A distant exoplanet known as Kepler-51d is challenging astronomers once again. New observations from the James Webb Space Telescope (JWST) revealed that the planet is covered by an exceptionally thick haze layer that blocks scientists from identifying the composition of its atmosphere. The discovery was detailed in The Astronomical Journal and adds to the mystery surrounding a rare class of worlds called super-puffs. Researchers say the planet’s structure, density, and orbit do not fit neatly within current models of planetary formation.
Located about 2,615 light-years away in the constellation Cygnus, the Kepler-51 system contains four known planets. Three of them are classified as super-puffs, giant planets with extremely low densities despite sizes comparable to Saturn.
Scientists relied on transit observations to study the planet. During a transit, part of the star’s light passes through the atmosphere before reaching telescopes, allowing astronomers to search for chemical signatures. Yet the expected atmospheric signals never appeared in the data collected by JWST.
A Giant Planet With An Unusually Low Density
Researchers describe Kepler-51d as celestial body a with a density similar to cotton candy. According to researchers at Penn State, it is the coolest and least dense of the system’s planets. Jessica Libby-Roberts, the study’s first author, explained that the three inner planets orbiting the exoplanet appear to have very small cores surrounded by enormous atmospheres.
Traditional formation models suggest gas giants develop dense cores that generate enough gravity to attract and retain thick gaseous envelopes. In the case of Kepler-51d, scientists say the planet does not appear to follow that process.
Its orbit creates another problem. The planet circles its host star at a distance comparable to the position of Venus in the solar system. As Libby-Roberts said:
“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.”
Researchers believe the planet may have originally formed farther away from its star before slowly moving closer over time, though they say many questions about its history still remain unanswered.
“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?” she added.
JWST Found a Massive Planetary Haze
Earlier observations conducted with the Hubble Space Telescopecovered wavelengths between 1.1 and 1.7 microns. The use of JWST’s Near-Infrared Spectrograph allowed astronomers to extend those observations to 5 microns in the infrared range. As explained in the latest research, the broader range should have revealed distinct atmospheric fingerprints. Instead, the telescope detected no clear molecular signatures.
“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, professor of astronomy and astrophysics at Penn State.
Scientists compared the haze to the atmosphere surrounding Titan, the largest moon of Saturn, which contains hydrocarbon compounds including methane. The haze enveloping Kepler-51d may extend nearly the radius of Earth itself, making it one of the largest haze structures ever observed around it.
Could Rings Explain the Discovery?
The team explored whether rings surrounding the alien world could explain the unusual observations. If tilted at a particular angle, rings could block additional starlight and make the planet appear larger and less dense during transits.
The paper explained the observed data did not fully match that scenario. Researchers identified a linear trend in which more light was blocked at longer wavelengths, a pattern they considered more consistent with atmospheric haze.
Libby-Roberts said rings would need to be short-lived, composed of very specific materials, and positioned at exactly the right angle to reproduce the observations. Scientists have not ruled out the possibility entirely, though the haze explanation currently fits the data more closely.
The research team is now analyzing observations of another planet in the same system, Kepler-51b, to determine whether similar atmospheric conditions exist on other super-puff planets.
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