Using the James Webb Space Telescope (JWST), astronomers have discovered a fourth world in a strange system of ultralight "super puff" planets.
The new extrasolar planet or "exoplanet" was discovered around the sun-like star Kepler-51, located around 2,615 light-years away in the constellation of Cygnus (the Swan).
Remarkably, the new world, designated Kepler-51e, isn't just the fourth exoplanet found orbiting this star; all these other worlds are cotton-candy-like planets. That means this could be a whole system of some of the lightest planets ever discovered.
"Super puff planets are very unusual in that they have very low mass and low density," team member Jessica Libby-Roberts of Penn State's Center for Exoplanets and Habitable Worlds said in a statement. "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."
Libby-Roberts added that the team theorizes that these cotton-candy planets have tiny cores and huge, puffy atmospheres of hydrogen or helium.
"How these strange planets formed and how their atmospheres haven’t been blown away by the intense radiation of their young star has remained a mystery," she added. "We planned 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!"
Kepler-51: A sweet star system
The fourth occupant of this strange planetary system was discovered when a team led by researchers at Penn State and Osaka Universities set out to investigate the properties of its lightweight sibling, Kepler-51d.
The team was shocked when Kepler-51d appeared to cross the face of its parent star, or make a transit, two hours ahead of schedule.
Transits are useful to astronomers because when starlight streams through a planet's atmosphere, different elements in that atmosphere absorb light at characteristic wavelengths. This means they leave their "fingerprint," allowing astronomers to determine the atmosphere's composition, among other characteristics of the planet, by analyzing the wavelengths of light detected.
Astronomers are accustomed to planets making transits that are either a few minutes early or a few minutes late, and the team's calculations were uncertain by 15 minutes. However, that can't account for a two-hour error.
They were expecting Kepler-51d to transit at 2 a.m. EDT in June 2023 after successfully using their three-planet model to predict the transit of Kepler-51d in May 2023. The researchers prepared to observe the event with both JWST and the Apache Point Observatory (APO) telescope. They were shocked when the transit didn't happen as predicted, going back to their data and finding it had already occurred.
"Thank goodness we started observing a few hours early to set a baseline because 2 a.m. came, then 3, and we still hadn’t observed a change in the star’s brightness with APO," Libby-Roberts said. "After frantically re-running our models and scrutinizing the data, we discovered a slight dip in stellar brightness immediately when we started observing with APO, which ended up being the start of the transit — 2 hours early, which is well beyond the 15-minute window of uncertainty from our models!"
Turning to archival data from space and ground-based telescopes to explain why they had almost missed the transit with the JWST, the team found that the best explanation was the presence of a hitherto undiscovered world.
"We were really puzzled by the early appearance of Kepler-51d, and no amount of fine-tuning the three-planet model could account for such a large discrepancy,” team member and associate professor of earth and space science at Osaka University Kento Masuda said. "Only adding a fourth planet explained this difference. This marks the first planet discovered by transit timing variations using JWST."
This world is impacting the orbits of the other planets in the system, including Kepler-51d, explaining why it was early for its transit.
"We conducted what is called a 'brute force' search, testing out many different combinations of planet properties to find the four-planet model that explains all of the transit data gathered over the past 14 years," Masuda explained. "We found that the signal is best explained if Kepler-51e has a mass similar 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 on a wider orbit, though we think these are less likely."
How does a star gather cotton candy planets?
When the team adjusted their models of the Kepler-51 system to account for the new planet, they also had to lower the expected masses of its other planets.
This also impacts theories about the other properties of these planets and how such an unusual planetary system may have formed. The researchers need Kepler-51e to transit its star before they can confirm it is a super puff planet.
"Super puff planets are fairly rare, and when they do occur, they tend to be the only one in a planetary system,” Libby-Roberts said. "If trying to explain how three super puffs formed in one system wasn’t challenging enough, now we have to explain a fourth planet, whether it’s a super puff or not. And we can’t rule out additional planets in the system either."
Because Kepler-51e has an orbit of 264 days, more observation time for the system will be required before the researchers can be sure how the new planet's gravity impacts its sibling worlds.
"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 concluded. "Continuing to look at transit timing variations might help us discover planets that are further away from their stars and might aid in our search for planets that could potentially support life."
The team's research was published on Tuesday (Dec. 3) in the Astronomical Journal.
