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Yale astronomers may have discovered the origin story for one of the universe's most dazzling phenomena—the double hot Jupiter—as well as a plan to find more of them.

Hot Jupiters are large, intensely hot planets (upwards of 3,000°F) that are roughly the size of Jupiter or Saturn. They orbit so close to their home star that, for them, a trip around the sun can be completed in as little as less than a day.

The 1995 discovery of the first hot Jupiter (known as 51 Pegasi b) earned two Swiss astronomers the Nobel Prize in physics in 2019 and led to a wave of exoplanet discoveries that continues to the present day.

Hot Jupiters are a rare class of planets, found orbiting only about 1% of stars. Rarer still are double hot Jupiters. They occur in binary systems—solar systems with two stars orbiting one another—with a single hot Jupiter forming around each of the stars.

In a in The Astrophysical Journal, Yale astronomer Malena Rice and her team show that the normal, long-term evolution of binary star systems can lead naturally to the formation of a hot Jupiter around each star.

It is a process formally known as von Zeipel-Lidov-Kozai (ZLK) migration. ZLK suggests that over a long timescale, a planet with an unusual orbit or angle can be influenced by the gravity of a somewhat distant secondary object. In this case, Rice said, the mechanism can create hot Jupiters.

"The ZLK mechanism is a dance of sorts," said Rice, an assistant professor of astronomy in Yale's Faculty of Arts and Sciences. "In a binary system, the extra star can shape and warp planets' orbits, causing the planets to migrate inward.

"We show how planets in binary systems can undergo a mirrored migration process, so that both stars end up with hot Jupiters."

For the study, the researchers ran numerical simulations to show the evolution of two stars and two planets in a binary system configuration, said Yurou Liu, a rising senior in Yale College and the study's first author.

"With the right code and enough computing power, we can explore how planets evolve over billions of years—movements that no human could watch in a lifetime, but that still could leave imprints for us to observe," Liu said.

In the case of hot Jupiters, Liu said their very existence challenges previous conceptions of planet formation, making their formation mechanisms even more interesting. "We would expect giant planets to form far away from their host stars," she said. "This makes hot Jupiters both accessible and mysterious—and a worthwhile subject to study."

The research team also came up with a suggestion for where to look for double hot Jupiters: the dozens of already-identified hot Jupiters that reside in star systems with a nearby second star.

"Our proposed mechanism works best when the stars are at a moderate separation," said Tiger Lu, who earned a Ph.D. in astrophysics at Yale this spring and is co-author of the study. "They need to be far enough apart that giant planets are still expected to form around each star, but close enough together for the two stars to influence each other during the system lifetime."