Did Webb Detect an Atmosphere on Hot Super-Earth 55 Cancri e?

The hot super-Earth 55 Cancri e, which whizzes around its yellow-orange star every 17 hours, has intrigued astronomers ever since its discovery in 2004. Now, new data from the James Webb Space Telescope, presented to the American Astronomical Society this week, is putting early speculation to the test, providing evidence for a thick atmosphere enveloping this hot world.

From Diamond in the Sky to Lava World

Initial estimates of the planet’s density as well of its star’s composition led some astronomers to suggest this world might be a diamond planet. That was before a better understanding of both the star and its planet turned astronomers in the opposite direction: In the next scenario, instead of the cold glint of diamond, the surface of 55 Cancri e glows hot with lava flows. Only a wisp of rock vapor lies between the magma ocean and the stars.

Two types of observations drove that previous conclusion. Measurements of the planet’s mass and radius combined to suggest a dense world, and the extreme temperature contrast between the planet’s night- and daysides, as measured by the Spitzer Space Telescope, ruled out a thick envelope of air.

At the same time, an infrared map of the planet’s dayside showed that the brightest point is offset in the east-west direction, instead of directly beneath the star’s light. Something is redistributing the heat, which led astronomers to suggest lava flows carry heat around the planet.

However, debates over the planet’s mass, radius, temperatures, and even its atmospheric composition have lingered, leaving the planet in limbo: Does it have an atmosphere or not?

Evidence of Air

James Webb Space Telescope (JWST) observations promise to decide the question, though whether they have done so just yet remains up for debate.

Renyu Hu (NASA / JPL) and colleagues asked the Webb mission to take a spectrum of the planet’s heat radiated through its day and night, observations that were taken by observing the planet as it orbited its star. Hu presented the results at the 243rd meeting of the American Astronomical Society in New Orleans. (The study itself is still under peer review.)

Hu acknowledged in his presentation that understanding the data was challenging. To start, 55 Cancri is a bright star. While the 6th-magnitude star is just barely visible to the unaided eye under clear and dark skies, it’s bright enough to overwhelm Webb’s sensitive instruments. The data were also noisy. “These data sets turned out to be more challenging than any other JWST datasets, at least  the ones that I've seen,” Hu said at the presentation.

Nevertheless, Hu’s team persisted, finding that the JWST spectra support a shroud of gases around the planet. This shroud is made of either carbon monoxide, carbon dioxide, or a mixture of the two gases along with nitrogen. The new data don’t negate the idea of lava flows, however. Hu and colleagues carried out detailed simulations showing that a magma ocean could in fact sustain this atmosphere.

The immense challenges in decoding the data made some astronomers in the audience cautious about embracing the results. “I think we want to be very careful about making claims of having detected an atmosphere without intensive scrutiny,” says Kevin Stevenson (Johns Hopkins University Applied Physics Laboratory).

“It certainly seems really interesting to see that carbon dioxide absorption feature in [Webb’s Near-Infrared Camera] data, and that’s really exciting if it holds,” Stevenson adds. “But I would love to see independent data reductions . . . These types of claims require extraordinary evidence.”

While making a definitive claim of the first atmosphere detected around a rocky world might still be premature, the data are in — and they're intriguing. Time will tell what we can conclude from Webb's unique view of this world.