Hot Jupiters are some of the most extreme planets in our galaxy.
These giant planets, similar in size to Jupiter, orbit very close to their stars, completing their orbits in just a few days.
This is a stark contrast to Jupiter in our solar system, which takes about 4,000 days to orbit the sun. Scientists believe hot Jupiters may have started as “cold Jupiters” far from their stars and then moved inward, but exactly how this happens has been a mystery.
Recently, astronomers from MIT, Penn State University, and other institutions discovered a planet that appears to be in the process of becoming a hot Jupiter.
This planet, named TIC 241249530 b, is located about 1,100 light-years from Earth. It has a highly unusual orbit, coming very close to its star before swinging far away, creating a long, elliptical path.
If it were in our solar system, it would come ten times closer to the sun than Mercury and then swing out past Earth’s orbit before circling back again. This makes its orbit the most eccentric, or elongated, ever observed.
Another unique feature of TIC 241249530 b is its “retrograde” orbit. Unlike the planets in our solar system, which orbit in the same direction as the sun spins, this planet orbits in the opposite direction of its star’s rotation.
To understand how TIC 241249530 b is evolving, the team ran simulations of its orbital dynamics. They found that the planet’s eccentric and retrograde orbit indicates it is likely undergoing “high-eccentricity migration.”
This process involves the planet’s orbit wobbling and shrinking over time due to gravitational interactions with another nearby star or planet.
In the case of TIC 241249530 b, it orbits a primary star that itself orbits a secondary star in a binary system. The gravitational pull between these two stars has gradually pulled the planet closer to its primary star over time. Currently, the planet’s orbit is elliptical and takes about 167 days to complete one orbit. The researchers predict that in about a billion years, the planet’s orbit will become much tighter and circular, turning it into a true hot Jupiter that orbits its star every few days.
“This new planet supports the theory that high eccentricity migration accounts for some hot Jupiters,” says Sarah Millholland, assistant professor of physics at MIT’s Kavli Institute for Astrophysics and Space Research. “We think that when this planet formed, it was a cold, distant world. But due to dramatic orbital changes, it will become a hot Jupiter with temperatures of several thousand degrees Kelvin in about a billion years.”
The discovery of TIC 241249530 b was first made using data from NASA’s Transiting Exoplanet Survey Satellite (TESS). TESS detects planets by looking for tiny dips in starlight as a planet passes in front of its star. Further measurements confirmed the planet’s size and its highly eccentric orbit.
Prior to this, only one other planet, HD 80606 b, was known to be an early-stage hot Jupiter. The discovery of TIC 241249530 b, with its even more extreme orbit, provides new insights into how these scorching planets form.
The researchers’ simulations showed that the planet likely started as a cold Jupiter far from its star. Over billions of years, interactions with the binary star system caused its orbit to become more stretched and eccentric. Eventually, the orbit will stabilize into a close, circular path, completing the planet’s transformation into a hot Jupiter.
“This discovery highlights how diverse exoplanets can be,” Millholland says. “They have wild orbits that tell a story of how they got that way and where they’re going. For this planet, its journey to becoming a hot Jupiter isn’t quite finished yet.”
