Kepler-51 is a 500-million-year-old G-type star hosting four low density planets, according to a new paper published in the Astronomical Journal.
This illustration depicts the Kepler-51 planetary system. Image credit: NASA / ESA / L. Hustak, J. Olmsted, D. Player & F. Summers, STScI.
Kepler-51 is located approximately 2,615 light-years away in the constellation of Cygnus.
Also known as KOI-620, this star was already known to host three Saturn-sized ‘super-puff’ exoplanets: Kepler-51b, c, and d.
First discovered by NASA’s Kepler space telescope in 2012, these worlds have orbital period ratios close to 1:2:3 (45 days, 85 days, and 130 days, respectively).
They are several times the mass of Earth, and have hydrogen/helium atmospheres.
“Super puff planets are very unusual in that they have very low mass and low density,” said Penn State astronomer Jessica Libby-Roberts.
“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.”
“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 Webb 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.”
To examine the evidence of the fourth planet in the system, Kepler-51e, the astronomers utilized an extensive transit timing data set spanning over 14 years from various facilities, including Webb, the Apache Point Observatory telescope and the Penn State Davey Lab telescope.
“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,” said Osaka University astronomer Kento Masuda.
“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.”
It is unclear if Kepler-51e is also a super puff planet, because the researchers have not observed a transit of Kepler-51e and therefore cannot calculate its radius or density.
According to the team, a wide range of masses (
“Super puff planets are fairly rare, and when they do occur, they tend to be the only one in a planetary system,” said Penn State astronomer Jessica Libby-Roberts.
“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.”
“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.”
“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.”
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Kento Masuda et al. 2024. A Fourth Planet in the Kepler-51 System Revealed by Transit Timing Variations. AJ 168, 294; doi: 10.3847/1538-3881/ad83d3
