ESA / Hubble, M. Kornmesser
The hot super-Earth 55 Cancri e, which orbits its yellow-orange star every 17 hours, has intrigued astronomers 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 of a thick atmosphere enveloping this hot world.
From the diamond in the sky to the world of lava
Initial estimates of the planet's density, as well as the composition of its star, led some astronomers to suggest that this world could be a diamond planet. That was before a better understanding of both the star and its planet caused astronomers to go in the opposite direction: in the following scenario, instead of the cold brilliance of diamond, the surface of 55 Cancri e sparkles with lava flows. Between the magma ocean and the stars there is only a wisp of rock vapor.
Two types of observations drove that earlier conclusion. Measurements of the planet's mass and radius combined to suggest a dense worldand the extreme temperature contrast between the day and night sides of the planet, measured by the Spitzer Space Telescope, ruled out a thick envelope of air.
Gregg Dinderman and Leah Tiscione/S&T; source: NASA / JPL-Caltech / Univ. OF Cambridge and NASA / L. Kreidberg (Center for Astrophysics, Harvard & Smithsonian)
At the same time, an infrared map of the planet's daytime side showed that the brightest point is offset in an east-west direction, rather than directly beneath the star's light. Something is redistributing heat, leading astronomers to suggest that lava flows transport heat around the planet.
However, debates about the planet mass, radius, temperaturesand even its atmospheric composition They have delayed, leaving the planet in limbo: Does it have an atmosphere or not?
Air evidence
Observations from the James Webb Space Telescope (JWST) I promise to decide the question.although it remains a matter of debate whether they have done so yet.
Renyu Hu (NASA/JPL) and his colleagues asked the Webb mission to take a spectrum of the heat radiated by the planet during the 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 a challenge. For starters, 55 Cancri is a shining star. While the sixth-magnitude star is barely visible to the naked eye under clear, dark skies, it is bright enough to overwhelm Webb's sensitive instruments. The data was also noisy. “These data sets turned out to be more challenging than any other JWST data set, at least the ones I have seen,” Hu said in the presentation.
However, Hu's team persisted and discovered that the JWST spectra support a layer of gases around the planet. This covering is made of carbon monoxide, carbon dioxide, or a mixture of the two gases along with nitrogen. However, the new data does not negate the idea of lava flows. Hu and his colleagues carried out detailed simulations that showed that a magma ocean could, in fact, support this atmosphere.
The immense challenges in decoding the data made some astronomers in the public cautious about accepting the results. “I think we have to be very careful about claiming that we have detected an atmosphere without intensive scrutiny,” says Kevin Stevenson (Johns Hopkins University Applied Physics Laboratory).
“It certainly looks really interesting to see that carbon dioxide absorption feature in the data (from Webb's near-infrared camera), and that's really exciting if it plays out,” Stevenson adds. “But I would love to see independent data reductions. . . These types of claims require extraordinary evidence.”
While making a definitive claim about the first atmosphere detected around a rocky world might still be premature, the data is available and intriguing. Time will tell what we can conclude from Webb's unique vision of this world.