NASA's James Webb Space Telescope has observed an unusually thick haze on the exoplanet Kepler-51d, obscuring its atmospheric composition. This super-puff planet, part of a rare low-density system around the star Kepler-51, challenges standard models of planetary formation. The findings, led by Penn State researchers, were published on March 16 in the Astronomical Journal.
Astronomers using NASA's James Webb Space Telescope (JWST) examined Kepler-51d, one of four planets orbiting the star Kepler-51, located 2,615 light years away in the constellation Cygnus. This planet belongs to the rare class of super-puffs: similar in size to Saturn but with only a few times Earth's mass, making it the least dense and coolest in its system. The three inner planets are thought to have tiny cores enveloped in huge atmospheres, resembling cotton candy in density, according to Jessica Libby-Roberts, a former Penn State postdoctoral fellow and lead author of the study, now at the University of Tampa. She noted, 'These ultra-low-density super-puff planets are rare, and they defy conventional understanding of how gas giants form. And if explaining how one formed wasn't difficult enough, this system has three!' Kepler-51d orbits at a distance akin to Venus from the Sun and lacks a dense core, unlike typical gas giants that form farther out. The star's activity raises questions about how the planet retains its atmosphere against stellar winds, Libby-Roberts added. JWST's Near-Infrared Spectrograph observations extended to 5 microns but detected no clear atmospheric signatures. Instead, a thick haze layer, comparable in scale to Earth's radius and possibly the largest detected on a planet, blocks the light. Suvrath Mahadevan, Penn State professor and co-author, compared it to haze on Saturn's moon Titan, saying, 'Kepler-51d seems to have a huge amount of haze -- almost the radius of Earth -- which would be one of the largest we've seen on a planet yet.' Transit spectroscopy, where starlight filters through the atmosphere, usually reveals composition, but the haze prevents this. Rings were considered but deemed unlikely due to a linear trend in blocked light at longer wavelengths. Future JWST observations of Kepler-51b may clarify if hazy atmospheres are common among super-puffs. Libby-Roberts reflected, 'We haven't found a solar system like ours yet, and being able to explain how all these different planets formed helps us understand... our place in the universe.' The research involved collaborators from multiple institutions, supported by NASA and Penn State.