The James Webb Space Telescope has taken the deepest look yet at Kepler-51d, an ultra-low-density exoplanet sometimes compared to cotton candy, and found that a thick atmospheric haze blocks nearly all useful chemical signatures. A new preprint published on May 27, 2025, reports that the planet’s transmission spectrum across wavelengths from 0.6 to 5.3 microns is largely featureless and sloped, leaving the origins of its extreme puffiness as uncertain as ever. With a mass of just 5.6 Earth masses packed into a radius of 9.32 Earth radii, Kepler-51d remains one of the most puzzling worlds known to science.
A Planet Too Fluffy to Explain
Kepler-51d is the outermost transiting planet in a system that has baffled astronomers for more than a decade. All three originally known planets in the Kepler-51 system have masses below 10 Earth masses and densities under 0.1 grams per cubic centimeter, making them far less dense than any planet in our solar system. Those extreme measurements were first quantified through transit timing variations, a technique that tracks gravitational tugs between planets to estimate their masses. That foundational work, published in a 2014 preprint analyzing the Kepler-51 system, established the so-called “super-puff” problem: how can planets this large weigh so little?
Follow-up observations only deepened the mystery. Using the Hubble Space Telescope, astronomers found that Kepler-51d and its sibling worlds were among the most inflated planets ever seen, with radii rivaling Jupiter’s despite their meager masses. A NASA feature on these cotton candy-like planets emphasized just how far they stray from standard models of planetary structure, which struggle to keep such low-density atmospheres from collapsing or escaping over time.
Two broad hypotheses have circulated since then. Either these planets formed in the outer reaches of their star system and migrated inward while retaining bloated hydrogen-helium envelopes, or some internal heating mechanism keeps their atmospheres inflated. Distinguishing between those scenarios requires reading the chemical fingerprints in their atmospheres, which is precisely what the haze keeps preventing.
JWST’s Sharpest View Still Hits a Wall
The new study used JWST’s NIRSpec-PRISM instrument to observe Kepler-51d during transit, capturing starlight filtered through the planet’s atmosphere across a wide infrared range. According to the NIRSpec-PRISM analysis, the resulting data show a sloped but largely featureless profile. The team concludes the atmosphere is consistent with low-metallicity hydrogen and helium gas topped by high-altitude submicron haze particles sitting at pressures around 1 to 100 microbars. An alternative explanation involving a tilted ring system was also evaluated but could not fully account for the observations.
This result echoes what Hubble found years earlier but at far greater sensitivity. Hubble’s Wide Field Camera 3 produced transmission spectra for both Kepler-51b and Kepler-51d that were, per a peer-reviewed analysis in the Astronomical Journal, essentially flat and featureless, interpreted as evidence of high-altitude aerosols at pressures below roughly 3 millibars. The JWST data push the constraint deeper, pinpointing haze at even lower pressures, yet the outcome is functionally the same: molecular absorption features that would reveal atmospheric composition remain hidden.
There is a subtle but meaningful difference between the two datasets. While Hubble’s spectrum was described as flat, the JWST spectrum is characterized as sloped. That slope hints at the scattering properties of the haze particles themselves, offering a narrow window into particle size and distribution even if the gas-phase chemistry stays obscured. Still, the gap between “sloped” and “informative” is wide, and the haze continues to act as a blanket over the data astronomers need most.
The preprint also compares Kepler-51d’s spectrum to models with different metallicities and cloud prescriptions, finding that several combinations can reproduce the observed slope. This degeneracy means that even with JWST’s precision, basic questions (such as whether the atmosphere is enriched in heavier elements relative to its star) remain unanswered. In effect, astronomers are staring at an exquisitely measured fog.
A Hidden Fourth Planet Reshapes the System
JWST’s observations of Kepler-51d also triggered a separate discovery that reshaped understanding of the entire system. When the telescope measured the planet’s transit time, the result deviated substantially from predictions based on a three-planet dynamical model, with the discrepancy spanning roughly 10 years of accumulated timing drift. That mismatch motivated a revised model, and the best fit required a previously unknown fourth body: Kepler-51e, whose gravitational influence on the other planets explained the timing anomaly.
