A newly studied exoplanet, Kepler-51d, is wrapped in an unusually dense layer of haze that may be hiding both what it is made of and how it formed. Using NASA’s James Webb Space Telescope (JWST), a team led by Penn State researchers took a closer look at this so-called “super-puff” planet, which already challenges standard ideas of how planets develop. What they found made things even more puzzling. The haze surrounding the planet appears to be the thickest ever detected on a world, making it extremely difficult to identify the chemical makeup of its atmosphere or trace its origins.
The findings were published March 16 in the Astronomical Journal.
A Cotton Candy-Like Planetary System
Kepler-51 is a star located about 2,615 light years away in the constellation Cygnus. It hosts four known planets, at least three of which belong to a rare class of ultra-low-density worlds known as super-puffs. These planets are similar in size to Saturn but have only a few times the mass of Earth. Among them, Kepler-51d stands out as both the coolest and the least dense.
“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!”
Why Kepler-51d Defies Planet Formation Models
Typically, gas giants form with dense cores that generate strong gravity, allowing them to pull in and hold onto thick atmospheres of gas. These planets usually develop far from their stars, where conditions favor gas accumulation, much like Jupiter and Saturn in our own solar system.
Kepler-51d does not follow this pattern. It appears to lack a dense core, and it orbits at a distance from its star comparable to Venus’s position relative to 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?”
What the Thick Haze Is Hiding
Because these planets are so low in density, scientists suspect they are made largely of lightweight gases like hydrogen and helium, along with additional elements. Identifying those elements could reveal where and how the planet formed.
Since Kepler-51d is too distant to image directly, researchers rely on a method called transit observation. When the planet passes in front of its star, some of the starlight passes through the planet’s atmosphere, carrying information about its composition.
“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.”
JWST Observations Blocked by Extreme Haze
Earlier observations with NASA’s Hubble Space Telescope captured near-infrared light between about 1.1 and 1.7 microns. JWST’s more advanced Near-Infrared Spectrograph extended that range to 5 microns, which should have provided a clearer atmospheric signature. Instead, researchers found no distinct signals.
“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.”
Could Rings Explain the Observations?
The team also explored other explanations, including the possibility that the planet has rings. If tilted at a certain angle, rings could block starlight and make the planet appear larger and less dense than it actually is. However, this scenario does not fully match the observed data.
“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.”
Looking Ahead at Other Super-Puff Planets
Further observations may help solve the mystery. Scientists are now analyzing JWST data from another planet in the same system, Kepler-51b, to determine whether all super-puff planets share similar hazy atmospheres or if Kepler-51d is an outlier.
“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.”
Research Team and Support
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, with additional backing from the Penn State Center for Exoplanets and Habitable Worlds. Computational work was carried out using the Penn State Institute for Computational and Data Sciences Advanced CyberInfrastructure.