JWST SPIES THICK AIR AROUND A ROCKY WORLD
IN THE FIRST TWO YEARS of results from NASA’s James Webb Space Telescope (JWST), astronomers have mostly come up empty in their search for atmospheres around rocky exoplanets. Multiple planets in the intriguing TRAPPIST-1 system have been examined by NASA’s new flagship observatory, with none revealing signs of an atmosphere. But the dry spell is over: JWST has finally found strong evidence of a thick atmosphere around a rocky world. The new observations, published May 8 in Nature, focus on super-Earth 55 Cancri e. It lies so close to its home star that it completes an orbit roughly every 17 hours — one of the fastest known orbital periods of any exoplanet. This leads to a molten surface with temperatures likely exceeding 3,000 degrees Fahrenheit (1,650 degrees Celsius) on a world that is intensely bombarded by radiation from its host star. As a result, the original atmosphere should be long gone. Yet a team led by researchers at NASA’s Jet Propulsion Lab (JPL) in Pasadena, California, discovered the likely presence of carbon dioxide and carbon monoxide gas around the planet. JWST’s suite of instruments includes a set of spectrometers. But the atmospheric gases weren’t detected by directly staring with them at the planet. The study instead made use of a technique called secondary eclipse spectroscopy, which is possible when a planet’s orbit takes it behind its host star from our point of view. When the planet is eclipsed, astronomers can isolate the light coming from the host star and subtract it from observations of the system when the planet is also visible, leaving behind the only the light coming from the planet. The team then ran simulations on a variety of possible atmospheric types that would give off similar signals. This analysis yielded no evidence for helium and hydrogen — the most common elements in the universe, which would have been part of the planet’s initial atmosphere when it formed. This means that what JWST detected is likely the planet’s secondary atmosphere. It seems that, despite the violent winds from its nearby star, continuous volcanic activity and the gravity of the planet have allowed the planet to regenerate and retain a relatively thick atmosphere. The team found that the best match for their data was a mix of carbon dioxide, carbon monoxide, and nitrogen gas. This would also track with a planetary atmosphere driven in part by volcanism. In short, 55 Cancri e shows that just because an exoplanet loses its atmosphere, that doesn’t mean that it can’t gain a new one. Earth’s atmosphere — as well as the atmospheres of Venus and Mars — are also secondary atmospheres, and knowledge of how common they are among exoplanets may impact our understanding of how common habitable planets may be in the universe. “I’m quite confident that JWST will provide a lot of information about the ability of rocky planets of various sizes and temperatures to develop a secondary atmosphere,” says Renyu Hu, a planetary scientist at JPL and lead author of the paper. “We can use that to calibrate our models, and eventually, we will have a better understanding of the emergence of habitable worlds.”