The discovery, announced in December 2024, means the system’s gravitational architecture is more complex than previously assumed. A fourth planet pulling on its neighbors changes the mass estimates for all the other worlds, which in turn affects density calculations. If Kepler-51d’s mass shifts even modestly under a revised dynamical model, its already extreme density could become slightly less extreme, or even more so. The reporting sources do not yet provide direct mass or radius measurements for Kepler-51e, so the full impact on the system’s density puzzle remains an open question.
The timing revision also illustrates how sensitive super-puff inferences are to small changes in orbital dynamics. Transit timing variations were originally used to establish the low masses of all three known planets, and the addition of a fourth perturber forces researchers to revisit those fits. As new JWST transits accumulate, the system may continue to evolve from a textbook example of super-puffs into a more nuanced case study in multi-planet interactions.
Slow Spin Adds Another Constraint
A separate analysis of the same JWST transit data examined Kepler-51d’s shape for signs of rotational flattening. Rapidly spinning planets bulge at the equator, producing a measurable oblateness in their transit light curves. According to a preprint on oblateness, the JWST data place tight upper limits on any such bulge, implying that Kepler-51d rotates relatively slowly for a planet of its size. If the planet were spinning quickly enough to help support its inflated atmosphere through centrifugal forces, that spin would likely leave a detectable imprint on the transit profile; instead, the signal is consistent with a nearly spherical shape.
This slow rotation has two important implications. First, it weakens scenarios in which rapid spin contributes meaningfully to the planet’s low density, nudging theorists back toward explanations rooted in atmospheric composition, internal heat, or formation history. Second, it suggests that tidal interactions with the host star may have already synchronized the planet’s rotation with its orbit, further constraining models of its age and past migration.
Haze, Chemistry, and the Limits of Current Models
While Kepler-51d’s spectrum remains stubbornly featureless, theorists are using the limited information it provides to refine models of atmospheric haze. A recent study of aerosol formation in low-gravity exoplanets, available as a preprint on haze microphysics, explores how tiny particles can loft to extremely high altitudes and produce the kind of scattering slopes JWST observes. Such work underscores that the “fog” obscuring super-puffs is not just a nuisance but a key part of the physics that may explain their inflated sizes.
The challenge is that many of these models rely on assumptions about chemistry and energy transport that cannot yet be tested directly for Kepler-51d. Without clear molecular absorption features (water, methane, or other common species), researchers must lean on more indirect clues, such as the wavelength dependence of scattering and the pressure levels inferred for the haze deck. Even then, multiple atmospheric configurations can match the data, leaving room for competing interpretations.
Behind the scenes, much of this rapid progress is enabled by the culture of open preprints in astronomy. The Kepler-51d studies, like many JWST results, first appear on arXiv’s member-supported platform before undergoing formal peer review, allowing the community to scrutinize methods, compare models, and plan follow-up observations in near real time. That open exchange is particularly valuable for complex systems like Kepler-51, where dynamics, rotation, and atmospheric physics intersect.
What Comes Next for a Cotton-Candy World
For now, Kepler-51d remains an icon of planetary weirdness: a world so large and light that even JWST struggles to peel back its atmospheric disguise. Future observations may attempt higher-resolution spectroscopy in narrower wavelength bands, search for thermal emission during secondary eclipse, or target its sibling planets to see whether their hazes behave similarly. Any divergence among the planets could hint at differences in metallicity, temperature, or evolutionary history that are currently hidden in the noise.
Even if the haze never fully lifts, Kepler-51d is already forcing theorists to stretch models of planetary formation, migration, and atmospheric escape to their limits. Each new constraint, from the sloped JWST spectrum to the hidden fourth planet and the slow spin, chips away at the space of viable explanations. In that sense, the cotton-candy planet is doing what the best astronomical oddities always do: revealing where our understanding is thinnest, and pointing the way toward the next generation of ideas and instruments needed to thicken it.
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*This article was researched with the help of AI, with human editors creating the final content.
