Hyderabad: Hot Jupiters are some of the most extreme planets in the galaxy. These scorching worlds are as massive as Jupiter, and they swing wildly close to their star, whirling around in a few days compared to our own gas giant’s leisurely 4,000-day orbit around the sun.
Scientists suspect, though, that hot Jupiters weren’t always so hot and in fact may have formed as “cold Jupiters,” in more frigid, distant environs. But how they evolved to be the star-hugging gas giants that astronomers observe today is a big unknown.
Now, astronomers at MIT, Penn State University, and elsewhere have discovered a hot Jupiter “progenitor” — a sort of juvenile planet that is in the midst of becoming a hot Jupiter. And its orbit is providing some answers to how hot Jupiters evolve.
The new planet, which astronomers labeled TIC 241249530 b, orbits a star that is about 1,100 light years from Earth. The planet circles its star in a highly “eccentric” orbit, meaning that it comes extremely close to the star before slinging far out, then doubling back, in a narrow, elliptical circuit. If the planet was part of our solar system, it would come ten times closer to the sun than Mercury, before hurtling out, just past Earth, then back around. By the scientists’ estimates, the planet’s stretched-out orbit has the highest eccentricity of any planet detected to date.
The new planet’s orbit is also unique in its “retrograde” orientation. Unlike the Earth and other planets in the solar system, which orbit in the same direction as the sun spins, the new planet travels in a direction that is counter to its star’s rotation.
The team ran simulations of orbital dynamics and found that the planet’s highly eccentric and retrograde orbit are signs that it is likely evolving into a hot Jupiter, through “high-eccentricity migration” — a process by which a planet’s orbit wobbles and progressively shrinks as it interacts with another star or planet on a much wider orbit.
In the case of TIC 241249530 b, the researchers determined that the planet orbits around a primary star that itself orbits around a secondary star, as part of a stellar binary system. The interactions between the two orbits — of the planet and its star — have caused the planet to gradually migrate closer to its star over time.
The planet’s orbit is currently elliptical in shape, and the planet takes about 167 days to complete a lap around its star. The researchers predict that in 1 billion years, the planet will migrate into a much tighter, circular orbit, when it will then circle its star every few days. At that point, the planet will have fully evolved into a hot Jupiter.
“This new planet supports the theory that high eccentricity migration should account for some fraction of hot Jupiters,” says Sarah Millholland, assistant professor of physics in MIT’s Kavli Institute for Astrophysics and Space Research. “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 kelvin. So it’s a huge shift from where it started.”
