an atmosphere.

For years, nobody could answer the question for TRAPPIST-1. The Hubble Space Telescope tried and came up empty — its instruments weren’t sensitive enough. Spitzer tried. Ground-based telescopes tried. Nothing. The planets were just too small, too dim, too far.

Then JWST launched on Christmas Day 2021, and everything changed.

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TRAPPIST-1 b: A World Without Air

The first JWST result on a TRAPPIST-1 planet came in March 2023, and it was both a triumph and a disappointment.

Tom Greene at NASA Ames Research Center led a team that pointed JWST’s mid-infrared camera at TRAPPIST-1 b — the innermost of the seven planets — and measured its secondary eclipse. A secondary eclipse is what happens when a planet passes behind its star. During those moments, the planet’s own light disappears from the signal, and by comparing the before-and-after brightness, astronomers can calculate exactly how much infrared light the planet was emitting from its dayside.

What they found was striking. TRAPPIST-1 b is emitting exactly the amount of heat you’d expect from a bare rock with no atmosphere at all. If the planet had any significant atmosphere — even a thin one — heat would circulate from the sunlit side to the dark side, cooling the dayside down and warming the nightside up. The dayside would appear cooler than expected. Instead, it appeared at full roasting temperature: all the absorbed starlight radiating back from a single, unshielded hemisphere.

The most straightforward interpretation, Greene and colleagues wrote in Nature in 2023, is that there is “little or no planetary atmosphere redistributing radiation.” The planet gets four times as much radiation from its star as Earth gets from the Sun. Whatever atmosphere it might once have had appears to have been stripped away long ago.

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TRAPPIST-1 c: The CO₂ That Wasn’t There

Three months later, in June 2023, a second team trained JWST on the next planet outward — TRAPPIST-1 c. This one was potentially more interesting. TRAPPIST-1 c orbits just outside the inner edge of the system’s habitable zone, at a distance reminiscent of Venus. The question was: could it be a Venus-twin, shrouded in a thick carbon dioxide atmosphere?

The measurement came back with a sharp answer: no.

Sebastian Zieba at the Max Planck Institute for Astronomy led the team that measured TRAPPIST-1 c’s dayside temperature: 380 ± 31 Kelvin. That sounds hot, but it’s actually too hot for a thick CO₂ atmosphere. A Venus-like atmosphere would trap and redistribute heat differently. The data ruled out cloud-free CO₂/O₂ atmospheres across a wide range of pressures, and even disfavored a sulfuric-acid-cloud Venus analogue at 2.6σ confidence.

Thinner atmospheres, or a bare-rock surface, remain consistent. But thick, Venus-like? The data said no.

This result had an additional sobering implication. If TRAPPIST-1 c — which receives less radiation than TRAPPIST-1 b and should theoretically be better able to retain an atmosphere — shows no evidence of a thick CO₂ envelope, that’s a signal about the whole system. The authors noted that this “suggests a relatively volatile-poor formation history, with less than a fraction of Earth’s oceans of water.” If all seven planets formed from similarly volatile-poor material, the prospects for the putatively habitable ones — d, e, f, and g — become more challenging.

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The Challenge of M-Dwarf Planets

Why might the TRAPPIST-1 planets be atmospheric deserts? The answer likely has everything to do with their star.

TRAPPIST-1 is what astronomers call an ultra-cool M-dwarf. It’s only slightly larger than Jupiter, with about 9% of the Sun’s mass and a surface temperature of around 2,566 Kelvin — cool enough that its light peaks in the near-infrared, barely visible to human eyes. This might sound benign. But M-dwarfs are notoriously active, especially when young. They flare constantly, blasting out ultraviolet and X-ray radiation that can strip away planetary atmospheres through a process called photoionization. TRAPPIST-1 is estimated to be around 7.6 billion years old — older than our Solar System — which means its planets have been marinating in that radiation for a very long time.

There’s a deeper problem, too. The seven TRAPPIST-1 planets are so close to their star that they are almost certainly tidally locked: one hemisphere perpetually facing the star, the other in permanent darkness. An atmosphere can help distribute heat across this temperature extreme, but if the atmosphere gets stripped away in the first billion years of bombardment, there’s nothing left to protect the surface.

This doesn’t mean habitability is impossible around M-dwarfs. But it raises hard questions about whether these systems — so common in the galaxy, so observationally accessible — are actually good bets for life.

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A Different Kind of Rocky World

While astronomers were finding bare rocks in the TRAPPIST-1 system, another team was looking at a very different rocky world. In May 2024, Renyu Hu at NASA’s Jet Propulsion Laboratory and colleagues published a result in Nature that stopped the exoplanet community in its tracks.

