JWST Found a Planet So Puffy It Shouldn't Exist — and Then Couldn't See Through It
The James Webb Space Telescope turned its most powerful infrared eye on a cotton-candy planet 2,615 light years away and saw nothing useful.
Kepler-51d is what astronomers call a super-puff: a gas giant roughly the size of Saturn but with only about five times Earth's mass, giving it a density around 0.04 grams per cubic centimeter — less than a tenth of water. It orbits a young Sun-like star in the constellation Cygnus that is only about 500 million years old, a childhood in cosmic terms. And it is wrapped in the thickest haze layer ever detected on an exoplanet, a shroud of submicron particles — smaller than smoke — extending roughly one Earth-radius into space.
A team led by Penn State astronomer Jessica Libby-Roberts pointed JWST's NIRSpec spectrograph at Kepler-51d and watched starlight filter through the planet's upper atmosphere across wavelengths from 0.6 to 5.3 microns. The instrument should have picked up absorption signatures from methane, water, carbon dioxide, and ammonia — the chemical fingerprint any gas giant atmosphere leaves on passing light. Instead, the team got a featureless sloped line. The haze absorbed everything.
"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 Penn State's Suvrath Mahadevan, a co-author of the study published March 16 in the Astronomical Journal.
The result is not a surprise in retrospect, but it is a frustrating one. Super-puff planets have defied explanation since they were first identified: standard formation models for gas giants require a dense core that generates enough gravity to hold a thick atmosphere, and Kepler-51d appears to lack that core entirely. It orbits at roughly Venus's distance from its star, close enough that stellar radiation and winds should have stripped its atmosphere over 500 million years — yet there it sits, fat and fluffy and opaque.
"What is it about this system that created these three really oddball planets, a combination of extremes that we haven't seen anywhere else?" Libby-Roberts said.
The team modeled three explanations for the low density: a hydrogen-helium envelope comprising more than 30% of the planet's mass, a high-altitude haze layer, or a tilted ring system. Their atmospheric retrieval best fits a low-metallicity gas envelope with high-altitude haze particles spanning pressures of 1 to 100 microbars. A tilted ring could also produce the signal, but the estimated lifetime of such rings is roughly 0.1 million years — cosmically brief, and requiring very specific composition and geometry to work.
One wrinkle: a separate research team is simultaneously publishing JWST observations of Kepler-51b, another super-puff in the same system. Two independent programs, two planets, the same star. Both teams are left with more questions than answers.
"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."
JWST's Mid-Infrared Instrument, which observes at longer wavelengths, may eventually penetrate the haze. For now, the view remains closed.