The research team initially aimed to observe Kepler-51d, one of the three super-puff planets, with JWST. However, during their observations, the planet transited its star two hours earlier than predicted by models. "Super puff planets are very unusual in that they have very low mass and low density," explained Jessica Libby-Roberts, a postdoctoral fellow at Penn State. "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." The unexpected timing anomaly led the researchers to hypothesize a gravitational influence from a fourth planet.

Transit observations, where a planet passes in front of its star as seen from Earth, help astronomers determine a planet's size and characteristics. Typically, slight timing variations in these events are caused by gravitational interactions with other planets. The three-planet model of the Kepler-51 system had reliably predicted past transits, but Kepler-51d's deviation signaled the need for a new explanation.

Observations using telescopes, including the Apache Point Observatory (APO) and JWST, confirmed Kepler-51d's transit earlier than expected. "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."

Kento Masuda, co-first author and associate professor at Osaka University, emphasized the importance of this finding. "Only adding a fourth planet explained this difference. This marks the first planet discovered by transit timing variations using JWST," Masuda said.

The team revisited earlier data from NASA's Kepler space telescope and TESS, as well as archival and new observations from Earth-based telescopes, to refine their models. Their analysis revealed that the fourth planet, Kepler-51e, has a likely orbital period of about 264 days. Unlike the inner three planets, Kepler-51e has not been observed transiting, complicating estimates of its density.

"This system is already rare for hosting three super-puff planets," Libby-Roberts noted. "Adding a fourth planet, whether it's a super puff or not, raises new questions about how these unusual planets form and persist despite the intense radiation from their young star."

Accounting for the gravitational effects of Kepler-51e required adjustments to the estimated masses of the inner planets, slightly increasing their values while maintaining their super-puff classification. However, it remains unclear whether Kepler-51e itself shares this classification.

The discovery highlights the need for further observations, particularly to understand how Kepler-51e influences its neighbors. "Kepler-51e has an orbit just inside the star's habitable zone," Libby-Roberts added. "Continuing to study transit timing variations might help us uncover more distant planets in this system and even identify worlds that could support life."

The researchers are also analyzing JWST data to study Kepler-51d's atmosphere, which could shed light on how super-puff planets form and evolve. This international effort involved researchers from Penn State, Osaka University, and the National Astronomical Observatory of Japan, among others.

Research Report:A Fourth Planet in the Kepler-51 System Revealed by Transit Timing Variations

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