The planet’s extreme orbit provides insights into how massive, hot planets are formed. Hot Jupiters are among the most extreme planets in the galaxy. These blazing worlds, comparable in mass to Jupiter, orbit extremely close to their star, completing a full revolution in just a few days—vastly shorter than Jupiter’s leisurely 4,000-day orbit around the Sun.

Scientists suspect that hot Jupiters may not always have been so hot; they might have originally formed as "cold Jupiters" in cooler, more distant regions. Understanding how these planets evolved into the star-hugging gas giants we observe today remains a significant mystery. Astronomers from MIT, Penn State University, and other institutions have now discovered a hot Jupiter "progenitor"—a young planet in the process of evolving into a hot Jupiter. This planet’s orbit is shedding light on the evolution of hot Jupiters.

Figure 1. Scientists Uncover Highly 'Eccentric' Planet Evolving into a Hot Jupiter

The newly identified planet, named TIC 241249530 b, orbits a star approximately 1,100 light-years from Earth. It follows a highly "eccentric" orbit, coming extremely close to its star before swinging far out and then returning in a narrow, elliptical path. If it were in our solar system, it would pass ten times closer to the Sun than Mercury before veering out past Earth and looping back. According to the scientists, this planet's elongated orbit has the highest eccentricity of any planet detected so far. Figure 1 shows Scientists Uncover Highly 'Eccentric' Planet Evolving into a Hot Jupiter.

The new planet's orbit is also distinctive due to its "retrograde" orientation. Unlike Earth and other planets in our solar system, which orbit in the same direction as the Sun’s rotation, this planet travels in the opposite direction to its star’s spin. The research team conducted simulations of orbital dynamics and found that the planet’s highly eccentric and retrograde orbit suggests it is likely evolving into a hot Jupiter through a process known as "high-eccentricity migration." This process involves the planet's orbit wobbling and gradually shrinking as it interacts with another star or planet on a much wider orbit.

In the case of TIC 241249530 b, the researchers found that the planet orbits a primary star, which itself orbits a secondary star, making it part of a stellar binary system. The interactions between the two orbits—the planet's and its star’s—have caused the planet to gradually migrate closer to its star over time. Currently, the planet’s orbit is elliptical, taking about 167 days to complete one orbit around its star. The researchers predict that in approximately 1 billion years, the planet will transition into a much tighter, circular orbit, completing its orbit in just a few days. By then, the planet will have fully evolved into a hot Jupiter.

“This new planet supports the theory that high-eccentricity migration accounts for some fraction of hot Jupiters,” says Sarah Millholland, assistant professor of physics at MIT’s Kavli Institute for Astrophysics and Space Research. “We believe that when this planet formed, it was a cold world. Due to its dramatic orbital dynamics, it will become a hot Jupiter in about a billion years, with temperatures reaching several thousand kelvin. This represents a significant transformation from its initial state.”

Extreme Seasons

The new planet was initially detected using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), an MIT-led mission that observes the brightness of nearby stars for "transits," or brief dips in starlight that may indicate a planet passing in front of and temporarily blocking the star’s light. On January 12, 2020, TESS recorded a potential transit of the star TIC 241249530. Gupta and his team at Penn State determined that this transit matched the profile of a Jupiter-sized planet crossing in front of the star. They then obtained radial velocity measurements from other observatories to assess the star's wobble, which indicates the gravitational influence of nearby objects.

These measurements confirmed the presence of a Jupiter-sized planet orbiting the star with a highly eccentric orbit, bringing it extremely close to the star before propelling it far out. Before this discovery, astronomers had identified only one other planet, HD 80606 b, believed to be an early hot Jupiter. Discovered in 2001, HD 80606 b previously held the record for the highest eccentricity among known planets. “This new planet undergoes extremely dramatic changes in starlight throughout its orbit,” Millholland says. “There must be truly extreme seasons and an intensely heated atmosphere each time it approaches close to the star.”

Orbital Dance

How could a planet end up in such an extreme orbit, and how might its eccentricity change over time? To find answers, Im and Millholland conducted simulations of planetary orbital dynamics to model the planet’s evolution and predict its behavior over hundreds of millions of years.

The team simulated the gravitational interactions among the planet, its star, and the nearby secondary star. Gupta and his colleagues had noted that the two stars orbit each other in a binary system, while the planet orbits the closer star. This arrangement is somewhat akin to a circus performer twirling one hula hoop around her waist while spinning another around her wrist.

Millholland and Im performed multiple simulations with various initial conditions to determine which scenarios, when projected forward over billions of years, matched the planetary and stellar orbits observed by Gupta’s team today. They then extended the simulations further into the future to forecast how the system might evolve over the next several billion years.

The simulations revealed that the new planet is likely in the process of evolving into a hot Jupiter. Several billion years ago, the planet began as a cold Jupiter, forming far from its star in a region cool enough for condensation and formation. Initially, it orbited the star in a circular path. Over time, this orbit became increasingly eccentric due to gravitational interactions with the star’s misaligned binary companion.

“It’s an extreme process with massive changes to the planet’s orbit,” Millholland says. “It’s like a grand dance of orbits unfolding over billions of years, and the planet is simply along for the ride.” According to the simulations, in another billion years, the planet’s orbit will stabilize into a close, circular path around its star, fully transforming into a hot Jupiter.

“This confirms that the planet will ultimately become a hot Jupiter,” Millholland adds. The team’s observations and simulations support the theory that hot Jupiters can form through high eccentricity migration—a process where a planet gradually settles into its current position through extreme orbital changes over time.

“This study, along with other statistical research, shows that high eccentricity migration likely accounts for some fraction of hot Jupiters,” Millholland notes. “This system demonstrates the incredible diversity of exoplanets. They are enigmatic worlds with wild orbits that reveal their evolutionary stories and future trajectories. For this planet, the journey is still unfolding.”

“It is extremely challenging to observe these hot Jupiter progenitors ‘in the act’ during their highly eccentric phases, so finding a system undergoing this process is very exciting,” 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 paves the way for a deeper understanding of the initial configurations of exoplanetary systems.”

Source: MIT News

Cite this article:

Janani R (2024), Astronomers Discover a Highly “Eccentric” Planet on Its Path to Becoming a Hot Jupiter, AnaTechMaz, pp. 75