Even JWST can’t see through this planet’s massive haze
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Even JWST can’t see through this planet’s massive haze
Date:
March 18, 2026
Source:
Penn State
Summary:
Kepler-51d is a giant, ultra-light “super-puff” planet wrapped in an unusually thick haze that’s blocking scientists from seeing what it’s made of. Observations from JWST revealed that this haze may be one of the largest ever detected, possibly stretching as wide as Earth itself. The planet’s low density and close orbit don’t match existing models of how gas giants form or survive. Now, researchers are left with more questions than answers about how such a strange world came to be.
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FULL STORY
The ultra-low-density planet Kepler-51d is surrounded by the thickest layer of haze found on a planet yet, according to a new study that used NASA’s James Webb Space Telescope to take a deeper look at the “super-puff” planet. The haze makes discerning the chemical elements in the planet’s atmosphere — and any clues to the planet’s formation — challenging. Credit: NASA, ESA, and L. Hustak, J. Olmsted, D. Player and F. Summers (STScI)
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-infra