Gas bubbling up from a lava-covered surface on the super-Earth exoplanet 55 Cancri e may feed an atmosphere rich in carbon dioxide or carbon monoxide, according to new research.
55 Cancri e is one of five planets orbiting the Sun-like star 55 Cancri A that is located 40 light-years away in the constellation of Cancer.
Discovered in 2004, this planet has a radius twice Earth’s, and a mass 8 times greater, making it a so-called super-Earth.
55 Cancri e orbits its host star at a distance of 0.015 AU — about 25 times closer than Mercury is to our Sun — every 18 hours.
The planet is also tidally locked, meaning that it doesn’t rotate like Earth does — instead there is a permanent ‘day’ side and a ‘night’ side.
Previous studies of 55 Cancri e using data from NASA’s Spitzer Space Telescope suggested the presence of a substantial atmosphere rich in volatiles like oxygen, nitrogen, and carbon dioxide.
But astronomers could not rule out another possibility: that the planet is bare, save for a tenuous shroud of vaporized rock, rich in elements like silicon, iron, aluminum, and calcium.
“The planet is so hot that some of the molten rock should evaporate,” said Dr. Renyu Hu, an astronomer at NASA’s Jet Propulsion Laboratory.
To distinguish between the two possibilities, Dr. Hu and colleagues used Webb’s NIRCam (Near-Infrared Camera) and MIRI (Mid-Infrared Instrument) instruments to measure 4- to 12-micron infrared light coming from 55 Cancri e.
Although Webb cannot capture a direct image of the planet, it can measure subtle changes in the light from the whole system as the planet orbits the star.
By subtracting the brightness during the secondary eclipse, when the planet is behind the star, from the brightness when the planet is right beside the star, the astronomers were able to calculate the amount of various wavelengths of infrared light coming from the dayside of the planet.
This method, known as secondary eclipse spectroscopy, is similar to that used by other research teams to search for atmospheres on other rocky exoplanets, like TRAPPIST-1b.
The first indication that 55 Cancri e could have a substantial atmosphere came from temperature measurements based on its thermal emission, the heat energy given off in the form of infrared light.
If the planet is covered in dark molten rock with a thin veil of vaporized rock, or has no atmosphere at all, the dayside should be around 2,200 degrees Celsius.
“Instead, the MIRI data showed a relatively low temperature of about 1,540 degrees Celsius,” Dr. Hu said.
“This is a very strong indication that energy is being distributed from the dayside to the nightside, most likely by a volatile-rich atmosphere.”
While currents of lava can carry some heat around to the nightside, they cannot move it efficiently enough to explain the cooling effect.
When the researchers looked at the NIRCam data, they saw patterns consistent with a volatile-rich atmosphere.
A thermal emission spectrum captured by Webb’s NIRCam instrument in November 2022, and MIRI instrument in March 2023, shows the brightness (y-axis) of different wavelengths of infrared light (x-axis) emitted by the super-Earth exoplanet 55 Cancri e. Image credit: NASA / ESA / CSA / J. Olmsted, STScI / R. Hu, JPL / A. Bello-Arufe, JPL / M. Zhang, University of Chicago / M. Zilinskas, SRON Netherlands Institute for Space Research.
“We see evidence of a dip in the spectrum between 4 and 5 microns — less of this light is reaching the telescope,” said Dr. Aaron Bello-Arufe, also from NASA’s Jet Propulsion Laboratory.
“This suggests the presence of an atmosphere containing carbon monoxide or carbon dioxide, both of which absorb these wavelengths of light.”
“A planet with no atmosphere or only vaporized rock in an atmosphere would not have this specific spectral feature.”
“This is exciting news,” said Dr. Yamila Miguel, an astronomer at Leiden Observatory and the Netherlands Institute for Space Research.
“We’ve spent the last ten years modeling different scenarios, trying to imagine what this world might look like. Finally getting some confirmation of our work is priceless.”
The authors think that the gases blanketing 55 Cancri e would be bubbling out from the interior, rather than being present since the planet’s formation.
“The primary atmosphere would be long gone because of the high temperature and intense radiation from the star,” Dr. Bello-Arufe said.
“This would be a secondary atmosphere that is continuously replenished by the magma ocean. Magma is not only crystals and liquid rock, there’s a lot of dissolved gas in it, too.”
In all likelihood, any atmosphere surrounding the planet would be more complex and quite variable as a result of interactions with the magma ocean.
In addition to carbon monoxide or carbon dioxide, there could be gases like nitrogen, water vapor, sulfur dioxide, some vaporized rock, and even short-lived clouds made of tiny droplets of lava condensed from the air.
While 55 Cancri e is far too hot to be habitable, the scientists think it could provide a unique window for studying interactions between the atmospheres, surfaces and interiors of rocky planets, and perhaps provide insights into the early Earth, Venus and Mars, which are thought to have been covered in magma oceans in the past.
“Ultimately, we want to understand what conditions make it possible for a rocky planet to sustain a gas-rich atmosphere, a key ingredient for a habitable planet,” Dr. Hu said.
The findings were published in the journal Nature.
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R. Hu et al. A secondary atmosphere on the rocky exoplanet 55 Cancri e. Nature, published online April 15, 2024; doi: 10.1038/s41586-024-07432-x