Currently, there are over 5600 confirmed exoplanets in more than 4000 star systems. Among these, about 300 to 500 are classified as hot Jupiters, which are large, Jupiter-like exoplanets that orbit very close to their stars. The process of how hot Jupiters end up in such close orbits is not fully understood, but scientists believe these planets initially form in distant orbits and gradually move inward. The early stages of this migration are rarely observed, but this new discovery provides valuable insights.
The exoplanet, named TIC 241249530 b, was initially detected by NASA's Transiting Exoplanet Survey Satellite (TESS) in January 2020. TESS observed a dip in a star's brightness, indicating a possible transit by a Jupiter-sized planet. To confirm this, astronomers used the WIYN 3.5-meter Telescope at KPNO with two specialized instruments.
First, they employed the NN-EXPLORE Exoplanet and Stellar Speckle Imager (NESSI) to eliminate atmospheric distortion and extraneous sources. Then, the NEID spectrograph was used to measure the radial velocity of TIC 241249530 b by analyzing the shifts in the star's spectrum caused by the orbiting exoplanet.
"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," explained Arvind Gupta, NOIRLab postdoctoral researcher and lead author of the paper published in Nature. Gupta noted the flexibility of NEID's observation scheduling as crucial for adapting 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," said Chris Davis, NSF's program director for NSF NOIRLab. "The collaboration between NSF and NASA on the NN-EXPLORE program continues to yield impressive results in exoplanet research."
Analysis of the spectrum showed that TIC 241249530 b is about five times more massive than Jupiter and has an extremely eccentric orbit with an eccentricity of 0.94, the highest ever found via the transiting method. For comparison, Pluto's orbit has an eccentricity of 0.25, and Earth's is 0.02.
If this planet were in our Solar System, its orbit would range from ten times closer to the Sun than Mercury to as far out as Earth's distance. This would cause extreme temperature variations, from a summer's day to temperatures hot enough to melt titanium.
The planet also orbits its star in the opposite direction of the star's rotation, a rare phenomenon that provides clues about its formation history. Its unique orbit suggests that it will eventually circularize due to tidal forces, offering insights into how hot Jupiters form and evolve.
"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," Gupta said. "Planets like this are incredibly rare and hard to find, and we hope it can help us unravel the hot Jupiter formation story."
Jason Wright, Penn State professor of astronomy and astrophysics, added, "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. 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 only the second known exoplanet to show the pre-migration phase of a hot Jupiter, confirming the theory that high-mass gas giants move from highly eccentric orbits to more circular ones as they evolve.
"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. "It's exactly what I was hoping to find."
Research Report:A hot Jupiter progenitor on a super-eccentric, retrograde orbit
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