NASA scientists have potentially uncovered evidence of atmospheric gases surrounding 55 Cancri e, a hot rocky exoplanet orbiting a star similar to our Sun located about 41 light-years away in the constellation Cancer.

This discovery, made with the James Webb Space Telescope (JWST), represents a significant leap in the study of exoplanets, offering the strongest indication to date of the existence of an atmosphere around a rocky planet beyond our Solar System.

The lead researcher, Renyu Hu from NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, emphasizes the significance of this discovery, stating that Webb is expanding the boundaries of exoplanet characterization, especially when it comes to rocky planets. The research, detailed in a paper published in Nature, underscores the unique science enabled by the Webb telescope.

A 7.5- to 11.8-micron light curve captured by the NASA James Webb Space Telescope's MIRI (Mid-Infrared Instrument) instrument in March 2023 shows the decrease in brightness of the 55 Cancri system as the rocky planet 55 Cancri e moves behind the star, a phenomenon known as secondary eclipse. The amount of mean infrared light emitted by the planet (the difference in brightness between the star and planet combined and the star alone) indicates that the diurnal temperature of the planet is about 2,800 degrees Fahrenheit. This temperature, low compared to that of a similar planet without an atmosphere, indicates that heat is distributed from the daytime to the nighttime side of the planet, possibly through a volatile-rich atmosphere
A 7.5- to 11.8-micron light curve captured by the NASA James Webb Space Telescope’s MIRI (Mid-Infrared Instrument) instrument in March 2023 shows the decrease in brightness of the 55 Cancri system as the rocky planet 55 Cancri e moves behind the star, a phenomenon known as secondary eclipse. The amount of mean infrared light emitted by the planet (the difference in brightness between the star and planet combined and the star alone) indicates that the diurnal temperature of the planet is about 2,800 degrees Fahrenheit. This temperature, low compared to that of a similar planet without an atmosphere, indicates that heat is distributed from the daytime to the nighttime side of the planet, possibly through a volatile-rich atmosphere. Credit: Joseph Olmsted (STScI) / NASA, ESA, CSA

55 Cancri e, also known as Janssen, is one of five planets orbiting the star 55 Cancri. With a diameter nearly twice that of Earth and a slightly higher density, it is categorized as a super-Earth—larger than Earth but smaller than Neptune, with a composition likely similar to rocky planets in our Solar System.

However, its proximity to its star—just 1.4 million miles away, about 1/25th the distance between Mercury and the Sun—suggests that its surface might be a bubbling ocean of magma, potentially tidally locked with a permanently illuminated day side and an eternally dark night side.

Despite its discovery in 2011, the question of whether 55 Cancri e has an atmosphere has remained unanswered due to the extreme temperatures and intense radiation from its star. Earlier studies with NASA’s retired Spitzer Space Telescope suggested a possible atmosphere containing volatiles like oxygen, nitrogen, and carbon dioxide. However, another theory posited that the planet might lack an atmosphere, with only a thin veil of vaporized rock.

A thermal emission spectrum of the exoplanet super-Earth 55 Cancri e, captured by the GRISM spectrometer (F444W) on the NIRCam (Near Infrared Camera) and the MIRI (Mid-Infrared Instrument) low-resolution spectrometer on NASA's James Webb Space Telescope, shows that the planet may be surrounded by an atmosphere rich in carbon dioxide or carbon monoxide and other volatiles, not just vaporized rock
A thermal emission spectrum of the exoplanet super-Earth 55 Cancri e, captured by the GRISM spectrometer (F444W) on the NIRCam (Near Infrared Camera) and the MIRI (Mid-Infrared Instrument) low-resolution spectrometer on NASA’s James Webb Space Telescope, shows that the planet may be surrounded by an atmosphere rich in carbon dioxide or carbon monoxide and other volatiles, not just vaporized rock. Credit: Joseph Olmsted (STScI) / NASA, ESA, CSA

To test these hypotheses, researchers used the Near-Infrared Camera (NIRCam) and the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope. By analyzing the planet’s light across different wavelengths, they aimed to determine its composition and structure.

This approach, called secondary eclipse spectroscopy, involves measuring changes in light as the planet moves behind its star, allowing scientists to detect atmospheric signatures.

The first indication of a substantial atmosphere came from MIRI’s measurements of the planet’s thermal emission, revealing a relatively low temperature of about 2,800 degrees Fahrenheit (1,540 degrees Celsius)—far lower than the expected 4,000 degrees Fahrenheit (2,200 degrees Celsius) if the planet lacked an atmosphere. This cooler temperature suggests that energy might be redistributed from the day side to the night side, likely due to an atmosphere rich in volatile compounds.

This artist's impression shows super-Earth 55 Cancri e in front of its parent star
This artist’s impression shows super-Earth 55 Cancri e in front of its parent star. Credit: M. Kornmesser / ESA/Hubble

Further analysis with NIRCam showed patterns consistent with an atmosphere containing gases like carbon monoxide or carbon dioxide, indicating a significant difference from a planet with no atmosphere or one composed only of vaporized rock.

The team believes that the gases covering 55 Cancri e might be the result of internal processes rather than remnants from the planet’s formation. As explained by Aaron Bello-Arufe, co-author of the study, this could represent a secondary atmosphere continually replenished by the planet’s magma ocean, with gas dissolved in the molten rock being released over time.

While 55 Cancri e is too hot to be habitable, the study of its atmosphere offers valuable insights into the interactions between atmospheres, surfaces, and interiors of rocky planets, providing clues about the early conditions on Earth, Venus, and Mars, which were likely covered in magma oceans at some point. Ultimately, this research could lead to a better understanding of what conditions allow rocky planets to retain atmospheres rich in gas—an essential ingredient for habitability.

Sources

NASA Goddard Space Flight Center | Hu, R., Bello-Arufe, A., Zhang, M. et al. A secondary atmosphere on the rocky Exoplanet 55 Cancri e. Nature (2024). doi.org/10.1038/s41586–024–07432-x

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