A remarkable planetary system featuring three known ultra-low density “super-puff” planets now appears to have at least one additional planet, based on recent findings from a research team including experts from Penn State and Osaka University. The team intended to investigate Kepler-51d, the third planet in the system, using NASA’s James Webb Space Telescope (JWST), but they were almost caught off guard when the planet unexpectedly transited its star two hours earlier than anticipated. After analyzing both new and previous data gathered from various space and ground-based telescopes, the researchers concluded that the most plausible explanation for the observed changes is the presence of a fourth planet affecting the orbits of the others through its gravitational influence.
The newly identified planet is discussed in a paper set to be published on December 3 in the Astronomical Journal.
“Super puff planets are quite unique due to their low mass and density,” explained Jessica Libby-Roberts, a Postdoctoral Fellow at Penn State’s Center for Exoplanets and Habitable Worlds and co-first author of the study. “The three previously recognized planets orbiting Kepler-51 are comparable in size to Saturn but only a few times the mass of Earth, resulting in a fluffy, cotton candy-like density. It is believed that they possess small cores surrounded by vast atmospheres of hydrogen and helium, yet the processes behind their formation and how their atmospheres have withstood the intense radiation from their youthful star remain unclear. Our plan was to use JWST to investigate one of these planets to better understand these mysteries; however, we now have the added complexity of explaining a fourth low-mass planet in the system!”
When a planet transits, or passes in front of, its star as seen from Earth, it obstructs part of the star’s light, leading to a slight dimming effect. The duration and degree of this dimming provide insights into the planet’s characteristics, including size. Although planets typically transit on a predictable schedule, variations can occur due to gravitational interactions from other planetary bodies, causing them to transit slightly earlier or later than expected. These subtle differences are known as transit timing variations and are factored into astronomical models to enhance the accuracy of transit predictions.
The researchers had no reason to doubt the existing three-planet model for the Kepler-51 system and utilized it successfully to anticipate the transit timing of Kepler-51b in May 2023, subsequently following up with observations at the Apache Point Observatory (APO).
“We attempted to observe a transit of Kepler-51d in 2022 using the Penn State Davey Lab telescope, but our view was blocked by clouds right as the transit was scheduled to commence,” said Libby-Roberts. “We might have noticed something unusual then, but at the time, we had no reason to suspect that Kepler-51d would not transit as predicted when we aimed to observe it with JWST.”
The research team’s three-planet model suggested that Kepler-51d would transit around 2 a.m. EDT in June 2023, leading them to prepare simultaneous observations with both JWST and APO.
“Fortunately, we began our observations a few hours earlier to establish a baseline, as 2 a.m. passed, then 3 a.m., and we had yet to see a change in the star’s brightness using APO,” Libby-Roberts noted. “After quickly re-running our models and examining the data, we noticed a slight reduction in the star’s brightness immediately as we started our observations with APO, which turned out to be the onset of the transit — a full two hours early, far exceeding the 15-minute margin of uncertainty in our models!”
The team confirmed that their observations from both APO and JWST had indeed captured the transit of Kepler-51d, despite its significant deviation from the expected timing.
“The early transit of Kepler-51d left us bewildered, and no adjustments to the three-planet model could account for such a significant discrepancy,” explained Kento Masuda, an associate professor of earth and space science at Osaka University and co-first author of the paper. “The inclusion of a fourth planet was the only explanation for this anomaly. This observation signifies the first planet detected via transit timing variations using JWST.”
To elucidate the dynamics of the Kepler-51 system, the research group revisited prior transit data collected by NASA’s Kepler spacecraft and the Transiting Exoplanet Survey Satellite (TESS). They also conducted new observations of the system’s inner planets, employing instruments like the Hubble Space Telescope and the California Institute of Technology’s Palomar Observatory telescope, while retrieving archival data from several ground-based facilities. Since the new planet, designated Kepler-51e, has not yet been observed to transit — possibly because it does not align well from our perspective — the team emphasized the necessity of gathering extensive data to forge robust models.
“We undertook a ‘brute force’ investigation, testing numerous combinations of planetary characteristics to identify the four-planet model that best explains the transit data collected over the past 14 years,” Masuda stated. “Our findings suggest that the signal indicates Kepler-51e has a mass similar to the other three planets and follows a nearly circular orbit of approximately 264 days — a trait consistent with other planetary systems. Alternative hypotheses we examined included a more massive planet on a wider orbit, but we consider these less probable.”
Incorporating a fourth planet into the model also revises the estimated masses of the known planets in the system. The researchers noted that this shift could alter various inferred characteristics about these planets and clarify their formation processes. While the inner three planets are now slightly more massive than originally believed, they remain classified as super puffs. However, it is uncertain whether Kepler-51e is also a super puff since it hasn’t been experienced passing in front of its star yet, preventing calculations of its radius and density.
“Super puff planets are relatively scarce, and when they do exist, they typically are the only such planet in a system,” Libby-Roberts highlighted. “If it was challenging to understand how three super puffs could form in one system, we now face the task of explaining a fourth planet, regardless of whether it’s a super puff. We must also consider the possibility of additional planets within the system.”
The researchers believe that due to the estimated 264-day orbit of Kepler-51e, additional observations will be vital in understanding its gravitational impact — or that of any further planets — on the three inner planets of the system.
“Kepler-51e follows an orbit that is slightly larger than Venus’s and lies just inside the star’s habitable zone. There could be much more going on beyond that orbit if we make the effort to search,” Libby-Roberts remarked. “Continuing our explorations of transit timing variations may aid in the discovery of more distant planets and assist in our quest for potentially life-supporting worlds.”
The team is currently processing the remaining JWST data, which could yield insights into the atmosphere of Kepler-51d. Enhanced comprehension of the compositions and traits of the inner three planets may further clarify the formation mechanisms behind these rare ultra-low density super puff planets, the researchers concluded.
In addition to Libby-Roberts and Masuda, the team behind the Kepler-51d study includes John Livingston from the National Astronomical Observatory of Japan, who coordinated the majority of the ground-based follow-up observations, along with several ground-based observers, the Kepler-51b team, and the Palomar team.
This research was supported by NASA through grants associated with JWST and the Hubble Space Telescope. The computational aspects of this study were conducted at Penn State’s Institute for Computational and Data Sciences Advanced CyberInfrastructure.
