Astronomers spot ‘highly eccentric’ planet on track to become hot Jupiter

Hot Jupiters are among the most extreme planets in the galaxy. These scorching worlds are as massive as Jupiter and they wobble around their star at breakneck speed, rotating every few days, while our gas giant orbits the Sun in a leisurely 4,000-day cycle.

Scientists suspect, however, that hot Jupiters weren’t always so hot, and may have formed as “cold Jupiters” in colder, more distant environments. But how they evolved into the star-enveloping gas giants astronomers see today is a big unknown.

Astronomers from MIT, Penn State, and elsewhere have discovered a hot Jupiter “progenitor,” a kind of juvenile planet that is in the process of becoming a hot Jupiter. And its orbit provides answers about 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 NSF NOIRLab, and collaborators from several other universities, institutions and observatories.

The new planet, named TIC 241249530 b by astronomers, orbits a star about 1,100 light-years from Earth. The planet orbits its star in a highly “eccentric” orbit, meaning it comes extremely close to the star before shooting out very far and then back out, following a narrow, elliptical path.

If the planet were part of our solar system, it would approach the Sun ten times closer than Mercury, before shooting out, passing Earth and turning around. According to scientists’ estimates, the planet’s elongated orbit has the highest eccentricity of any planet detected to date.

The new planet’s orbit is also unique because of its “retrograde” orientation. Unlike Earth and other planets in the solar system, which orbit in the same direction as the Sun, the new planet is moving in a direction opposite to that of its star.

The team ran simulations of orbital dynamics and found that the planet’s highly eccentric, retrograde orbit is a sign that it is likely evolving toward a hot Jupiter, through “high-eccentricity migration” – a process by which a planet’s orbit wobbles and gradually narrows as it interacts with another star or planet in a much wider orbit.

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

The planet’s orbit is currently elliptical, and it takes about 167 days to completely orbit its star. Researchers predict that in 1 billion years, the planet will evolve into a much tighter circular orbit, rotating around its star every two to three days. By then, the planet will have fully evolved into a hot Jupiter.

“This new planet supports the theory that high-eccentricity migration should be responsible 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 an icy world. And because of the spectacular orbital dynamics, it will become a hot Jupiter in about a billion years, with temperatures of several thousand kelvins. So that’s a huge change 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 to detect “transits,” or brief dips in starlight that could signal the presence of a planet passing in front of and temporarily blocking a star’s light.

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

These measurements confirmed that a Jupiter-sized planet was orbiting the star and that its orbit was highly eccentric, bringing the planet extremely close to the star before throwing it very far away.

Before this detection, astronomers knew of only one other planet, HD 80606 b, which was thought to be a primitive hot Jupiter. This planet, discovered in 2001, had until now held the record for the greatest eccentricity.

“This new planet has dramatic changes in brightness throughout its orbit,” Millholland says. “There must be really drastic seasons and an absolutely scorching 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 could its eccentricity change over time? To answer these questions, Im and Millholland ran simulations of planetary orbital dynamics to model how the planet would change over its history and how it might persist for hundreds of millions of years.

The team modeled the gravitational interactions between the planet, its star, and the second 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 a bit like a circus performer twirling a hoop around her waist while spinning a second hoop around her wrist.

Millholland and Im ran several simulations, each with a different set of starting conditions, to see which condition, when carried forward several billion years, produced the configuration of planetary and stellar orbits that Gupta’s team observed today. They then ran the best match even further into the future to predict how the system will evolve over the next billions of years.

These simulations revealed that the new planet is likely evolving into a hot Jupiter: billions of years ago, the planet formed as a cold Jupiter, far from its star, in a region cool enough to condense and take shape. As it formed, the planet likely orbited the star in a circular path. This conventional orbit, however, gradually became elongated and eccentric as it was subjected to the gravitational forces of the star’s misaligned orbit with its second binary star.

“It’s a pretty extreme process because the changes in the planet’s orbit are huge,” Millholland says. “It’s a big dance of orbits that happens over billions of years, and the planet just gets caught up in the whirlwind.”

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

“Then the planet will become a complete hot Jupiter,” Millholland says.

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

“It is clear, not only from this study, but also from other statistical studies, that high-eccentricity migration should be responsible for some fraction of hot Jupiters,” Millholland notes.

“This system highlights the incredible diversity of exoplanets. These are other mysterious worlds that can have wild orbits and tell the story of their evolution and destination. For this planet, its journey is not quite over yet.”

This article is republished with kind permission from MIT News (web.mit.edu/newsoffice/), a popular site covering the latest research, innovation, and teaching at MIT.