Kepler-51 is a young G dwarf star located approximately 2,615 light-years away in the constellation Cygnus. It remarkably hosts three superpuffs and one low-mass non-transiting exoplanet, making it a fascinating subject in the field of astronomy. Among these, Kepler-51d stands out as the system’s coolest transiting planet and is recognized as one of the least dense superpuffs discovered to date. However, it is enveloped in an extremely dense atmospheric fog, leaving scientists pondering its origin and formation.
This diagram illustrates Kepler-51 and its three giant planets. Image credit: NASA / ESA / L. Hustak, J. Olmsted, D. Player & F. Summers, STScI.
Also referred to as KOI-620, Kepler-51 is an intriguing star hosting four exoplanets, out of which at least three are classified as Saturn-sized “superpuff” worlds. Kepler-51d is the coolest and least dense planet within this unique system.
Jessica Libby Roberts, an astronomer at the University of Tampa, notes, “The three inner planets orbiting Kepler-51 have small cores and large atmospheres that are thought to possess densities similar to cotton candy.”
She adds, “These ultra-low-density, super-puff planets are rare and challenge conventional theories regarding the formation of gas giant planets. It’s not just one but three in this system, raising critical questions.”
Typically, gas giant planets boast dense cores, resulting in strong gravitational forces that attract and retain gas. They usually form farther from their stars, much like our solar system’s gas giants, which reside beyond the asteroid belt.
In a twist, Kepler-51d does not feature a dense core and orbits similarly to the distance Venus is from the Sun.
Interestingly, Dr. Libby Roberts mentions, “Kepler-51 is a relatively active star, and its stellar wind should easily blow gas away from the planet. However, we are still uncertain about the extent of Kepler-51d’s mass loss over its lifespan.”
There are theories suggesting that Kepler-51d may have migrated inward after forming farther away, but numerous questions remain about the origin of this planet and its companions.
“What specific processes in this system led to the formation of these three unusual planets? It’s an extreme combination that hasn’t been observed elsewhere,” Dr. Roberts reflects.
Due to their ultra-low density, researchers suspect that these super-puff planets are rich in the lightest elements, hydrogen and helium, while also possessing other elements.
Analyzing the atmospheric composition of Kepler-51d will provide valuable insights into its formation environment.
Although direct imaging of such distant planets is not feasible, scientists can analyze the starlight that diminishes as the planet transits in front of its host star.
Dr. Libby Roberts explains, “Starlight passes through the planet’s atmosphere before reaching our telescopes. Similar to how different colors on Earth absorb various wavelengths, certain atmospheric molecules can block specific wavelengths of light.”
“By examining spectral data, we can create a unique fingerprint that reveals the atmospheric composition of the planet.”
Dr. Libby Roberts and her team previously observed Kepler-51d using the NASA/ESA Hubble Space Telescope, focusing on near-infrared wavelengths of approximately 1.1 to 1.7 microns. Enhanced technology in the Near Infrared Spectrometer (NIRSpec) aboard the NASA/ESA/CSA James Webb Space Telescope has permitted scientists to extend observations down to 5 microns, potentially offering a more detailed atmospheric “fingerprint.”
However, no obvious decrease in the star’s intensity was detected at any specific wavelength. Subrath Mahadevan, a professor at Pennsylvania State University, states, “The thick fog surrounding the planet absorbs the observed wavelengths, obscuring underneath features.”
He compares it to the haze found on Saturn’s largest moon, Titan, which contains hydrocarbons like methane, but on a much grander scale.
“Kepler-51d seems to possess a massive fog layer, nearly equivalent to Earth’s radius—one of the largest we’ve seen alongside our planet.”
For a detailed exploration of this groundbreaking research, please visit the result published in this week’s Astronomy Magazine.
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Jessica E. Libby Roberts et al. 2026. NIRSpec-PRISM transmission spectrum of the James Webb Space Telescope Superpuff, Kepler-51d. A.J. 171, 221; doi: 10.3847/1538-3881/ae33c0
Source: www.sci.news
