Universe Today
There are some strange types of exoplanets out there with no counterparts in our Solar System. One of those types is super-puff planets. These oddballs have radii larger than Neptune, but only have a few Earth masses. This means they have large volumes and low density. How this peculiar type of exoplanet forms is unclear, and current models of gas giant formation can't account for them.
Kepler-51 is 500 million year old Sun like star about 2620 light-years away that hosts 3 super-puff planets. One of them, Kepler-51d, is the coolest and the least dense of the three. It's the subject of new research in The Astronomical Journal. In it, the researchers test the three hypotheses that try to explain Kepler-51d, and super-puffs in general.
The research is "The James Webb Space Telescope NIRSpec-PRISM Transmission Spectrum of the Super-puff, Kepler-51d," and the lead author is Jessica Libby-Roberts. Libby-Roberts is from the Department of Astronomy & Astrophysics and the Center for Exoplanets and Habitable Worlds, both at Pennsylvania State University.
“We think the three inner planets orbiting Kepler-51 have tiny cores and huge atmospheres giving them a density akin to cotton candy,” said lead author Libby-Roberts in a press release. “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!”
Retaining a large puffy atmosphere requires a massive core with enough gravity to prevent the atmosphere from being stripped away. Typically, these types of planets are also further from their stars, which also makes it more difficult for the star to remove their atmospheres. But Kepler-51d is only as far from its star as Venus is from the Sun. And since Kepler-51 is young, only about 500 million years old, it's more active than older stars like 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,” said Libby-Roberts. “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?"
Kepler-51d is one of the lowest-density examples of this type of exoplanet, along with being the coolest one in the system. It's planetary mass is about 5.6 Earth masses, and its radius is about 9.3 Earth radii. That means that it's nearly 10 times Earth's radius but just over 5 times Earth's mass. A planet this large, this light, and this cold defies our understanding of planet formation. The authors write that "... the observed properties of this planet are not readily explained by most planet formation theories."
The exoplanet's characteristics make it a valuable scientific target to test the different hypotheses that try to explain super-puffs.
As the study's title makes clear, this research is based on the JWST's NIRSpec instrument. When NIRSpec captured the transmission spectrum from Kepler-51d's atmosphere, it was featureless. There were no strong signatures of molecular absorption. The spectrum looks like an unremarkable slope.
This is Kepler-51d’s transmission spectrum observed with JWST/NIRSpec-PRISM covering 0.6–5.3 μm. This range of wavelengths is typically rich with chemical fingerprints. If they were present and detectable, molecules like H2O, CO2, and NH3 would be visible in the spectrum. The problem is that haze can obscure all of these features, creating the slope in the spectrum. Image Credit: Libby-Roberts et al. 2026. AnJ.
"At 350 K, we expect to observe a rich assortment of molecular features (methane, water, carbon dioxide, and ammonia) assuming an aerosol-free chemical equilibrium atmosphere for Kepler-51d—notably given its extreme scale height of ∼1700 km. Instead, the lack of any clear detectable features in an extended H/He-rich atmosphere between 0.6 and 5.3 μm is a first for JWST," the researchers write. But some molecules containing carbon, oxygen, nitrogen and other chemicals must be present to seed the formation of the haze.
There are three working hypotheses that try to explain super-puffs like Kepler-51d.
The first is that the planet has a massive Hydrogen/Helium envelope. Planets don't typically retain these atmosphere because they're too light. The loss of these atmospheres explains the observed Fulton gap, or radius gap, in the exoplanet population. While the exoplanet's atmospheric spectrum is featureless, forward modelling shows that it's likely to have low-metallicity for several reasons, which supports the H/He envelope hypothesis. But to retain these atmosphere, scientists think a planet has to be massive and not too close to its star, which goes against this hypothesis.
The second hypothesis is that Kepler-51d has high-altitude photochemical hazes. This is consistent with sub-micron sized haze particles in the exoplanet's upper atmosphere. Spectra from other super-puffs show the same thing. Since hazes act to block out any molecular features in the spectrum, the JWST's results support this.
The third is that the planet actually has a ring system that's tilted toward us. That would make the planet appear to be larger than it is. In turn, that would make the density seem much lower. The researchers found that a ring system can fit the data, but that it would have to be a very short-lived system. This is because the planet is so close to its star that any ring system would be unstable. Since the planet is only about 500 million years old, and the ring system could only survive for about 100,000 years, it means we would be very fortunate to be observing it at just the right time for a ring system to exist. This is a very low probability, and is why the researchers don't favour this explanation.
Rings are made of dust and would also block light in a consistent pattern. “Instead, we see a linear trend, with more light being blocked at longer wavelengths,” Libby-Roberts said.
*This artist's illustration shows a super-puff planet. With low masses and large radii, they defy our models of planet formation. Image Credit: By Pablo Carlos Budassi - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=136006077*
The researchers conclude that the high-altitude photochemical hazes hypothesis fits the evidence best.
“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 study co-author Suvrath Mahadevan. Mahadevan is a Professor of Astronomy and Astrophysics in the Penn State Eberly College of Science. “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.”
Lead author Libby-Roberts echoed Mahadevan's comments. "Rings would have to be short-lived, composed of very particular materials, and situated in just the right angle, which seems unlikely, but we can’t completely rule it out. If we could observe the planet at even longer wavelengths, such as with JWST’s Mid Infrared Instrument, we might be able to detect the materials that would be in a ring or see the full extent of the haze layer.”
Missions like Kepler TESS have shown us how diverse the exoplanet population is. Our models of planet formation took shape mostly based on what we see in the Solar System. But they're being tested by the discovery of super-puff planets like Kepler-51d.
“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, and we have these alien worlds that really challenge our understanding of planet formation. We haven’t found a solar system like ours yet, and being able to explain how all these different planets formed helps us understand how we fit into the big picture and our place in the universe.”
Without detailed knowledge of Kepler-51d's composition and structure, researchers can't explain how the super-puff formed. But the JWST's NIRSpec spectrum can help determine rule out some scenarios and constraining others. The next step is to examine the other super-puff planets in the system, with both NIRSpec and MIRI.
"Future observations other super-puff planets in the Kepler-51 system with JWST could provide additional insights into how these planets (Kepler-51d included) formed and whether they all possess a substantial haze layer," the researchers write. "For now, Kepler-51d is the only known planet with a featureless sloped JWST transmission spectrum spanning 0.6–5.3 μm," they conclude.
