The study, led by Penn State researchers, was published on July 17 in the journal Nature.

"We caught this massive planet making a sharp, hairpin turn during its close passage to its star," said Suvrath Mahadevan, the Verne M. Willaman Professor of Astronomy at Penn State and an author of the paper. "Such highly eccentric transiting planets are incredibly rare - and it's really amazing that we were able to discover the most eccentric one."

Mahadevan noted that the term "eccentric" describes the shape of a planet's orbit, which ranges from zero (a perfect circle) to one (a highly stretched ellipse). The exoplanet, TIC 241249530, has an orbital eccentricity of 0.94, surpassing that of any other transiting exoplanet known. For context, Pluto's orbit has an eccentricity of 0.25, while Earth's is 0.02. This extreme orbit results in significant temperature variations on the planet, from moderate to extremely hot.

Additionally, the team discovered that the planet orbits backwards, in a direction opposite to the rotation of its host star. This uncommon characteristic provides clues about the planet's formation history.

"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," said Arvind Gupta, NOIRLab postdoctoral researcher and lead author of the paper, who conducted the research as a doctoral student at Penn State. "Planets like this are hard to find and we hope it can help us unravel the hot Jupiter formation story."

Currently, over 5,600 confirmed exoplanets exist in more than 4,000 star systems. Among these, about 300 to 500 are classified as hot Jupiters - large, Jupiter-like exoplanets that orbit very close to their star. The formation process of hot Jupiters remains a mystery, though astronomers believe they start in distant orbits and gradually move inward. Observing the early stages of this migration is rare, but this discovery offers new insights.

"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 said.

The discovery was made possible by three instruments developed at Penn State: the NASA-funded NEID spectrograph, the Habitable Zone Planet Finder spectrograph, and a photometric diffuser. These tools enable detailed observation and analysis of the exoplanet's light.

The planet was first detected by NASA's Transiting Exoplanet Survey Satellite (TESS) in January 2020, which observed a dip in a star's brightness consistent with a Jupiter-sized planet transiting the star. To confirm this and rule out other causes, astronomers used two instruments on the WIYN 3.5-meter Telescope at the U.S. National Science Foundation's Kitt Peak National Observatory.

Using the NN-EXPLORE Exoplanet and Stellar Speckle Imager (NESSI) to minimize atmospheric interference, the team ensured no nearby stars could have affected the TESS data. Further observations with the HPF and NEID spectrographs examined how TIC 241249530's light spectrum shifted due to the exoplanet's orbit.

"It's so exciting to see such great science coming out of NEID within just a few years of operations," said Andrea Lin, a co-author of the paper and a doctoral student at Penn State who contributed to the construction and commissioning of the NEID spectrograph. "We're just getting started and I'm looking forward to seeing what we can accomplish in the future."

The star's velocity changes, analyzed over the planet's six-month orbit, confirmed the exoplanet is about five times more massive than Jupiter and follows an extremely eccentric path.

"This is the most eccentric transiting planet known and will prove to be as important as the previous record holder, HD80606b, which likewise has a wacky orbit highly misaligned with its host star's spin," said Jason Wright, Penn State professor of astronomy and astrophysics, who supervised Gupta's doctoral research. "These two highly eccentric planets have been 'caught in the act' of evolving towards hot Jupiter status. Like HD80606b, this planet is many times Jupiter's mass, suggesting this channel for forming hot Jupiters might be one only the most massive planets can take."

These findings support the theory that higher-mass gas giants become hot Jupiters as they migrate from eccentric orbits to closer, more circular ones.

"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," Wright said. "Telescopes like NASA's James Webb Space Telescope have the sensitivity to probe the changes in the atmosphere of this newly discovered exoplanet as it undergoes rapid heating, so there is still much more for the team to learn about the exoplanet."

Research Report:A hot-Jupiter progenitor on a super-eccentric retrograde orbit

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