Exactly how planets end up so close to their stars remains unknown, but this world's strange orbit may hold clues.
This artist's impression shows a Jupiter-like exoplanet that is on its way to becoming a hot Jupiter—a large, Jupiter-like exoplanet that orbits very close to its star. Credit: NOIRLab/NSF/AURA/J. da Silva Credit: NOIRLab/NSF/AURA/J. da Silva
Hot Jupiters — massive worlds orbiting close to their host star — probably weren't always so hot. Astronomers think these exoplanets formed cold, in the outer regions of their star systems, then migrated inward, toward their star. Researchers have discovered a young planet in the process of doing just that, on its way to becoming a fully-fledged hot Jupiter.
The planet, dubbed TIC 241249530 b, is in a tight elliptical orbit that approaches its host star before rapidly moving away. To put the orbit into perspective, the planet passes its sun at a distance about ten times closer than that between Mercury and our Sun, before passing the distance of Earth's orbit and then returning again, all in about 167 days.
That orbit is likely evolving, too, changing the planet from cold to hot. “We think that when this planet formed, it would have been a frigid world. And because of the dramatic orbital dynamics, it will become a hot Jupiter in about a billion years, with temperatures of several thousand[degrees]. So it’s a big change from where it started,” said Sarah Millholland, a co-author of the study and an astrophysicist at MIT, in a paper published in the journal Astrophysics. Press release.
An unusual orbit
Not only is the shape of TIC 241249530 b's orbit interesting, but the planet also orbits Retrograde, or in the opposite direction to that of its host star. This is unlike the planets in our solar system, which all orbit in the same direction as the Sun's rotation.
After running simulations on the planet, researchers determined that its orbit is evolving toward that of a hot Jupiter through a process called high-eccentricity migration. This process wobbles and shrinks the planet's orbit through interactions with another planet or star(AK2) In the case of TIC 241249530 b, its host star orbits a second star as part of a binary system. And interactions between the orbits of the planet and its host star have been moving the likely future hot Jupiter ever closer to its host over time. The team described these orbits interacting with each other as if someone were simultaneously spinning two hula hoops — one around their wrist and the other around their waist.
Related: Hot Jupiters could form close to their stars
Very few young and hot Jupiters
The team estimates that within a billion years, TIC 241249530 b will orbit its star in a more circular orbit every few days. When this happens, it will officially be considered a hot Jupiter.
The planet was first discovered by NASA's Transiting Exoplanet Survey Satellite (TESS) in January 2020. Prior to this, only one other early hot Jupiter, HD 80606 b, was known.
“It’s clear from not just this study, but from other statistical studies as well, that high-eccentricity migration should account for a fraction of hot Jupiters,” Millholland said. “This system highlights how incredibly diverse exoplanets can be. They’re other mysterious worlds that can have crazy orbits that tell a story of how they got that way and where they’re headed. In the case of this planet, its journey isn’t quite over yet.”
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