UNIVERSITY PARK, Pa. — A thick layer of haze around the ultra-low-density planet Kepler-51d likely obscures not only the strange planet’s composition, but also its origin, according to a new study. A team led by Penn State researchers used NASA’s James Webb Space Telescope (JWST) to take a deeper look at the “super-puff” planet that defies planetary formation models. However, the thickest layer of haze found on a planet yet makes discerning the chemical elements in the planet’s atmosphere — and any clues to the planet’s formation — challenging.
A paper describing the planet appeared today (March 16) in the Astronomical Journal .
Kepler-51 is a star located about 2,615 light years away from Earth in the constellation Cygnus. The system has four known planets, at least three of which are unusual low-density super-puff planets that are about the size of Saturn but only a few times the mass of Earth. Kepler-51d is the coolest and least dense of the planets in this system.
“We think the three inner planets orbiting Kepler-51 have tiny cores and huge atmospheres giving them a density akin to cotton candy,” said Jessical Libby-Roberts, Center for Exoplanets and Habitable Worlds Postdoctoral Fellow at Penn State at the time of the research and first author of the paper. “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!”
According to Libby-Roberts, gas giants typically have a dense core and therefore a strong gravitational pull that attracts and retains gasses. These planets typically form further away from their star, whose gravitational pull also attracts gasses, much like the gas giants of our solar system are located outside the asteroid belt. But Kepler-51d doesn’t have a dense core, and its distance to its star is approximately the same distance as Venus is from the sun.
“Kepler-51 is a relatively active star, and its stellar winds should easily blow away the gasses from this planet, though the extent of this mass-loss over Kepler-51d’s lifetime remains unknown,” said Libby-Roberts, who is now an assistant professor of physics and astronomy at the University of Tampa. “It’s possible that the planet formed further away and moved inward, but we are still left with a ton of questions about how this planet — and the other planets in this system — formed. What is it about this system that created these three really oddball planets, a combination of extremes that we haven’t seen anywhere else?”
Because of the ultra-low densities, the researchers said they believe these super-puff planets have a substantial amount of the lightest elements, hydrogen and helium, but they also expect to find other elements. By clarifying the elements that make up the atmosphere of Kepler-51d, the researchers can also infer information about the environment and location in which the planet formed.
Although planets at these distances cannot be directly imaged from Earth, researchers can observe the star’s light, which dims when a planet passes in front of — or transits — its host star from the perspective of Earth.
“A star’s light is filtered through the atmosphere of the planet before it reaches our telescopes,” Libby-Roberts said. “If a certain molecule is present in the atmosphere that absorbs a certain wavelength of light — like how different colored objects on earth absorb different wavelengths of light — it can block the light at that wavelength. If we look across a range of wavelengths, across a spectrum, we get a sort of fingerprint of the planet’s atmosphere that reveals its composition.”
Libby-Roberts and her colleagues previously observed Kepler-51d with NASA’s Hubble Space Telescope, which observed near-infrared wavelengths of about 1.1 to 1.7 microns. The improved technology of JWST’s Near-Infrared Spectrograph allowed the team to extend observations to 5 microns, with the potential to provide a more nuanced atmosphere “fingerprint.” However, the researchers did not see any clear dips in the star’s intensity at specific wavelengths.
“We think that the planet has such a thick haze layer that is absorbing the wavelengths of light we looked at, so we can’t actually see the features underneath,” said Suvrath Mahadevan, Verne M. Willaman Professor of Astronomy and Astrophysics in the Penn State Eberly College of Science and an author of the paper. “It seems very similar to the haze we see on Saturn’s largest moon Titan, which has hydrocarbons like methane, but at a much larger scale. 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.”
The researchers did consider alternatives to a large haze layer but largely ruled them out. For example, if the planet had rings and was tilted, it’s possible the rings would block the star’s light, making the planet appear larger — and in this case less dense — than it really is. However, the researchers said that rings would block light at different wavelengths in a consistent manner and would have to be angled in a very specific way to create these results.
“Instead, we see a linear trend, with more light being blocked at longer wavelengths,” Libby-Roberts said. “This is unusual, and the simplest explanation is thick haze. Rings would have to be short-lived, composed of very particular materials, and situated in just the right angle, which seems unlikely, but we can’t completely rule it out. If we could observe the planet at even longer wavelengths, such as with JWST’s Mid Infrared Instrument, we might be able to detect the materials that would be in a ring or see the full extent of the haze layer.”
The researchers said that observations of other super-puff planets could also shed light on Kepler-51d. For example, another research team is currently analyzing JWST observations of Kepler-51b, which could help clarify if all super-puff planets are hazy, if they all have similar structure and environments they were formed in, or if 51d is unique.
“Before astronomers found planets outside our solar system, we thought we had a pretty good grasp on how planets formed,” Libby-Roberts said. “But we started to find exoplanets that didn’t match our solar system at all, and we have these alien worlds that really challenge our understanding of planet formation. We haven’t found a solar system like ours yet, and being able to explain how all these different planets formed helps us understand how we fit into the big picture and our place in the universe.”
In addition to Libby-Roberts and Mahadevan, the research team includes Renyu Hu, associate professor of astronomy and astrophysics at Penn State, and Caleb Cañas at NASA Goddard Space Flight Center, who earned his doctoral degree in astronomy and astrophysics at Penn State. The team also includes Aaron Bello-Arufe, Kazumasa Ohno and Armen Tokadjian at the California Institute of Technology; Zachory K. Berta-Thompson and Catriona Murray at the University of Colorado Boulder; Yayaati Chachan at the University of California, Santa Cruz; Yui Kawashima at Kyoto University; Kento Masuda at Osaka University; Leslie Hebb at Hobart and William Smith Colleges; Caroline Morley at the University of Texas at Austin; Guangwei Fu and Kevin B. Stevenson at Johns Hopkins University; and Peter Gao at the Carnegie Institution for Science.
NASA supported this research through a JWST grant and the Penn State Center for Exoplanets and Habitable Worlds provided additional support. Computations were performed on the Penn State Institute for Computational and Data Sciences Advanced CyberInfrastructure.
The Astronomical Journal
Observational study
Not applicable
The James Webb Space Telescope NIRSpec-PRISM Transmission Spectrum of the Super-Puff, Kepler-51d
16-Mar-2026