TRAPPIST-1c Has No Thick Atmosphere Surprising Scientists

The First Close Look

When the James Webb Space Telescope turned its gold mirrors toward a dim red star forty light years away, astronomers were ready for nearly anything. A thick Venus-like haze. A thin wisp of carbon dioxide. A bare, airless rock baked by its sun.

What they were not ready for was how quickly the answer arrived.

The planet TRAPPIST-1c, a rocky world roughly the same size as Earth, orbits its star every 6.7 days. Its dayside temperature, measured directly by JWST at 15 microns, came back at 380 Kelvin, plus or minus 31 degrees. That is hot. Hot enough to rule out a thick, carbon dioxide rich atmosphere. Hot enough to suggest that this planet, so similar to Earth in size and mass, is almost certainly a bare rock.

The paper, published in Nature by Sebastian Zieba, Laura Kreidberg, Elsa Ducrot, and Michael Gillon (Zieba et al., 2023), is the second such result from the TRAPPIST-1 system. The innermost planet, TRAPPIST-1b, already showed no signs of an atmosphere. Now the next one in line appears similarly naked.

This is not what anyone expected.

Why This Star System Matters More Than Any Other

The TRAPPIST-1 system is a planetary laboratory unlike anything else in the galaxy. Seven rocky planets, all roughly Earth sized, orbit a single ultracool dwarf star. Three of them sit in the habitable zone, where liquid water could exist on the surface. The planets are close enough to study with JWST, and their host star is small and dim enough that the telescope can actually detect their thermal emission.

Before JWST launched, astronomers had models. They had simulations. They had educated guesses about what these planets might look like. The most popular scenario involved thick carbon dioxide atmospheres, similar to Venus but perhaps even denser. The logic was simple: small planets orbiting active stars should lose their hydrogen and helium quickly, but heavier molecules like CO2 should stick around. A CO2 atmosphere would trap heat, warm the surface, and potentially create a runaway greenhouse effect.

TRAPPIST-1c was the perfect test case. It receives about the same amount of stellar radiation as Venus does from our Sun. If any planet in the system had a thick atmosphere, this one seemed like a safe bet.

JWST proved that bet wrong.

What JWST Actually Saw

The measurement itself is a technical achievement worth understanding. JWST did not take a picture of TRAPPIST-1c. It measured the combined light of the star and planet, then subtracted the star's contribution. The leftover signal, 421 parts per million of the star's brightness, came from the planet's dayside thermal emission.

That number, 421 plus or minus 94 parts per million, corresponds to a dayside brightness temperature of 380 Kelvin. For context, that is about 107 degrees Celsius, or 224 degrees Fahrenheit. Hot enough to melt lead. Hot enough to rule out any significant greenhouse effect.

The authors ran models to see what kinds of atmospheres could produce that temperature. They tested cloud free mixtures of oxygen and carbon dioxide at various pressures. They tested a Venus analog with sulfuric acid clouds. They tested thin atmospheres and bare rock surfaces.

The results were unambiguous. A thick CO2 atmosphere, even at just 0.1 bar of pure CO2, would have produced a lower dayside temperature because the atmosphere would redistribute heat to the nightside. The data rule out cloud free O2/CO2 mixtures with surface pressures from 10 bar down to 0.1 bar. The Venus analog failed at 2.6 sigma confidence, meaning there is only about a 1 in 100 chance that the data are consistent with such an atmosphere.

The simplest explanation is that TRAPPIST-1c has no thick atmosphere at all. It is a bare rock, like Mercury, but hotter.

The Numbers That Matter

The abstract gives specific constraints that are worth understanding:

• ▸Surface pressures above 0.1 bar of pure CO2 are ruled out
• ▸Mixtures with as little as 10 ppm CO2 at 10 bar are also ruled out
• ▸A Venus-like atmosphere with sulfuric acid clouds is disfavored at 2.6 sigma
• ▸The measured dayside brightness temperature of 380 +/- 31 K is consistent with a bare rock

These are not subtle effects. The authors did not need to squint at the data or apply complex statistical corrections. The signal was clear enough that the simplest interpretation wins.

What this means in practical terms: if TRAPPIST-1c ever had a thick atmosphere, it lost it. The planet is either geologically dead, or its volatiles were stripped away by the star's radiation and stellar wind.

The Volatile Budget Problem

Here is where the story gets interesting for the rest of the system.

The authors estimate that TRAPPIST-1c has less than 9.5 Earth oceans of water, with an uncertainty range of minus 2.3 to plus 7.5 Earth oceans. That might sound like a lot, but it is actually quite low for a planet of this size. Earth has about one ocean of surface water, but Earth also has significant water in its mantle. Venus probably had oceans early in its history. Mars had lakes and rivers.

TRAPPIST-1c appears to be dry.

If all seven planets in the system formed from the same disk of material, and if they all experienced similar volatile delivery and loss processes, then the habitable zone planets might also be dry. That would be bad news for anyone hoping to find life in this system.

But there is a catch.

