Colossal Planet in Rare Orbit Offers Clues to Origins of ‘Hot Jupiters’

New observations of a gas planet orbiting a distant star are helping astronomers figure out how some of the universe’s hottest planets came to be.

Using a telescope at the U.S. National Science Foundation’s Kitt Peak National Observatory, astronomers discovered the extreme orbit of an exoplanet that’s on its way to becoming a ‘hot Jupiter’ — a giant planet orbiting so close to its star that its surface temperatures can reach thousands of degrees.

The astronomers found that the exoplanet follows one of the most drastically stretched-out, or ‘eccentric,’ orbits of any known planet. Further, the exoplanet orbits its star backward relative to its star’s rotation.

The planet’s extreme orbit suggests that hot Jupiters form in cooler regions further from their stars and migrate inward due to gravitational interactions, called tidal forces, with their stars. The astronomers report their findings on July 17 in Nature.

“The origins of hot Jupiters have been puzzling astronomers for nearly three decades. Multiple theories have been proposed, but none have solid supporting evidence,” says study co-author Jiayin Dong, a research fellow at the Flatiron Institute’s Center for Computational Astrophysics. “This discovery is one of the clearest pieces of evidence in support of hot Jupiters forming due to high-eccentricity tidal migration.”

Of the 5,600 confirmed exoplanets, around 300 to 500 classify as hot Jupiters. These colossal planets orbit very close to their star, some even as close as Mercury is to our sun, making them among the easiest exoplanets to spot. How hot Jupiters end up in such close orbits is a mystery, but astronomers postulate that they begin in orbits far from their star and then migrate inward over time. The early stages of this process have rarely been observed. Still, with this new analysis of an exoplanet with an unusual orbit, astronomers are one step closer to unraveling the hot-Jupiter mystery.

The discovery of this exoplanet, named TIC 241249530 b, originated in January 2020 with the detection by NASA’s Transiting Exoplanet Survey Satellite (TESS) of a dip in a star’s brightness consistent with a single Jupiter-sized planet passing in front of it. This approach to hunting planets is called the transit method. The team of astronomers behind the new paper confirmed the nature of the fluctuations and eliminated other possible causes using two instruments on the WIYN 3.5-Meter Telescope at the Kitt Peak National Observatory, a program of NSF NOIRLab.

The team first utilized the NN-EXPLORE Exoplanet and Stellar Speckle Imager (NESSI) in a technique that helps to “freeze out” distortions caused by Earth’s atmosphere and eliminate any extraneous light sources that might taint the measurements. Then, using the NASA-funded NEID spectrograph, the team measured the movements of TIC 241249530 b by carefully observing how its host star’s spectrum, or the wavelengths of its emitted light, shifted as a result of the exoplanet orbiting it.

Arvind Gupta, a NOIRLab postdoctoral researcher and lead author of the new Nature paper, praised NESSI and NEID as critical to the team’s efforts to characterize and confirm the exoplanet’s signal. “NESSI gave us a sharper view of the star than would have been possible otherwise, and NEID precisely measured the star’s spectrum to detect shifts in response to the orbiting exoplanet,” he says. Gupta particularly noted the unique flexibility of NEID’s observation scheduling framework as it allows for the swift adaptation of the team’s observing plan in response to new data.

“The WIYN telescope is playing a crucial role in helping us understand why the planets found in other solar systems can be so different from system to system,” says Chris Davis, program director for NOIRLab. “The collaboration between NSF and NASA on the NN-EXPLORE program continues to yield impressive results in exoplanet research.”

Detailed spectrum analysis confirmed that the exoplanet is approximately five times as massive as Jupiter. The spectrum also revealed that the exoplanet is orbiting along a highly eccentric path. The eccentricity of a planet’s orbit is measured on a scale from 0 to 1, with 0 being a perfectly circular orbit and 1 being extremely elliptical. This exoplanet has an orbital eccentricity of 0.94, making it more eccentric than the orbit of any other exoplanet ever found via the transit method. For comparison, Pluto’s highly elliptical orbit around the sun has an eccentricity of 0.25; Earth’s eccentricity is 0.02.

If this planet were part of our solar system, its closest approach would be one-tenth the distance from the sun to Mercury. Its orbit’s furthest extent would be around Earth’s position. The exoplanet’s extreme orbit would cause temperatures on the planet to vary between the warmth of a summer day and titanium-melting heat.

To add to the unusual nature of the exoplanet’s orbit, the team also found that it’s orbiting backward, meaning in a direction opposite to the rotation of its host star. This feature is something astronomers rarely see, and it informs the team’s interpretation of the exoplanet’s formation history.

The exoplanet’s unique orbital characteristics also hint at its future trajectory. The exoplanet’s highly eccentric orbit and extremely close approach to its host star will likely cause its orbit to gradually shrink and become more circular as it loses energy to tidal forces acting between the planet and its star. Discovering this exoplanet before this migration has taken place is valuable, as it lends crucial insight into how hot Jupiters form, stabilize and evolve, Gupta says.

“While we can’t exactly press rewind and watch the process of planetary migration in real time, this exoplanet serves as a sort of snapshot of the migration process,” he says. “Planets like this are incredibly rare and hard to find, and we hope it can help us unravel the hot-Jupiter formation story.”

“We’re especially interested in what we can learn about the dynamics of this planet’s atmosphere after it makes one of its scorchingly close passages to its star,” says Jason Wright, a professor of astronomy and astrophysics at Pennsylvania State University who supervised the project while Gupta was a doctoral student at the university. “Telescopes like NASA’s James Webb Space Telescope have the sensitivity to probe the changes in the atmosphere of the newly discovered exoplanet as it undergoes rapid heating, so there is still much more for the team to learn about the exoplanet.”

TIC 241249530 b is the second exoplanet found to demonstrate the hot-Jupiter pre-migration phase. Together, these two examples observationally support the idea that higher-mass gas giants evolve to become hot Jupiters as they migrate from highly eccentric orbits toward tighter, more circular orbits.

“Astronomers have been searching for exoplanets that are likely precursors to hot Jupiters, or that are intermediate products of the migration process, for more than two decades, so I was very surprised — and excited — to find one,” Gupta says. “It’s exactly what I was hoping to find.”