Observations with the NASA/ESA/CSA James Webb Space Telescope may have detected atmospheric gases surrounding 55 Cancri e, a hot, rocky exoplanet 41 light years from Earth.
Published in the journal Nature, this is the best evidence to date for the existence of a rocky planet atmosphere outside our Solar System, according to ESA.
55 Cancri e is one of five known planets orbiting a Sun-like star in the constellation Cancer. With a diameter almost twice that of Earth and a slightly higher density, the planet is classified as a super-Earth: larger than Earth, smaller than Neptune and probably similar in composition to the rocky planets in our Solar System. .
However, describing 55 Cancri e as rocky could give the wrong impression. The planet orbits so close to its star (about 2.25 million kilometers, or one-twentieth the distance between Mercury and the Sun) that its surface is likely molten: a bubbling ocean of magma. In such a tight orbit, the planet is also likely tidally locked, with a day side facing the star at all times and a night side in perpetual darkness.
Despite numerous observations since it was discovered transiting in 2011, the question of whether 55 Cancri e has an atmosphere (or even might have one, given its high temperature and the continued onslaught of stellar radiation and wind from its star) has remained unanswered. .
“I have been working on this planet for more than a decade,” Diana Dragomir, an exoplanet researcher at the University of New Mexico and co-author of the study, said in a statement. “It’s been really frustrating that none of the observations we’ve received have solidly resolved these mysteries. I’m thrilled we’re finally getting some answers!”
Unlike the atmospheres of gas giants, which are relatively easy to detect (the first was detected by the NASA/ESA Hubble Space Telescope more than two decades ago), the thinner, denser atmospheres surrounding rocky planets remain hard to reach.
Previous 55 Cancri e studies using data from NASA’s now-retired Spitzer Space Telescope suggested the presence of a substantial atmosphere rich in volatiles (molecules found in gas form on Earth) such as oxygen, nitrogen and carbon dioxide. But researchers could not rule out another possibility: that the planet is naked, except for a thin mantle of vaporized rock, rich in elements such as silicon, iron, aluminum and calcium. “The planet is so hot that some of the molten rock should evaporate,” Hu explained.
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, he can measure subtle changes in the light from the entire system as the planet orbits the star.
By subtracting the brightness during the secondary eclipse, when the planet is behind the star (starlight only), from the brightness when the planet is right next to 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 day side 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, such as TRAPPIST-1 b.
COOLER THAN EXPECTED
The first indication that 55 Cancri e might have a substantial atmosphere came from temperature measurements based on its thermal emission, the thermal energy emitted 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 temperature should be around 2,200 degrees Celsius.
“Instead, the MIRI data showed a relatively low temperature of around 1,540 degrees Celsius,” Hu said. “This is a very strong indication that energy is being distributed from the day side to the night side, most likely by a volatile-rich atmosphere.” While lava flows can transport some heat to the night side, they cannot move it efficiently enough to explain the cooling effect.
When the team examined the NIRCam data, they saw patterns consistent with a volatile-rich atmosphere. “We see evidence of a drop in the spectrum between 4 and 5 microns: less light reaches the telescope,” explained co-author Aaron Bello-Arufe, also from JPL. “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 characteristic.
The team believes that the gases covering 55 Cancri e would be bubbling up from within, rather than being present since the planet’s formation. “The primary atmosphere would have disappeared long ago due to the star’s high temperature and intense radiation,” Bello-Arufe said. “This would be a secondary atmosphere that the magma ocean continually replenishes. Magma is not just crystals and liquid rock, it also contains a lot of dissolved gas.”
