Artist’s impression of the Sun-like star Kepler 51 and three giant planets that NASA’s Kepler space telescope discovered in 2012-2014.
New research suggests that the super-bloated planet Kepler-51d has a layer of thick photochemical haze that absorbs wavelengths of light and prevents us from observing its composition.
There are some strange types of exoplanets out there, with no counterparts in our Solar System. One of these types is super-swollen planets. These peculiar bodies have radii larger than Neptune’s, but only a few Earth masses. This means they have large volumes and low density. How this peculiar type of exoplanet forms is still unclear, and current models of gas giant formation cannot explain them.
Kepler-51 is a 500-million-year-old Sun-like star located about 2,620 light-years away that hosts three super-bloated planets. One of them, Kepler-51d, is the coldest and least dense of the three. It’s the subject of new research in The Astronomical Journal. In it, researchers test three hypotheses that try to explain Kepler-51d and super-swollen planets in general.
The investigation, titled “NIRSpec-PRISM Transmission Spectrum from the James Webb Space Telescope of the Ignited Superplanet Kepler-51d”, is led by Jessica Libby-Roberts. Libby-Roberts is from the Department of Astronomy and Astrophysics and the Center for Exoplanets and Habitable Worlds, both at Pennsylvania State University.
“We believe that the three inner planets orbiting Kepler-51 have tiny cores and huge atmospheres, giving them a density similar to cotton candy” said lead author Libby-Roberts in a press release. “These ultra-low-density flammable superplanets are rare and defy conventional understanding of how gas giants form. And, as if explaining the formation of one wasn’t difficult enough, This system has three!”
To maintain a dense, flammable atmosphere, a massive core with enough gravity is needed to prevent the atmosphere from being stripped away. Typically, these types of planets are also further away from their starswhich also makes it difficult for the star to remove their atmospheres. But Kepler-51d is at a distance from its star equivalent to the distance between Venus and the Sun. And because Kepler-51 is young, only about 500 million years old, it is more active than older stars like the Sun.
“Kepler-51 is a relatively active star, and its stellar winds should easily disperse gases from this planet, although the extent of this mass loss over Kepler-51d’s lifetime remains unknown,” Libby-Roberts said. “It is possible that the planet formed further away and moved inward, but we still have a lot of questions about how this planet — and the other planets in this system — formed. What is it about this system that created these three peculiar 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, in addition to being the coldest in the system. Its planetary mass is approximately 5.6 land massesand its radius is about 9.3 Earth radii. This means it has almost 10 times the radius of Earth, but just over 5 times the Earth’s mass. A planet so big, so light and so cold challenges our understanding of planetary formation. The authors write that “…the observed properties of this planet are not easily explained by most theories of planetary formation.”
The exoplanet’s characteristics make it a valuable scientific target to test the different hypotheses that try to explain the super-bloated.
As the study title makes clear, this research is based on JWST’s NIRSpec instrument. When NIRSpec captured the transmission spectrum of Kepler-51d’s atmosphere, this did not present notable characteristics. There was no strong evidence of molecular absorption. The spectrum resembles a common slope.
“At 350 K, we expect to observe a rich variety of molecular features (methane, water, carbon dioxide, and ammonia), assuming an aerosol-free chemical equilibrium atmosphere for Kepler-51d—remarkably given its extreme scale height of approximately 1700 km. Instead, the absence of any detectable characteristics “Clear light in an extensive H/He-rich atmosphere between 0.6 and 5.3 μm is unprecedented for the JWST,” the researchers write. But some molecules containing carbon, oxygen, nitrogen and other chemical compounds must be present to initiate haze formation.
There are three working hypotheses that try to explain this super swelling.
The first is that the planet has a massive hydrogen/helium shell. Planets do not normally retain these atmospheres because they are too light. The loss of these atmospheres explains the observed Fulton gap, or radius gap, in the exoplanet population. Although the exoplanet’s atmospheric spectrum is homogeneous, prospective modeling shows that it probably has a low metallicity for several reasons, which corroborates the H/He envelope hypothesis. However, to retain this atmosphere, scientists believe that a planet needs to be massive and not be too close to its star, which contradicts this hypothesis.
The second hypothesis is that Kepler-51d has photochemical hazes at high altitudes. This is consistent with the presence of submicron fog particles in the exoplanet’s upper atmosphere. The spectra of other superswells show the same pattern. Since the fogs act by blocking any molecular features in the spectrum, the JWST results support this hypothesis.
The third hypothesis is that the planet, in reality, has a inclined ring system in our direction. This would make the planet appear larger than it actually is, which in turn would make its density appear much smaller. The researchers found that a ring system could explain 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, this means we would be very lucky to observe it at the exact time a ring system existed. This probability is very low, which is why researchers do not consider this explanation plausible.
The rings are made up of dust and would also consistently block light. “Instead, we observed a linear trendwith more light being blocked at longer wavelengths,” said Libby-Roberts.
The researchers conclude that the hypothesis of photochemical hazes at high altitudes best fits the evidence.
“We believe that the planet has a layer of fog so thick that absorbs wavelengths of light that we observe, so we can’t see the features beneath it,” said study co-author Suvrath Mahadevan. Mahadevan is a professor of Astronomy and Astrophysics at Pennsylvania State University’s Eberly College of Science. “It looks very similar to the fog we see on Titan, Saturn’s largest moon, which contains hydrocarbons like methane, but on a much larger scale. Kepler-51d appears to have an enormous amount of haze — almost the size of the Earth’s radius — which would be one of the largest ever observed on a planet.”
Lead author Libby-Roberts corroborated Mahadevan’s comments. “The rings would have to be short-lived, composed of very specific materials and located at the exact angle, which seems unlikelybut we cannot completely rule out this possibility. If we could observe the planet at even longer wavelengths, as with JWST’s Mid-Infrared Instrument, we could detect materials that would be in a ring or see the full extent of the haze layer.”
Missions like Kepler TESS have shown the diversity of the exoplanet population. Our models of planetary formation have been shaped primarily based on what we observe in the Solar System. But are being tested for the discovery of super-bloated planets like Kepler-51d.
“Before astronomers found planets outside our solar system, we thought we had a pretty good understanding of how planets formed,” Libby-Roberts said. “But we have started to find exoplanets that don’t fit into anything in our solar systemand we have these alien worlds that really challenge our understanding of planetary 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 bigger picture and what our place is in the universe.”
Without detailed knowledge of Kepler-51d’s composition and structure, researchers cannot explain how the superswell formed. But JWST’s NIRSpec spectrum can help rule out some scenarios and narrow down others. The next step is examine the system’s other super-bloatedboth with NIRSpec and MIRI.
“Future observations of other superplanets in the Kepler-51 system with JWST may provide additional information about how these planets (including Kepler-51d) formed and whether they all have a substantial haze layer,” the researchers write. “For now, Kepler-51d is the only known planet with a tilted JWST transmission spectrum without distinctive features, covering the range from 0.6 to 5.3 μm,” they conclude.
