Ever had your plans thrown off by something happening 2 hours earlier than you thought it would? Confused 12- and 24-hour clocks, perhaps?

When it happened to a team of astronomers, they discovered a new planet.

The exoplanet – some 2,556 light-years away from Earth – was spotted because another planet in the same star system passed in front of its sun 2 hours earlier than predicted.

They’ve published their discovery of the exoplanet in The Astronomical Journal.

The researchers were planning to observe an exoplanet, Kepler-51d, as it transited in front of its star (Kepler-51).

The researchers knew of 3 planets already in the system: Kepler-51b, c, and d. All 3 are “super puff planets”, meaning they’re gas giants made from very thin gas.

“Super puff planets are very unusual in that they have very low mass and low density,” says co-first author Dr Jessica Libby-Roberts, a postdoctoral fellow at the Pennsylvania State University, USA.

“We think they have tiny cores and huge atmospheres of hydrogen or 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.”

Kepler-51d orbits the star every 130 days, and the researchers were planning to learn more about its weird origins by observing one of these transits.

They used the James Webb Space Telescope (JWST) and the Apache Point Observatory (APO) telescope to capture its stellar transit at 2am on 26 June 2023.

“Thank goodness we started observing a few hours early to set a baseline, because 2am came, then 3am, and we still hadn’t observed a change in the star’s brightness with APO,” says Libby-Roberts.

“After frantically re-running our models and scrutinising 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!”

The researchers turned to archival data to see what might have thrown their calculations off.

“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,” says co-first author Associate Professor Kento Masuda, from Osaka University, Japan.

They came to the conclusion that there must be a fourth planet in the system, the gravity of which was throwing Kepler-51d’s timing off.

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

The new planet – Kepler-51e – has not been directly observed passing in front of its star, like the other 3 in the system. This might be because its orbit doesn’t align with Earth in a way that allows us to see it.

“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,” says 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.”

The predicted orbit would put the planet just inside the star’s habitable zone, according to Libby-Roberts.

“Other possible solutions we found involve a more massive planet on a wider orbit, though we think these are less likely,” says Masuda.

Astronomers use transits to calculate the size and density of exoplanets, so they don’t know whether or not Kepler-51e is also a super puff planet yet. They’ll need to observe the system for longer to see if they can spot a transit.

“Super puff planets are fairly rare, and when they do occur, they tend to be the only one in a planetary system,” says 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.”

The researchers are now probing the JWST data to learn more about the atmosphere of the planet they first set out to study – Kepler-51d.