Researchers using the James Webb Space Telescope (JWST) and the Apache Point Observatory (APO) telescope have discovered a new planet orbiting within the habitable zone of star Kepler 51.
Previous scans of the distant system identified three low-density “super puff” planets named Kepler-51a, Kepler-51b, and Kepler-51c in the star’s orbit. The newly discovered Kepler-51e is the first planet found orbiting within the star’s habitable zone.
“Kepler-51e has an orbit slightly larger than Venus and is just inside the star’s habitable zone,” explained Jessica Libby-Roberts, Center for Exoplanets and Habitable Worlds Postdoctoral Fellow at Penn State and co-first author of the paper detailing the discovery.
While the 2,556 light years between Kepler-51e and Earth make it difficult to study, the team believes that finding a planet solely through its gravitational effects on observable planets may offer a new method for locating exoplanets where life may exist.
Habitable Zone Planet Discovery by James Webb Space Telescope Was an Accident
Notably, the researchers behind the newly detected planet say their discovery was coincidental. They were initially planning to study and further characterize the properties of Kepler 51-a using the JWST and the APO when their target failed to appear at the time their simulations predicted.
“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!”
“Thank goodness we started observing a few hours early to set a baseline,” Lilly-Roberts added. Still, the presence of Kepler-51d hours before their models had predicted left the team with a problem: they had to figure out why it was late.
According to Kento Masuda, associate professor of earth and space science at Osaka University and co-first author of the paper, the team was “really puzzled by the early appearance of Kepler-51d.” Furthermore, Masuda says that no amount of fine-tuning the three-planet model could account for such a large discrepancy.
After trying various simulations, the team found that the three known planets must be experiencing a gravitational influence from something else within the system. While some models favored a fourth planet that was more massive than the others and orbited at a much farther distance, the model that best fit their readings put the new planet just within the arc of the star’s habitable zone.
“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 said. “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.”
“This marks the first planet discovered by transit timing variations using JWST,” Masuda added.
Finding Opens Up More Questions About Kepler-51 System
Although finding a planet within a star’s habitable zone offers the tantalizing possibility of finding life beyond Earth, it has also left the research team with a new set of mysteries to explain. That’s because the previously discovered planets were extremely large but had a low density, earning them the nicknames “super puff” or “cotton candy” planets.
“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 explained. “We think they have tiny cores and huge atmospheres of hydrogen of helium, but 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.”
“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!” Libby-Roberts added.
The team says that super puff planets are “fairly rare.” According to the study authors, finding three of them in the same system was always considered unusual since they tend to be the only one of their kind within a planetary system.
“If trying to explain how three super puffs formed in one system wasn’t challenging enough, now we have to explain a fourth planet,” Lilly Roberts said, “whether it’s a super puff or not. And we can’t rule out additional planets in the system either.”
The Search For Planets That Could Potentially Support Life
Next, the team says they hope to refine their calculations with additional observations to characterize the size and orbit of Kepler-51e better. Since the planet does not appear to transit its star like the other known system planets, researchers will have to rely on calculating its gravitational effects on the planets they can see. This includes looking for discrepancies in the timing of those planets’ transits.
According to Libby-Roberts, the team also hopes to look farther away from the host star in case even more hidden planets are waiting to be discovered. “A lot more could be going on beyond that distance if we take the time to look,” she said.
In the study’s conclusion, the authors note that understanding the dynamics of the planets in the Kepler-51 system will not only help understand cotton candy planets but also refine the process of looking for planets within a star’s habitable zone where life could exist.
“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,” Libby-Roberts said.
The study “A Fourth Planet in the Kepler-51 System Revealed by Transit Timing Variations” was published in the Astronomical Journal.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.