The planet is called 55 Cancri e — or, to use its official name, Janssen. It orbits a star called Copernicus, 41 light-years away in Cancer, completing a full orbit in just 17.5 hours. It is 1.95 times Earth’s radius and 8.8 times Earth’s mass. Its surface temperature on the dayside exceeds 2,000 Kelvin — hot enough to melt iron, hot enough to keep rock liquid. It is, essentially, a world covered in magma.

And it appears to have an atmosphere.

The Hu team used both NIRCam and MIRI aboard JWST to collect 55 Cancri e’s thermal emission spectrum from 4 to 12 micrometers — a far more detailed measurement than the single-band eclipse photometry used on TRAPPIST-1. What they saw ruled out one scenario that many astronomers had considered likely: a tenuous “rock vapor” atmosphere, where the surface is so hot that it constantly evaporates, surrounding itself in a thin shroud of vaporized minerals. Such an atmosphere would produce a distinctive infrared signature.

The JWST spectrum didn’t match that. Instead, it was consistent with a real, substantial volatile atmosphere — probably rich in carbon dioxide or carbon monoxide. Not a primordial hydrogen-helium atmosphere (that would have been blown away long ago by the star’s radiation), but a secondary atmosphere: gases that have been continuously outgassed from the planet’s interior. From a magma ocean. Through volcanoes.

This is profound. It means that even on an extraordinarily hostile world — far hotter than anything in the habitable zone, tidally locked, with a star that has been bombarding it for billions of years — a secondary atmosphere can form and persist if the planet has enough geological activity to keep replenishing it. The mantle keeps churning. The magma keeps outgassing. The atmosphere keeps rebuilding.

It’s not habitable, not by any stretch. But it is alive, in a geophysical sense. And it changes how we think about atmospheres on rocky worlds.

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What This Tells Us About Earth

There’s a thread here that connects to our own planet’s history.

Earth’s original atmosphere — the one that condensed from the solar nebula — was mostly hydrogen and helium. It was swept away early in the Solar System’s history. What we breathe now is almost entirely a secondary atmosphere: gases that have been slowly released from Earth’s interior through billions of years of volcanism, modified by chemistry and life.

Carbon dioxide, nitrogen, water vapor — these are geological gases, shaped into something breathable over eons by photosynthesis and chemistry and carbon-silicate cycles. Life didn’t just evolve in an atmosphere; it built one.

The detection of a secondary atmosphere on 55 Cancri e is the first confirmation, in the wild, that other rocky worlds can build and maintain secondary atmospheres too. The planet is extreme, and the atmosphere is almost certainly not life-friendly. But the principle is demonstrated: rocky worlds can outgas. Rocky worlds can hold air.

That makes the habitable-zone planets of TRAPPIST-1 — the ones with moderate temperatures where water could be liquid — somewhat more hopeful in retrospect. The question is no longer whether rocky planets can have secondary atmospheres. The question is whether they can have them in the right conditions, long enough, for something interesting to happen.

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The Road Ahead

JWST has been observing the TRAPPIST-1 system extensively. Observations of planets d, e, f, and g — the potentially habitable ones — have been conducted and results are being analyzed. These are harder measurements than the inner planets: the planets emit less heat, the signal is smaller, the noise is larger. But the telescope is up to the task. Results are expected in the next year or two.

Astronomers are also refining the 55 Cancri e picture. A single detection is exciting; spectroscopic fingerprints of specific molecules would be revolutionary. Follow-up observations are searching for the precise atmospheric composition — to distinguish CO₂ from CO, to look for other gases, to understand what the magma ocean is outgassing and why.

The broader enterprise — characterizing rocky exoplanet atmospheres — is just getting started. JWST’s design lifetime is 20 years. The sample of planets accessible to this kind of study is growing. And in the 2030s, proposed next-generation observatories like the Habitable Worlds Observatory could push this characterization technique all the way to Earth-like planets in truly habitable orbits around Sun-like stars.

We are in the first chapter of a story that will take decades to tell. But the opening pages are already extraordinary. We have measured the heat of worlds light-years away. We have found bare rocks and magma oceans. We have glimpsed, for the first time, the air of another rocky planet.

Somewhere in the data streaming back from JWST, the answer to one of humanity’s oldest questions may be quietly hiding, waiting to be read.

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Key papers:

• Greene et al. (2023). “Thermal emission from the Earth-sized exoplanet TRAPPIST-1 b using JWST.” Nature 618, 39–42. DOI: 10.1038/s41586-023-05951-7
• Zieba et al. (2023). “No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c.” Nature 620, 746–749. DOI: 10.1038/s41586-023-06232-z
• Hu et al. (2024). “A secondary atmosphere on the rocky exoplanet 55 Cancri e.” Nature 630, 609–612. DOI: 10.1038/s41586-024-07432-x
• Gillon et al. (2017). “Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1.” Nature 542, 456–460. DOI: 10.1038/nature21360