For years, 55 Cancri e looked like a test of how thoroughly a nearby star could strip a rocky planet. It completes an orbit every 17.7 hours, close enough for the surface to be molten, and its original hydrogen and helium envelope was expected to have escaped long ago.
A new analysis of James Webb Space Telescope observations offers a more complicated picture: carbon monoxide high in the atmosphere, changing between observations, with atmospheric models favouring a hydrogen-rich composition linked to a reduced magma ocean. This is one study, not settled consensus. It is also a preprint submitted to Nature Astronomy, not a peer-reviewed result.
Webb measured the planet by waiting for it to disappear
Ignas Snellen of Leiden Observatory and colleagues reanalysed five Webb NIRCam observations from November 2022 and April 2023. Each visit covered a secondary eclipse, when the planet passed behind its star. By comparing the star-only signal during eclipse with the combined light before and after it, the team isolated the dayside’s faint infrared contribution.
Earlier analyses smoothed the spectra into broad wavelength bins. Snellen’s team retained NIRCam’s native resolving power across 3.9 to 5 micrometres and cross-correlated the data with the pattern expected from individual molecular lines. The researchers report an approximately eight-sigma carbon monoxide emission signal in one visit, possible three-sigma signals in two others and no signal in the remaining two.
The strongest feature appeared at roughly the planet’s expected orbital speed and vanished while the star hid the planet. Those checks support a planetary origin. They do not, by themselves, reveal the atmosphere’s principal ingredient or prove that its interior is releasing gas.
Hydrogen is a modelled explanation, not a direct detection
Carbon monoxide seen in emission requires upper layers that are hotter than the gas beneath them, a structure known as a thermal inversion. The unusually strong signal is difficult to reproduce in a static, hydrostatic atmosphere. In the preprint’s modelling, it requires a sharp temperature rise near pressures of roughly one to 10 millibars and at least 1,000 times less carbon dioxide than carbon monoxide. More carbon dioxide would blanket the narrow carbon monoxide lines.
Across 8,136 self-consistent atmospheric models, hydrogen-rich cases tended to produce the steepest inversions and the largest ratios of carbon monoxide to carbon dioxide. The authors interpret that preference as consistent with gas released from a relatively oxygen-poor, or reduced, molten interior.
The distinction matters: Webb did not directly detect molecular hydrogen, photograph gas bubbles or watch material rise from the magma. The telescope measured changing infrared emission. Hydrogen richness, outgassing and the interior’s chemical state are inferences used to explain it.
A changing atmosphere may be the larger result
The variation across five visits may prove as important as the strongest detection. The preprint considers changes in atmospheric temperature, intermittent outgassing, clouds and atmospheric escape. A proposed feedback cycle would begin as gases released from the magma form clouds. Those clouds could cool the surface and briefly reduce further outgassing; after they dissipate, the release could resume.
Changing flows in the magma ocean might also alter local temperatures and the supply of gas. Another possibility is a variable outflow above the bound atmosphere, though carbon monoxide would need shielding and continual replenishment to survive the star’s ultraviolet radiation for long enough.
The observations cannot yet choose between these scenarios. The underlying light curves also retain instrumental variations of about 100 to 200 parts per million, although the authors say their line-by-line cross-correlation is comparatively insensitive to such broad, wavelength-independent effects.
The bare-rock expectation was never the only one
Astronomers had good reason to think 55 Cancri e lost its primordial atmosphere, but that was not the same as agreement that the planet had no atmosphere worth studying. Earlier Spitzer measurements produced competing clues about the planet’s heat distribution, surface and possible blanket of gas.
In 2024, a team led by Renyu Hu of NASA’s Jet Propulsion Laboratory reported in Nature that Webb observations from four to 12 micrometres favoured a substantial secondary atmosphere rich in carbon monoxide or carbon dioxide over a bare surface carrying only vaporised rock. The paper, summarised by NASA, also proposed that dissolved gases could bubble out of a magma ocean and continually renew the atmosphere.
Uncertainty about composition persisted. A 2025 Astronomy & Astrophysics study led by Leoni Janssen tested more than 25,000 models and statistically favoured a hydrogen-free, nitrogen-dominated atmosphere, while finding several alternatives remained viable. The new preprint examines the same five NIRCam eclipses at higher spectral resolution and reaches a different preference. That contrast shows how strongly the answer still depends on the data treatment and model assumptions.
Repeated observations can test the exhaling-world picture
NASA’s catalogue puts 55 Cancri e at about 1.9 times Earth’s radius and eight times its mass. At roughly 41 light-years away, orbiting a bright star, it is unusually accessible for repeated measurements of a rocky exoplanet. Its short year also gives observers frequent eclipses.
More eclipse spectra could show whether the carbon monoxide signal returns at predictable intervals and whether its speed, abundance and temperature structure vary together.
For now, the careful conclusion is that 55 Cancri e may have a restless connection between molten rock and gas, while the hydrogen-rich version of its atmosphere remains an informed model rather than a completed portrait.
