Astronomers using the James Webb Space Telescope (JWST) have unveiled a new planet in the enigmatic Kepler-51 system, a stellar neighborhood already famous for its collection of ultra-low-density “super-puff” planets. This discovery has reignited discussions about how such peculiar planets—often compared to cotton candy due to their astonishingly low densities—could form and survive in the extreme environments close to their host stars.
The newly discovered planet, named Kepler-51e, was revealed through subtle yet telling anomalies in the transit of one of the system’s previously known planets. This detection adds a new layer of complexity to an already baffling planetary system and marks a milestone in exoplanet research conducted with JWST.
Kepler-51: A Stellar System That Defies Expectations
The Kepler-51 system, located about 2,600 light-years away in the constellation Cygnus, has puzzled astronomers since its discovery. The system is home to planets classified as super-puffs, a rare type of exoplanet with densities so low they resemble cotton candy. These planets are approximately the size of Saturn but possess masses only a few times greater than Earth’s. Such a configuration indicates they have tiny solid cores surrounded by massive, hydrogen- and helium-rich atmospheres.
“Super-puff planets are very unusual in that they have very low mass and low density,” said Jessica Libby-Roberts, a postdoctoral fellow at Penn State’s Center for Exoplanets and Habitable Worlds. “The three previously known planets that orbit the star Kepler-51 are about the size of Saturn but only a few times the mass of Earth, resulting in a density like cotton candy.”
What makes the Kepler-51 system even more puzzling is the apparent stability of these planets’ massive atmospheres despite the intense radiation from their young, active star. These conditions should, in theory, strip away the atmospheres of such planets over time, yet the super-puffs remain intact. This contradiction has made the Kepler-51 system a focal point for understanding extreme planetary evolution.
Anomalies in Kepler-51d’s Orbit Lead to a New Discovery
The discovery of Kepler-51e occurred unexpectedly during observations of Kepler-51d, one of the previously known super-puff planets in the system. Using JWST, astronomers planned to observe a transit of Kepler-51d—a moment when the planet passes in front of its star from our perspective on Earth. Such transits provide vital data about a planet’s size and orbit. However, the planet’s transit happened two hours earlier than models had predicted.
“Thank goodness we started observing a few hours early,” Libby-Roberts said. “After frantically re-running our models, we discovered a slight dip in stellar brightness that matched the start of the transit—2 hours early.” This discrepancy was far beyond the typical margin of error and hinted at the gravitational influence of another planetary body in the system.
Further analysis confirmed that these unexpected timing variations, also known as Transit Timing Variations (TTVs), were caused by the gravitational pull of an unseen planet. This marks a significant achievement for JWST, as it is the first planet detected through TTVs using this groundbreaking telescope.
“Only adding a new planet explained this difference,” explained Kento Masuda, co-first author and an associate professor at Osaka University. “This marks the first planet discovered by transit timing variations using JWST.
What We Know About Kepler-51e
Preliminary data suggests that Kepler-51e has an orbital period of about 264 days, placing it just inside the star’s habitable zone, where conditions might support liquid water on a planet’s surface. However, researchers have not yet observed this planet transiting its star, making it difficult to determine its exact size and density. While it is uncertain whether Kepler-51e is also a “super-puff,” its presence reshapes our understanding of this system’s dynamics.
Adjusting for Kepler-51e’s gravitational influence, researchers have also refined the mass estimates of the other planets in the system. Though slightly more massive than initially thought, the three inner planets remain classified as super-puffs.
“We planned to use JWST to study one of these planets to help answer these questions, but now we have to explain a new low-mass planet in the system,” said Libby-Roberts. This unexpected discovery highlights how JWST continues to exceed expectations, not only characterizing known worlds but also uncovering hidden ones.
Unlocking the Secrets of the Kepler-51 System
The Kepler-51 system remains one of the most intriguing collections of planets discovered to date. The presence of multiple super-puff planets challenges existing models of planetary formation and raises new questions about how these planets have managed to retain their enormous atmospheres despite the intense radiation from their host star. Adding to this complexity, Kepler-51e deepens the mystery by suggesting that more planets could remain undetected in this system.
As researchers continue to analyze JWST’s data, they are particularly focused on understanding the atmospheres of the super-puff planets. By studying their chemical compositions, scientists hope to determine whether these planets formed closer to their star or migrated inward from colder regions of the system.
“Kepler-51e has an orbit slightly larger than Venus and is just inside the star’s habitable zone, so a lot more could be going on beyond that distance if we take the time to look,” Libby-Roberts said. The possibility of additional, more distant planets in the system could help refine theories about planet formation and evolution.
