Kepler-51 is a young G-dwarf star hosting three super-puffs and one low-mass non-transiting exoplanet. Kepler-51d, the coolest transiting planet in this system and also one of the lowest-density super-puffs known to date, appears cloaked in the thickest atmospheric haze yet observed, leaving astronomers uncertain about its origins.
This illustration depicts Kepler-51 and its three giant planets. Image credit: NASA / ESA / L. Hustak, J. Olmsted, D. Player & F. Summers, STScI.
Kepler-51 is located approximately 2,615 light-years away in the constellation of Cygnus.
Also known as KOI-620, the star hosts four exoplanets, at least three of which are Saturn-sized ‘super-puff’ worlds.
Kepler-51d is the coolest and least dense of the planets in this planetary system.
“We think the three inner planets orbiting Kepler-51 have tiny cores and huge atmospheres giving them a density akin to cotton candy,” said Dr. Jessica Libby-Roberts, an astronomer at the University of Tampa.
“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!”
Gas giants typically have a dense core and therefore a strong gravitational pull that attracts and retains gasses.
These planets typically form further away from their star, whose gravitational pull also attracts gasses, much like the gas giants of our solar system are located outside the asteroid belt.
But Kepler-51d doesn’t have a dense core, and its distance to its star is approximately the same distance as Venus is from 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,” Dr. Libby-Roberts said.
“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?”
Because of the ultra-low densities, the researchers believe these super-puff planets have a substantial amount of the lightest elements, hydrogen and helium, but they also expect to find other elements.
By clarifying the elements that make up the atmosphere of Kepler-51d, they can also infer information about the environment and location in which the planet formed.
Although planets at these distances cannot be directly imaged from Earth, they can observe the star’s light, which dims when a planet passes in front of its host star from the perspective of Earth.
“A star’s light is filtered through the atmosphere of the planet before it reaches our telescopes,” Dr. 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.”
Dr. Libby-Roberts and her colleagues previously observed Kepler-51d with the NASA/ESA Hubble Space Telescope, which observed near-infrared wavelengths of about 1.1 to 1.7 microns.
The improved technology of the Near-Infrared Spectrograph (NIRSpec) aboard the NASA/ESA/CSA James Webb Space Telescope allowed the scientists to extend observations to 5 microns, with the potential to provide a more nuanced atmosphere ‘fingerprint.’
However, they did not see any clear dips in the star’s intensity at specific wavelengths.
“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 Professor Suvrath Mahadevan.
“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.”
The results were published this week in the Astronomical Journal.
_____
Jessica E. Libby-Roberts et al. 2026. The James Webb Space Telescope NIRSpec-PRISM Transmission Spectrum of the Super-puff, Kepler-51d. AJ 171, 221; doi: 10.3847/1538-3881/ae33c0
