Hot Jupiters represent some of the most extreme planets in our galaxy. These scorching worlds are as massive as Jupiter, but they move very close to their stars, completing an orbit in a few days, unlike our Jupiter which circles the Sun in about 4,000 days.

Scientists suspect that Hot Jupiters were not always so hot and may have originated as “cold Jupiters” in colder, more distant parts of the universe. However, how they evolved into the hot giants we observe today remains a big question.

Astronomers from MIT, Penn State University, and other institutions have discovered a “progenitor” of Hot Jupiters — a young planet in the process of becoming a Hot Jupiter. Its orbit provides answers to questions about the evolution of these planets.

The new planet, designated as TIC 241249530 b, orbits a star about 1,100 light-years from Earth. The planet has a highly "eccentric" orbit, meaning it comes very close to the star before moving away, forming a narrow, elliptical loop. If it were part of our solar system, it would come 10 times closer to the Sun than Mercury before looping back just beyond Earth and circling again. Scientists estimate that this planet has the most eccentric orbit ever discovered.

The orbit of this planet is also unique because of its "retrograde" orientation. Unlike Earth and other planets in our system, which orbit in the same direction as the Sun rotates, this planet travels in the opposite direction of its star's rotation.

The team conducted simulations of orbital dynamics and found that the highly eccentric and retrograde orbit are signs that the planet is likely evolving into a Hot Jupiter through a process known as "high-eccentricity migration" — a process in which the planet's orbit wobbles and gradually shrinks 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 system. Interactions between the two orbits — the planet's and its star's — have caused the planet to gradually move closer to its star over time.

Currently, the planet's orbit is elliptical, taking about 167 days to circle the star. Researchers predict that in a billion years, the planet will transition to a much tighter, circular orbit, orbiting the 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 explain some Hot Jupiters,” says Sarah Millholland, an 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 world. And because of 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.”

Millholland and her colleagues published their findings today in the journal Nature. Her co-authors are Haedam Im, a student at MIT, lead author Arvind Gupta of Penn State University and NSF NOIRLab, and collaborators from multiple universities, institutions, and observatories.

Radical Seasons
The new planet was first spotted in data collected by NASA's Transiting Exoplanet Survey Satellite (TESS), a mission led by MIT, which monitors the brightness of nearby stars looking for "transits," or short dips in star light that can signal the presence of a planet passing in front of the star and temporarily blocking its light.

On January 12, 2020, TESS recorded a possible transit of the star TIC 241249530. Gupta and his colleagues at Penn State determined that the transit was consistent with a Jupiter-sized planet crossing in front of the star. They then obtained radial velocity measurements of the star from other observatories, which estimate the star’s wobble, or the degree to which it moves back and forth, in response to other nearby objects that might be gravitationally affecting the star.

These measurements confirmed that a Jupiter-sized planet orbits the star and that its orbit is highly eccentric, bringing the planet very close to the star before flinging it far away.

Prior to this discovery, astronomers knew of only one other planet, HD 80606 b, thought to be an early Hot Jupiter. That planet, discovered in 2001, held the record for the highest eccentricity until now.

“This new planet experiences really dramatic changes in star light during its orbit,” says Millholland. “There must be really radical seasons and an absolutely scorched atmosphere every time it passes close to the star.”

Orbital Dance
How could the planet have fallen into such an extreme orbit? And how might its eccentricity evolve over time? To find out, Im and Millholland conducted simulations of the planet’s orbital dynamics to model how the planet might have evolved over its history and how it might continue over 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 like a circus performer twirling a hula hoop around their waist, while spinning another hula hoop around their wrist.

Millholland and Im ran multiple simulations, each with different initial conditions, to see which condition, when run forward through several billion years, produces the planet and star orbital configuration that Gupta’s team observed today. They then ran the best match 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 process of evolving into a Hot Jupiter: Several billion years ago, the planet formed as a cold Jupiter, far from its star, in a region cold enough to condense and form. Newly formed, the planet likely orbited the star in a circular path. This conventional orbit, however, gradually stretched and became eccentric as it experienced gravitational forces due to the misaligned orbit of its star with the other, binary star.

“It’s a pretty extreme process where the changes in the planet’s orbit are massive,” says Millholland. “It’s a grand dance of orbits taking place over billions of years, and the planet is just going along for the ride.”

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

“Then the planet will fully become a Hot Jupiter,” says Millholland.

The team’s observations, along with the planet’s evolutionary simulations, 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’s clear not just from this, but from other statistical studies, that high-eccentricity migration should explain some Hot Jupiters,” notes Millholland. “This system highlights how incredibly diverse exoplanets can be. These are mysterious other worlds that can have wild orbits that tell a story of how they got to be that way and where they’re going. For this planet, its journey is not yet complete.”

“It’s really difficult to catch these progenitors of Hot Jupiters ‘in the act’ as they go through their super eccentric episodes, so it’s very exciting to find a system undergoing this process,” says Smadar Naoz, a professor of physics and astronomy at the University of California, Los Angeles, who was not involved in the study. “I believe this discovery opens the door to a deeper understanding of the initial configuration of exoplanetary systems.”

The new research further expands the understanding of how complex gravitational relationships among stars and planets can shape the evolution of planetary systems. It is key to uncovering the secrets of the formation and transformation of celestial bodies and can provide important insights into the development of exoplanets that might be suitable for life. In the context of broader space research, discoveries like TIC 241249530 b help us better understand our own solar system and its place in galactic history.

Source: Massachusetts Institute of Technology