Our Solar System is, honestly, kind of boring by galactic standards. Nothing out here is melting in real time. 55 Cancri e is a good reminder of that. It's a rocky planet almost twice Earth's size, sitting so close to its star that a full year passes in under a day. Most of its surface is thought to be molten rock rather than anything solid. And new data from the James Webb Space Telescope suggests this planet, against pretty much every expectation, might still be holding onto a thin atmosphere.
41 LIGHT YEARS, 18 HOURS
55 Cancri e orbits a Sun-like star called 55 Cancri A, about 41 light-years away in the constellation Cancer. It was found in 2004 using the radial velocity method, which picks up a planet indirectly by measuring the wobble it causes in its star's light. Astronomers originally clocked its orbit at 2.8 days. A correction in 2011 cut that down to the real figure: about 18 hours.
For context, Mercury takes 88 days to orbit our Sun, and it's already considered absurdly close. 55 Cancri e sits roughly 26 times closer to its star than that, at around 0.015 AU. At that distance the planet is almost certainly tidally locked, meaning one side bakes under permanent daylight while the other never sees the star at all.
TWICE THE SIZE, AND MOSTLY LIQUID
The planet has a mass around eight times Earth's and a radius close to 1.9 Earth radii, which puts it in the "super-Earth" bracket, a size class we don't have an example of at home. Its density points to a rocky composition, but the word "rocky" undersells what's actually going on. Dayside temperatures land somewhere between 2,000 and nearly 3,000 Kelvin, hot enough to keep silicate rock liquid across much of the surface. Even the nightside, which never faces the star, stays above 1,600 K.
That rules out water, ice, anything you'd call habitable. What it doesn't rule out is an atmosphere, and that's the part worth paying attention to.
WHAT JWST ACTUALLY SAW
Before JWST, nobody was confident 55 Cancri e had an atmosphere at all. A planet this close to its star should get its gas envelope stripped away fast by radiation and stellar wind. Earlier Spitzer data hinted at heat moving from the dayside to the nightside, which would suggest an atmosphere doing the redistributing, but later reanalysis questioned that.
Then JWST looked at it with two different instruments, NIRCam and MIRI, essentially reading the heat signature coming off the dayside. One team picked up an absorption feature in the mid-infrared that lines up with carbon monoxide or carbon dioxide, possibly sitting on a background of nitrogen. They also ruled out an atmosphere made purely of vaporized rock, the kind of thin mineral haze you'd expect from a planet with no real gas envelope.
If that holds up, it's genuinely surprising. A planet this hot, this close in, keeping a secondary atmosphere against odds that should favor total erosion. Either the atmosphere is being topped up somehow, volcanic outgassing from the molten surface is the leading guess, or our models for how rocky planets lose their atmospheres near a star need work.
NOBODY'S CALLING THIS SETTLED
It's worth saying plainly: this is still an open question. A second team, observing at a different wavelength, found the planet's brightness shifting noticeably between visits. Sometimes the data looked consistent with a thin CO/CO2 atmosphere, other times it looked more like bare rock with nothing on it. One idea is uneven outgassing, the planet essentially burping gas unevenly as the molten surface churns. Another is a patchy ring of volcanic dust drifting near the star, occasionally getting in the way of the signal.
Follow-up papers through 2025 haven't fully agreed either. Some support the CO/CO2 reading, others argue the same numbers could fit different gas mixtures entirely, including stranger candidates like phosphine or silicon compounds. Right now the honest summary isn't "we found the atmosphere." It's closer to "there's very likely something there, and figuring out what will take more telescope time."
Why bother with a planet nobody could live on
55 Cancri e was never going to be a habitability story, and it isn't one. What makes it useful is that it's one of the most observable rocky exoplanets we have. Bright host star, favorable orbit, close enough for detailed spectroscopy. That makes it a testbed for the models astronomers will eventually apply to cooler, smaller rocky planets, the ones that might actually turn out to be temperate.
Every atmosphere we manage to characterize on a planet like this sharpens the tools we'll use on the next one. Future observations, longer-wavelength MIRI photometry and full-orbit phase curves that track heat movement, should help settle some of the current disagreement. For a planet whose year is shorter than a night's sleep, we're still fairly early into understanding it.
Sources referenced:
● Hu et al. (2024), JWST NIRCam and MIRI observations of 55 Cancri e (GO 1952)
● Patel et al. (2024), JWST variability study of 55 Cancri e (GO 2084), arXiv:2407.12898
● Zilinskas et al., "Characterising the atmosphere of 55 Cancri e," Astronomy & Astrophysics, 2025
● NASA Science Exoplanet Catalog, 55 Cancri e planet profile
IMAGE SOURCES
● INFOGRAPHICS MADE BY ME
●PLANETARY PICTURES FROM NASA website
