An unusual system with three ultra-low-density planets, also called a ‘superballoon,’ has at least one more planet, according to astronomers at Penn State and Osaka University.
The research team set out study Kepler-51d, the third planet in the systemwith NASA’s James Webb Space Telescope (JWST), but they almost missed the opportunity when the planet unexpectedly passed in front of its star two hours earlier than predicted by the models.
After examining new and archival data from a variety of space and ground-based telescopes, Researchers discovered that the best explanation is the presence of a fourth planetwhose gravitational attraction affects the orbits of the other planets in the system.
The discovery of the new planet is detailed in an article that appears in The Astronomical Journal.
“Superbright planets are very unusual in that they have very low mass and density,” said Jessica Libby-Roberts, a postdoctoral researcher at Penn State’s Center for Exoplanets and Habitable Worlds and co-senior author of the paper.
A density like cotton candy
“The three previously known planets orbiting 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. We think they have tiny nuclei and huge atmospheres of hydrogen and heliumbut how these strange planets formed and how their atmospheres have not been destroyed by the intense radiation of their young star remains a mystery. “We plan to use JWST to study one of these planets to help answer these questions, but now we have to explain a fourth low-mass planet in the system.”
When a planet passes in front of (or transits) its star when viewed from Earth, it blocks some of the star’s light, causing a slight decrease in the star’s brightness. The duration and amount of that decline give clues to the planet’s size and other characteristics.
Planets transit when they complete an orbit around their star, but sometimes they transit a few minutes earlier or later because the gravity of other planets in the system pulls on them. These small differences are known as transit time variations and are built into astronomers’ models to allow them to accurately predict when planets will transit.
The researchers said they had no reason to believe that the three-planet model of the Kepler-51 system was inaccurate, and they successfully used the model to predict Kepler-51b’s transit time in May 2023 and followed it up with the Apache telescope. Point Observatory (APO) to observe it in time.
“Also We are trying to use the Penn State Davey Lab telescope to observe a transit of Kepler-51d in 2022, but some poorly timed clouds blocked our view just as the transit was predicted to begin,” Libby-Roberts said. “We may have realized something was wrong at the time, but we had no reason to suspect that Kepler-51d would not transit as expected when we planned to observe it with JWST.”
The team’s three-planet model predicted that Kepler-51d would transit around 2 a.m. EDT in June 2023, and researchers prepared to observe the event with both JWST and APO.
“Thank goodness we started observing a few hours earlier to establish a baseline, because 2 a.m. came, then 3 a.m., and we still hadn’t observed a change in the star’s brightness with APO,” Libby-Roberts said. “After frantically rerunning our models and analyzing the data, We discovered a slight drop in stellar brightness immediately when we started observing with APOwhich ended up being the start of the transit, 2 hours earlier, which is well beyond the 15-minute uncertainty window of our models.”
When the researchers analyzed the new data from APO and JWST, they confirmed that they had captured the transit of Kepler-51d, although considerably earlier than expected.
“We were really puzzled by the early appearance of Kepler-51d, and no adjustment of the three-planet model could explain such a large discrepancy,” said Kento Masuda, associate professor of Earth and space sciences at Osaka University and co-author. main article. “Only the addition of a fourth planet explained this difference. This is the first planet discovered by variations in transit time using JWST.”
To help explain what is happening in the Kepler-51 system, The research team reviewed previous transit data from NASA’s Kepler space telescope and NASA’s Transiting Exoplanet Survey Satellite (TESS).. They also made new observations of the system’s inner planets, including with the Hubble Space Telescope and the California Institute of Technology’s Palomar Observatory telescope, and obtained archival data from several ground-based telescopes.
As the new planet, Kepler-51e, has not yet been observed in transit (perhaps because it does not pass in the line of sight between its star and Earth), the researchers noted how important it was to obtain as much data as possible to support their new models.
“We perform what is called a ‘brute force’ search, trying many different combinations of planetary properties to find the four-planet model that explains all the transit data collected over the past 14 years,” Masuda said.
“We found that the signal is best explained if Kepler-51e has a similar mass to the other three planets and follows a fairly circular orbit of about 264 days, something we would expect based on other planetary systems. Other possible solutions we found involve a more massive planet in a wider orbit, although we think they are less likely.”
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