NASA finds a hidden planet in the Kepler-51 system

An extraordinary planetary system, Kepler-51, continues to intrigue astronomers with its unique characteristics. Researchers at Penn State and Osaka Universities have discovered a fourth planet in this system.

The Kepler-51 system was already known for its three ultra-low-density “super-puff” planets, but this latest finding adds another layer of complexity.

The team identified the fourth planet, Kepler-51e, after observing unexpected transit timing variations (TTVs) in the system. This discovery challenges existing planetary formation theories and opens up new questions about the dynamics and evolution of this peculiar system.

Understanding super-puff planets

Super-puff planets are distinctly characterized by their incredibly low mass and density. They are roughly the size of Saturn but only a few times the mass of Earth, making them as light and fluffy as cotton candy.

The super-puffs in the Kepler-51 system are believed to have tiny cores and vast atmospheres of hydrogen and helium.

They are a cosmic enigma that has astronomers questioning how they formed and how their atmospheres survive the intense radiation from their star. The discovery of a fourth planet adds more intrigue to these questions.

A surprise in Kepler-51

The research team, led by Jessica Libby-Roberts, a postdoctoral fellow at the Center for Exoplanets and Habitable Worlds at Penn State, ventured to study Kepler-51d using NASA’s James Webb Space Telescope.

However, the planet decided to steal the show by passing in front of its star two hours earlier than anticipated – an event that nearly caught the researchers off guard.

This unexpected transit time suggested another player in the system: a fourth hidden planet.

Planetary transits and their secrets

The transit of a planet, when it passes in front of its star as observed from Earth, can reveal a wealth of information. The duration and magnitude of the star’s dip in brightness help astronomers to deduce the planet’s size and other properties.

Occasionally, a planet may transit earlier or later due to the gravitational pull from other planets, resulting in transit timing variations.

These minor discrepancies are usually accounted for in models to accurately predict when planets will transit.

An observational twist in Kepler-51

Despite their accurate predictions for the transit of Kepler-51b in May 2023, the researchers were thrown for a loop by Kepler-51d. This planet was expected to transit around 2 a.m. EDT in June 2023.

But it made its grand entrance two hours early, which challenged the researchers to revise their models.

“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,” said Kento Masuda, associate professor of Earth and Space Science at Osaka University and co-first author of the paper.

“Only adding a fourth planet explained this difference. This marks the first planet discovered by transit timing variations using JWST.”

To better understand the Kepler-51 system, the researchers adopted a “brute force” search. They tested different combinations of planet properties to construct a four-planet model that explained all of the transit data collected over 14 years.

This rigorous approach led them to identify the fourth planet, Kepler-51e.

Details of Kepler-51e

Kepler-51e’s existence and properties compelled the team to adjust their models and rethink the expected masses of the other planets in the system.

However, without direct observation of Kepler-51e’s transit, it’s unclear whether this newest family member is also a super-puff.

Although Kepler-51e is currently stealing the spotlight, Libby-Roberts noted that there’s a lot more to discover in this planetary system.

“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,” said Libby-Roberts.

“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.”

Road ahead with Kepler-51 and its planets

The researchers are now analyzing the remaining James Webb Space Telescope data.

The findings could provide information about the atmosphere of Kepler-51d and enhance our understanding of how ultra-low density super-puff planets are formed.

The mysteries of the universe, and the super-puff planets of Kepler-51, continue to intrigue and captivate the minds of researchers.

The study is published in The Astronomical Journal.

Image Credit: NASA, ESA, and L. Hustak, J. Olmsted, D. Player and F. Summers (STScI)

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