The Formation History Puzzle

The authors suggest that TRAPPIST-1c's lack of atmosphere points to a volatile poor formation history. The planet probably formed closer to the star, where temperatures were too high for water ice to condense. It may have accreted from dry material, or it may have lost its volatiles early on due to the star's intense radiation.

This is consistent with what we see in our own solar system. The inner planets Mercury, Venus, Earth, and Mars all have different volatile budgets. Mercury is essentially a metal ball with a thin exosphere. Venus has a thick atmosphere but almost no water. Earth has oceans and a nitrogen oxygen atmosphere. Mars has a thin carbon dioxide atmosphere and frozen water at its poles.

The diversity in our own system suggests that volatile delivery and retention are complex processes. A planet's final atmosphere depends on its formation location, its accretion history, its geological activity, its magnetic field, and the evolution of its host star.

TRAPPIST-1c is just one data point. But it is a data point that challenges our models.

What This Does Not Prove

It is important to be precise about what this paper does and does not show.

The paper shows that TRAPPIST-1c does not have a thick CO2 atmosphere. It does not show that the planet has no atmosphere at all. Thin atmospheres, with surface pressures below 0.1 bar, are still possible. A thin atmosphere of nitrogen, oxygen, or even water vapor could exist without producing a detectable signal in this measurement.

The paper also does not show that the other planets in the system are bare rocks. TRAPPIST-1b, the innermost planet, also appears to be airless based on earlier JWST observations. But planets further out, including the three in the habitable zone, could still have atmospheres. They receive less stellar radiation, which means atmospheric escape is slower. They may have formed with different volatile budgets.

The paper does not rule out life. Life as we know it requires liquid water, which requires an atmosphere to maintain surface pressure and temperature. If the habitable zone planets are bare rocks, they cannot support life. But if they have thin atmospheres, or if they have different compositions, the door remains open.

The paper does show that our models of planet formation and atmospheric retention need revision. We cannot simply assume that small planets orbiting cool stars will retain thick CO2 atmospheres. The reality is more complicated.

How This Changes the Search for Life

The TRAPPIST-1 system was supposed to be the best place to search for biosignatures in the coming decade. Seven rocky planets, three in the habitable zone, all close enough to study with JWST and future telescopes like the Extremely Large Telescope and the Habitable Worlds Observatory.

If the habitable zone planets are also bare rocks, that search becomes much harder. Bare rocks do not produce biosignatures. They do not have clouds, weather, or surface liquid water. They are geologically dead.

But if the habitable zone planets have thin atmospheres, or if they have different compositions than their inner siblings, the search becomes more interesting. We would need to understand why some planets retained atmospheres and others did not. That would tell us something fundamental about planet formation and the conditions necessary for habitability.

The authors are careful not to overinterpret their results. They note that if all planets in the system formed the same way, the volatile reservoir would be limited. But they also acknowledge that the planets could have formed differently, or that volatile delivery could have been uneven.

The next step is obvious: point JWST at the habitable zone planets and measure their thermal emission. If they are also hot and bare, the system is likely dead. If they show signs of cooler temperatures or atmospheric absorption, the search for life continues.

The Broader Context

This result fits into a larger pattern that is emerging from JWST's first year of exoplanet observations. The telescope has already shown that TRAPPIST-1b is a bare rock. It has shown that the hot Jupiter WASP-39b has a complex atmosphere with sulfur dioxide. It has shown that the super Earth LHS 475b is probably airless.

The pattern so far: small planets around M dwarfs tend to lose their atmospheres. Large planets around Sun like stars tend to keep them.

This makes sense from a physical perspective. M dwarfs are active stars that produce high levels of X ray and ultraviolet radiation. Their stellar winds are strong. Planets in close orbits are bombarded by particles and photons that can strip away atmospheres over time.

But the pattern is not universal. Some M dwarf planets might retain atmospheres if they have strong magnetic fields, or if they formed with massive volatile inventories, or if they are geologically active enough to outgas new atmospheres.

We are still in the early stages of understanding this diversity. JWST has only observed a handful of small planets. The sample size will grow over the next few years as more observations are completed and analyzed.

What This Actually Means

• ▸TRAPPIST-1c has no thick CO2 atmosphere. The dayside temperature is 380 K, too hot for a greenhouse effect. This rules out the most commonly assumed atmospheric scenario for this planet.

• ▸The planet is probably a bare rock. Thin atmospheres below 0.1 bar are still possible, but the simplest explanation is that the surface is exposed to space.

• ▸The volatile budget of the system may be limited. TRAPPIST-1c has less than 9.5 Earth oceans of water. If the other planets formed similarly, they may also be dry.

• ▸The habitable zone planets are not ruled out yet. They could have different formation histories, different volatile inventories, or different atmospheric retention mechanisms. But the trend is worrying.

• ▸JWST can do this measurement for other planets. The technique is proven. The telescope works. We will know within a few years whether any TRAPPIST-1 planet has a detectable atmosphere.

• ▸Our models of planet formation need updating. The assumption that small planets around cool stars retain thick CO2 atmospheres is not supported by the data. Something is missing from our understanding of how these systems form and evolve.