Astronomers are now increasingly tracking down the gas shells of Earth-like exoplanets: the Webb telescope has provided convincing evidence of the atmosphere of a hot super-Earth. The results suggest that it is formed by gases escaping from the seething embers of the hellish planet. The results can now contribute to understanding the atmospheric evolution of rocky planets, say the researchers.
Is there an atmosphere and if so, what are its characteristics? Now that thousands of planets have been discovered around distant stars, this question is now increasingly the focus of research. In the case of gas giants, astronomers have already gained insights into their shells in numerous cases, because their size and structure make them comparatively easy to recognize and examine. Researching the atmospheres of rocky planets, on the other hand, is much more difficult. The planet 55 Cancri e has been in the sights of astronomy for some time. It orbits a sun-like star located about 41 light-years away from us in the constellation Cancer.
The results of the investigation so far have shown that it is a so-called super-Earth: 55 Cancri e is a rocky planet that has approximately twice the diameter of Earth. However, it does not offer living conditions. Because 55 Cancri e orbits its host star so closely that it becomes extremely heated. Its surface therefore probably consists largely of bubbling magma. This certainly applies to one of its halves: it is assumed that the planet has one side permanently facing the star, while the other remains in constant darkness. Previous studies of 55 Cancri e using data from NASA’s Spitzer Space Telescope had also provided evidence of a possible atmosphere. However, no clear conclusions have been possible so far.
Hell planet with an atmosphere?
That’s why researchers led by Renyu Hu from the California Institute of Technology in Pasadena have now evaluated observations from the much more powerful James Webb Space Telescope (JWST). Specifically, it was data from its near-infrared camera and mid-infrared instrument. Although the JWST cannot provide a direct image of 55 Cancri e, it is able to measure subtle changes in the system’s light. To get clues about the planet’s characteristics, the team used a method called secondary eclipse spectroscopy: by subtracting the brightness of the planet passing behind the star from the brightness when it is directly next to the star calculate the emissions of infrared light of different wavelengths.
As the researchers report, the clear indication of an atmosphere emerged from the temperature measurements based on heat radiation. According to them, if 55 Cancri e had no atmosphere or was only covered by a thin veil of vaporized rock, the dayside temperature should actually be around 2,200 degrees Celsius. “Instead, the MIRI data showed a relatively low temperature of only around 1,500 degrees Celsius,” reports Hu. “This is a clear indication that energy is being transferred from the day side of the planet to the night side, most likely through an atmosphere with a lot of volatile components,” explains the astronomer. Further study results supported this: Evidence of volatile substances emerged in the spectral data, reports the team: “They indicate the presence of an atmosphere that contains carbon monoxide or carbon dioxide – substances that absorb certain wavelengths of light,” says co-author Aaron Bello – Calls from the California Institute of Technology in Pasadena.
Fed by bubbling magma
This is the strongest evidence to date of an atmosphere on a rocky planet outside our solar system, the astronomers conclude. According to them, however, 55 Cancri e does not have a so-called primary atmosphere. Because an original gas shell would have long since disappeared due to the star’s high temperature and intense radiation. Instead, it can be assumed that it is constantly being recreated: This secondary atmosphere is created from the gases that bubble from the magma ocean of 55 Cancri e, the researchers explain. The results can therefore now contribute to understanding the interactions between atmospheres, surfaces and the interior of rocky planets.
This also focuses on Earth, Venus and Mars. Because it is assumed that they were also once covered by magma. “Ultimately, we want to understand what conditions allow a rocky planet to maintain a gas-rich atmosphere: the most important ingredient for habitable planets,” says Hu. Finally, co-author Brice-Olivier Demory from the University of Bern highlights the groundbreaking significance of the study: “The observations of 55 Canceri e also bode well for the JWST’s ability to characterize cooler – and possibly life-friendly – rocky planets Sun-like stars orbit”.
Source: NASA, University of Bern, specialist article: Nature, doi: 10.1038/s41586-024-07432-x