Kepler-51, a young G-dwarf star, is orbited by three exoplanets classified as ‘super-puffs’ and one low-mass planet that does not transit. Among these, Kepler-51d stands out as the system’s coolest transiting planet and one of the least dense super-puffs discovered to date; its atmosphere, obscured by an exceptionally thick haze, presents a profound enigma regarding its formation.
This illustration depicts Kepler-51 and its three giant planets. Image credit: NASA / ESA / L. Hustak, J. Olmsted, D. Player & F. Summers, STScI.
Situated approximately 2,615 light-years distant, Kepler-51 resides within the constellation Cygnus.
This stellar system, also identified as KOI-620, is host to four exoplanets, a notable portion of which—at least three—are characterized as Saturn-sized celestial bodies known as ‘super-puffs’.
Kepler-51d distinguishes itself as the planet with the lowest temperature and least density within this planetary configuration.
“We surmise that the three inner planets revolving around Kepler-51 possess diminutive cores enveloped by substantial atmospheres, affording them a density comparable to cotton candy,” stated Dr. Jessica Libby-Roberts, an astronomer affiliated with the University of Tampa.
“Such planets with ultra-low densities, or ‘super-puffs,’ are exceedingly uncommon and challenge our established paradigms of gas giant formation.”
“And to find not just one, but three of these objects in a single system presents an even greater explanatory hurdle!”
Conventional gas giants are characterized by a dense central core, which generates a potent gravitational field capable of attracting and retaining vast amounts of gas.
These planets typically coalesce in regions more remote from their parent star, where the star’s gravitational influence also plays a role in gas accretion, mirroring the positioning of our solar system’s gas giants beyond the asteroid belt.
However, Kepler-51d lacks a dense core, and its orbital distance from its star is roughly equivalent to Venus’s proximity to the Sun.
“Kepler-51 is a relatively energetic star, and its stellar winds ordinarily possess sufficient force to dissipate the gaseous envelopes of planets like this one; however, the degree of such mass loss over the evolutionary history of Kepler-51d remains indeterminate,” Dr. Libby-Roberts commented.
“The possibility exists that this planet originated further out and subsequently migrated inwards, yet this scenario still leaves us with a multitude of unresolved questions concerning the genesis of this planet—and indeed, all the planets within this system.”
“What specific characteristics of this system have facilitated the formation of these three exceptionally anomalous planets, a confluence of extreme properties not previously observed elsewhere?”
Given their exceedingly low densities, the research team posits that these super-puff planets comprise a significant proportion of the lightest elements, namely hydrogen and helium; however, they also anticipate the presence of additional elemental constituents.
By delineating the elemental makeup of Kepler-51d’s atmosphere, scientists canFurthermore, glean insights into the environmental conditions and the specific locus where the planet initially formed.
While direct imaging of planets at such distances from Earth is unattainable, astronomers can analyze the starlight, observing a dimming effect as a planet traverses the face of its host star from our vantage point.
“A star’s emitted light undergoes filtration as it percolates through the planet’s atmosphere before reaching our observational instruments,” explained Dr. Libby-Roberts.
“Should a particular molecule be present within the atmosphere that absorbs specific wavelengths of light—akin to how objects of different colors on Earth absorb distinct light wavelengths—it can effectively occlude the light at that particular spectral band.”
“By examining this phenomenon across a spectrum of wavelengths, we can discern a unique atmospheric fingerprint that reveals the planet’s composition.”
Dr. Libby-Roberts and her collaborators had previously conducted observations of Kepler-51d utilizing the NASA/ESA Hubble Space Telescope, which focused on near-infrared wavelengths ranging from approximately 1.1 to 1.7 microns.
The advanced capabilities of the Near-Infrared Spectrograph (NIRSpec) aboard the NASA/ESA/CSA James Webb Space Telescope have enabled the scientific team to extend their observational range up to 5 microns, thereby offering the potential for a more detailed atmospheric spectral analysis.
Nevertheless, no definitive attenuation of stellar intensity at specific wavelengths was detected.
“Our hypothesis is that the planet possesses such an extensive atmospheric haze layer that it is absorbing the wavelengths of light we examined, thereby obscuring any discernible features beneath,” commented Penn State Professor Suvrath Mahadevan.
“This scenario bears a strong resemblance to the haze observed on Saturn’s largest moon, Titan, which contains hydrocarbons such as methane, but on a vastly magnified scale.”
“Kepler-51d appears to be enshrouded in an immense quantity of haze—almost equivalent to Earth’s radius in extent—which would represent one of the most substantial hazes documented on an exoplanet to date.”
The findings were disseminated this week in the prestigious 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
