This Cotton Candy Planet Is So Weird Even James Webb Can’t See Inside

A bizarre, ultra-light planet is cloaked in such a thick haze that even JWST can’t reveal its composition. Its unusual size and orbit are forcing scientists to rethink how planets form.

A newly published study suggests that a dense haze surrounding the ultra-low-density planet Kepler-51d may be concealing both its composition and how it formed. Researchers led by Penn State used NASA’s James Webb Space Telescope (JWST) to investigate this unusual “super-puff” planet, which already challenges standard models of planetary formation. What they found added to the mystery. The haze enveloping the planet appears to be the thickest ever observed, making it extremely difficult to detect the chemical signatures in its atmosphere and uncover clues about its origin.

The study was published March 16 in the Astronomical Journal.

Kepler-51 System and Its Unusual Planets

Kepler-51 is a star about 2,615 light-years from Earth in the constellation Cygnus. It hosts four known planets, at least three of which are classified as super-puffs. These worlds are roughly the size of Saturn but contain only a few times the mass of Earth. Among them, Kepler-51d is 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 Breaks Planet Formation Rules

In most cases, gas giants form with solid, dense cores that create strong gravity, allowing them to pull in and retain thick envelopes of gas. These planets typically develop farther from their stars, where gas is more abundant, similar to Jupiter and Saturn beyond the asteroid belt.

Kepler-51d does not follow this pattern. It appears to lack a dense core and orbits its star at a distance similar to Venus’s position relative to the sun.

“Kepler-51 is a relatively active star, and its stellar winds should easily blow away the gases 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 Scientists Expect Inside the Atmosphere

Because these planets are so low in density, researchers think they are dominated by light elements such as hydrogen and helium, along with other materials. Identifying those elements would provide important clues about where the planet formed and the conditions it experienced.

Although Kepler-51d cannot be directly imaged, scientists can study it by observing its host star. When the planet passes in front of the star, the starlight dims slightly and filters through the planet’s atmosphere.

“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 Data Suggests an Extremely Thick Haze

Earlier observations using NASA’s Hubble Space Telescope examined near-infrared light between about 1.1 and 1.7 microns. JWST’s more advanced Near-Infrared Spectrograph extended this range to 5 microns, which should have revealed more detailed features of the atmosphere. Instead, researchers found no clear absorption 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.”

Alternative Explanation Considered and Rejected

The team also evaluated whether rings could explain the observations. If the planet had rings tilted at a certain angle, they might block starlight and make the planet appear larger and less dense than it actually is. However, this explanation does not match the observed pattern of light.

“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.”

Future Observations May Solve the Mystery

Further studies of similar planets could help clarify what is happening. Another team is currently analyzing JWST observations of Kepler-51b to determine whether all super-puff planets share similar hazy atmospheres or if Kepler-51d is an exception.

“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.”

Reference: “The James Webb Space Telescope NIRSpec-PRISM Transmission Spectrum of the Super-puff, Kepler-51d” by Jessica E. Libby-Roberts, Aaron Bello-Arufe, Zachory K. Berta-Thompson, Caleb I. Cañas, Yayaati Chachan, Renyu Hu, Yui Kawashima, Catriona Murray, Kazumasa Ohno, Armen Tokadjian, Suvrath Mahadevan, Kento Masuda, Leslie Hebb, Caroline Morley, Guangwei Fu, Peter Gao and Kevin B. Stevenson, 16 March 2026, The Astronomical Journal.
DOI: 10.3847/1538-3881/ae33c0

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 funded the research through a JWST grant, with additional support 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.

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