Using NASA’s James Webb Space Telescope, scientists may have identified atmospheric gases on 55 Cancri e, a super-hot, rocky exoplanet. This discovery could represent the most definitive evidence of an atmosphere on any rocky planet outside our solar system. Credit: SciTechDaily.com
Gas bubbling up from a lava-covered surface on 55 Cancri e may feed an atmosphere rich in carbon dioxide or carbon monoxide.
These days, detecting a planetary atmosphere tens or even hundreds of light-years from Earth might not sound like such a big deal. Scientists have found signs of atmosphere surrounding dozens of exoplanets over the past two decades. The catch is, all those planets have thick, hydrogen-dominated atmospheres that are relatively easy to study. The much thinner blankets of gas that almost certainly surround some small, rocky exoplanets have remained elusive.
Researchers think they may have finally caught a glimpse of a volatile-rich atmosphere surrounding a rocky planet. Light emitted by the hot, highly-irradiated
This artist’s concept shows what the exoplanet 55 Cancri e could look like. Also called Janssen, 55 Cancri e is a so-called super-Earth, a rocky planet significantly larger than Earth but smaller than Neptune, which orbits its star at a distance of only 1.4 million miles (0.015 astronomical units), completing one full orbit in less than 18 hours. (Mercury is 25 times farther from the Sun than 55 Cancri e is from its star.) The system, which also includes four large gas-giant planets, is located about 41 light-years from Earth, in the constellation Cancer. Credit: NASA, ESA, CSA, Ralf Crawford (STScI)
Webb Space Telescope Hints at Possible Atmosphere Surrounding Rocky Exoplanet
Researchers using first was detected by NASA’s
This light curve shows the change in brightness of the 55 Cancri system as the rocky planet 55 Cancri e, the closest of the five known planets in the system, moves behind the star. This phenomenon is known a secondary eclipse.
When the planet is next to the star, the mid-infrared light emitted by both the star and the dayside of the planet reaches the telescope, and the system appears brighter. When the planet is behind the star, the light emitted by the planet is blocked and only the starlight reaches the telescope, causing the apparent brightness to decrease.
Astronomers can subtract the brightness of the star from the combined brightness of the star and planet to calculate how much infrared light is coming from the dayside of the planet. This is then used to calculate the dayside temperature and infer whether or not the planet has an atmosphere.
Credit: NASA, ESA, CSA, Joseph Olmsted (STScI), Aaron Bello-Arufe (NASA-JPL)
Measuring Subtle Variations in Infrared Colors
To distinguish between the two possibilities, the team used Webb’s NIRCam (Near-Infrared Camera) and MIRI (Mid-Infrared Instrument) to measure 4- to 12-micron infrared light coming from the planet.
Although Webb cannot capture a direct image of 55 Cancri e, it can measure subtle changes in light from the system as the planet orbits the star.
By subtracting the brightness during the secondary eclipse (see image above), when the planet is behind the star (starlight only), from the brightness when the planet is right beside the star (light from the star and planet combined), the team was 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-1 b.
A thermal emission spectrum captured by Webb’s NIRCam (Near-Infrared Camera) in November 2022, and MIRI (Mid-Infrared 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. The spectrum shows that the planet may be surrounded by an atmosphere rich in carbon dioxide or carbon monoxide and other volatiles, not just vaporized rock.
The graph compares data collected by NIRCam (orange dots) and MIRI (purple dots) to two different models. Model A, in red, shows what the emission spectrum of 55 Cancri e should look like if it has an atmosphere made of vaporized rock. Model B, in blue, shows what the emission spectrum should look like if the planet has a volatile-rich atmosphere outgassed from a magma ocean that has a similar volatile content as Earth’s mantle. Both MIRI and NIRCam data are consistent with the volatile-rich model.
The amount of mid-infrared light emitted by the planet (MIRI) shows that its dayside temperature is significantly lower than what it would be if it did not have an atmosphere to distribute heat from the dayside to the nightside. The dip in the spectrum between 4 and 5 microns (NIRCam data) can be explained by absorption of those wavelengths by carbon monoxide or carbon dioxide molecules in the atmosphere.
Credit: NASA, ESA, CSA, Joseph Olmsted (STScI), Renyu Hu (NASA-JPL), Aaron Bello-Arufe (NASA-JPL), Michael Zhang (University of Chicago), Mantas Zilinskas (SRON)
Cooler Than Expected
The first indication that 55 Cancri e could have a substantial atmosphere came from temperature measurements based on its thermal emission (see image above), or 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 no atmosphere at all, the dayside should be around 4,000 degrees DOI: 10.1038/s41586-024-07432-x
The James Webb Space Telescope is the world’s premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (