James Webb space telescope points to possible atmosphere around rocky exoplanet

Researchers from NASA’s James Webb Space Telescope may have discovered atmospheric gases around 55 Cancri e, a hot rocky exoplanet located 41 light-years from Earth. This is the best evidence yet for the existence of an atmosphere of a rocky planet outside our solar system. Renyu Hu of NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California, is lead author of a paper published today in Nature. “Webb pushes the boundaries of exoplanet characterization to rocky planets,” Hu said. “It really makes a new kind of science possible.”

Super hot super Earth 55 Cancri e

55 Cancri e (image below, details/download), also known as Janssen, is one of five known planets orbiting the sun-like star 55 Cancri, in the constellation Cancer. With a diameter almost twice that of Earth and a density slightly greater, 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. 55 However, describing Cancri e as “rocky” might give the wrong impression. The planet orbits its star so closely (about 1.4 million miles, or one-twenty-fifth the distance between Mercury and the Sun) that its surface is likely molten—a bubbling ocean of magma. With such a tight orbit, the planet is also likely tidally locked, with a day side always facing the star and a night side in perpetual darkness.

Despite numerous observations since the planet’s discovery in 2011, the question of whether 55 Cancri e has an atmosphere – or even might have one given its star’s high temperature and persistent stellar radiation and wind – has remained unanswered. “I’ve been working on this planet for more than a decade,” said Diana Dragomir, an exoplanet researcher at the University of New Mexico and co-author of the study. “It was really frustrating that none of the observations we got were able to solve these mysteries properly. I’m glad we’re finally getting answers!” Unlike the atmospheres of gas giant planets, which are relatively easy to discover (the first was discovered more than twenty years ago by NASA’s Hubble Space Telescope), the thinner and denser atmospheres around rocky planets have remained elusive.

Previous studies of 55 Cancri e using data from NASA’s now retired Spitzer Space Telescope suggested the presence of a substantial atmosphere rich in volatiles (molecules that exist in gaseous form on Earth) such as oxygen, nitrogen and carbon dioxide. But researchers couldn’t rule out another possibility: that the planet is barren except for a thin layer of evaporated rock rich in elements like silicon, iron, aluminum and calcium. “The planet is so hot that some of the molten rock should evaporate,” Hu explained.

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 the 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 the system’s light as the planet orbits the star. By subtracting the brightness during the secondary eclipse (image below, details/download), 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 the planet together), the team was able to calculate the amount of infrared light of different wavelengths 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, 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 (image below, details/download), 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, then the dayside temperature should be around 4,000 degrees Fahrenheit (~2,200 degrees Celsius). “Instead, the MIRI data showed a relatively low temperature of about 2,800 degrees Fahrenheit [~1540 graden Celsius],” said Hu. “This is a very strong indication that energy is distributed from the day side to the night side, probably by a volatile atmosphere.” Although lava flows can move some heat to the night side, they cannot do so efficiently enough to reduce the cooling effect explain.

When the team looked at the NIRCam data, they saw patterns consistent with a volatile atmosphere. “We see evidence of a dip in the spectrum between 4 and 5 microns – less of this light reaches the telescope,” explains co-author Aaron Bello-Arufe, also of NASA JPL. “This suggests the presence of an atmosphere containing carbon monoxide or carbon dioxide, which absorb these wavelengths of light.” A planet with no atmosphere or an atmosphere consisting only of vaporized rock would not have this specific spectral feature. “We have spent the past ten years modeling different scenarios to imagine what this world could look like,” says co-author Yamila Miguel of the Leiden Observatory and the Netherlands Institute for Space Research (SRON). “Finally getting confirmation of our work is priceless!”

Bubbling Magma Ocean

The team thinks the gases covering 55 Cancri e are bubbling out of its interior, rather than having been present since the planet’s formation. “The primary atmosphere would have long since disappeared due to the star’s high temperature and intense radiation,” Bello-Arufe said. “This would be a secondary atmosphere that is continuously replenished by the magma ocean. Magma is not just made up of crystals and liquid rock; it also contains a lot of dissolved gas.” Although 55 Cancri e is far too hot to be habitable, researchers believe it could provide a unique window for studying interactions between atmospheres, surfaces and interiors of rocky planets, and perhaps provide insight into Earth’s early conditions. Venus and Mars, which are thought to have been covered by magma oceans far in the past. “Ultimately, we want to understand under what conditions a rocky planet can maintain a gas-rich atmosphere: a key ingredient for a habitable planet,” Hu said. This research was conducted as part of Webb’s General Observers (GO) Program 1952. Additional observations of the secondary eclipse of 55 Cancri e are currently being analyzed.

Source: NASA