Astronomers have spotted a “highly eccentric” planet on its way to becoming a hot Jupiter

Hot Jupiters are some of the most extreme planets in the galaxy. These scorching worlds are as massive as Jupiter and swing wildly close to their star, turning around in a matter of days compared to our gas giant’s leisurely 4,000-day orbit around the Sun.

However, scientists believe that hot Jupiters were not always so hot and may have actually formed as “cold Jupiters” in cooler, more distant environments. But how they evolved into the star-hugging gas giants that astronomers observe today is largely unknown.

Now astronomers at MIT, Penn State University and elsewhere have discovered a hot Jupiter “progenitor” — a kind of juvenile planet that’s just becoming a hot Jupiter. And its orbit provides some answers to how hot Jupiters evolve. The findings are published in the journal Nature.

Co-authors include MIT undergraduate Haedam Im, lead author Arvind Gupta of Penn State University and the NSF NOIRLab, and collaborators from many other universities, institutions, and observatories.

The new planet, which astronomers have named TIC 241249530 b, orbits a star about 1,100 light-years from Earth. A planet orbits its star in a highly “eccentric” orbit, meaning it gets extremely close to the star before being thrown far out and then doubling back in a narrow elliptical orbit.

If the planet were part of our solar system, it would come ten times closer to the Sun than Mercury before hurtling outward, just past Earth, and then back. According to scientists’ estimates, the elongated orbit of the planet has the highest eccentricity of all the planets detected so far.

The new planet’s orbit is also unique in its “retrograde” orientation. Unlike Earth and other planets in the Solar System, which orbit in the same direction as the Sun, the new planet moves in a direction that opposes the rotation of its star.

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 in which a planet’s orbit oscillates and gradually shrinks. interacts with another star or planet in a much wider orbit.

In the case of TIC 241249530 b, scientists determined that the planet orbits a primary star that itself orbits a secondary star as part of a stellar binary system. Interactions between the two orbits—the planet and its star—caused the planet to gradually move closer to its star over time.

The planet’s orbit is currently elliptical, and the planet takes around 167 days to orbit its star. Scientists predict that in 1 billion years, the planet will migrate into a much tighter circular orbit, circling its star every few days. At this point, the planet will fully evolve 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 at MIT’s Kavli Institute for Astrophysics and Space Research.

“We think that when this planet formed, it would have been a cold world. And because of the dramatic orbital dynamics, in a billion years it will become a hot Jupiter with temperatures of several thousand kelvins. So it’s a huge shift from where it started.”

“Radical Seasons”

The new planet was first spotted in data taken by NASA’s Transiting Exoplanet Survey Satellite (TESS), an MIT-led mission that monitors the brightness of nearby stars for “transits,” or brief dips in starlight, that could signal the presence of a transiting planet. before and temporarily blocking the starlight.

On January 12, 2020, TESS captured a possible transit of the star TIC 241249530. Gupta and his Penn State colleagues found that the transit corresponded to the passage of a Jupiter-sized planet in front of the star. They then obtained measurements from other observatories of the star’s radial velocity, which estimates the star’s wobble, or the degree to which it moves back and forth in response to other nearby objects that might be pulling on the star gravitationally.

These measurements confirmed that there is a Jupiter-sized planet orbiting the star and that its orbit is highly eccentric, bringing the planet extremely close to the star before ejecting it far out.

Before this detection, astronomers knew of only one other planet, HD 80606 b, which was thought to be an early hot Jupiter. Discovered in 2001, this planet still holds the record for the highest eccentricity.

“This new planet is experiencing really dramatic starlight changes in its orbit,” says Millholland. “There must be a really radical period and a completely scorched atmosphere every time it passes close to the star.”

“Dance of the Orbits”

How could a planet fall into such an extreme orbit? And how might his eccentricity evolve over time? For answers, Im and Millholland ran planetary orbital dynamics simulations to model how the planet might have evolved over its history and how it might continue for hundreds of millions of years.

The team modeled the gravitational interactions between the planet, its star, and another nearby star. Gupta and his colleagues observed that the two stars orbit each other in a binary system while the planet simultaneously orbits the closer star. The configuration of the two orbits is somewhat similar to a circus performer twirling a hula hoop around her waist and a second hoop around her wrist.

Millholland and Im ran several simulations, each with a different set of initial conditions, to see which condition, when run over several billion years, produced the configuration of planetary and stellar orbits that Gupta’s team currently observes. They then took the best fit even further into the future to predict how the system would evolve over the next few billion years.

These simulations revealed that the new planet is likely in the midst of evolving into a hot Jupiter: A few billion years ago, the planet formed as a cold Jupiter, far from its star, in a region cool enough to condense and take shape. The newly formed planet probably orbited the star in a circular orbit. However, this conventional orbit gradually stretched and became eccentric as it was acted upon by the gravitational forces caused by the star’s misaligned orbit with its other, binary star.

“It’s a pretty extreme process in that the changes in the planet’s orbit are massive,” says Millholland. “It’s a big dance of orbits that takes place over billions of years, and the planet just goes along.”

In another billion years, simulations show the planet’s orbit will settle into a tight circular orbit around its star.

“Then the planet fully becomes a hot Jupiter,” says Millholland.

The team’s observations, along with their simulations of planet evolution, support the theory that hot Jupiters can form through high-eccentricity migration, a process in which a planet gradually moves into place through extreme changes in its orbit over time.

“It is clear not only from these but also from other statistical studies that high-eccentricity migration should represent a fraction of hot Jupiters,” notes Millholland.

“This system highlights how incredibly diverse exoplanets can be. They’re mysterious other worlds that can have wild orbits that tell the story of how they got there and where they’re going. For this planet, its journey isn’t quite finished yet.” “

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation, and teaching.