The first exoplanet discovered in 1995 was a hot Jupiter, a Jupiter mass planet orbiting its star every few days. Today, models indicate that hot Jupiters form far from their stars, similar to Jupiter in the Solar System, and later move inward. Two main migration channels are considered: high-eccentricity migration, in which gravitational perturbations excite the orbit before tidal forces circul Tokyo, Japan (SPX) Dec 16, 2025
The first exoplanet discovered in 1995 was a hot Jupiter, a Jupiter mass planet orbiting its star every few days. Today, models indicate that hot Jupiters form far from their stars, similar to Jupiter in the Solar System, and later move inward. Two main migration channels are considered: high-eccentricity migration, in which gravitational perturbations excite the orbit before tidal forces circularize it close to the star, and disk migration, in which the planet drifts inward through the protoplanetary disk.
Observationally, separating these mechanisms has been difficult. In the high-eccentricity case, gravitational interactions can tilt the planet's orbit relative to the star's rotation axis, producing a measurable spin - orbit misalignment. Tidal forces, however, can realign the orbit and stellar spin over time, so aligned systems are not automatically evidence of disk migration. This degeneracy has limited attempts to identify planets that reached close-in orbits through disk-driven migration.
A team led by PhD student Yugo Kawai and Assistant Professor Akihiko Fukui at the Graduate School of Arts and Sciences, the University of Tokyo, proposed a new way to distinguish migration histories by exploiting the timescale of high-eccentricity migration itself. In the high-eccentricity scenario, a planet first occupies a highly elongated orbit before repeated close passages near the star allow tides to remove orbital energy and angular momentum, gradually circularizing the orbit. The circularization timescale depends on factors such as planetary mass, orbital period, stellar properties, and the efficiency of tidal dissipation.
If a hot Jupiter formed through high-eccentricity migration, its eccentricity must damp within a time shorter than the age of the planetary system. The researchers calculated tidal circularization timescales for more than 500 known hot Jupiters and compared these values with estimated system ages. They then searched for planets whose observed circular orbits could not be reconciled with the time available for tidal circularization under the high-eccentricity scenario.
This analysis identified about 30 hot Jupiters whose orbits are already circular even though their computed circularization times exceed the inferred ages of their host systems. For these planets, high-eccentricity migration would not have had enough time to erase orbital eccentricity, making that pathway unlikely. Their present-day architectures instead point to an origin via disk migration, in which the eccentricity remains low as the planet migrates inward through the gas disk.
The same subset of planets exhibits additional properties expected for disk migration. None of these hot Jupiters shows a significant spin - orbit misalignment, consistent with migration in a disk that remains roughly aligned with the stellar equator and does not strongly disturb the orbital plane. In several systems, the hot Jupiter orbits alongside nearby planetary companions, a configuration that high-eccentricity migration would tend to disrupt by scattering or ejecting other planets.
The team concludes that these hot Jupiters preserve a clearer dynamical record of their migration paths. By isolating planets whose orbits still reflect their original migration processes, astronomers can better connect current orbital architectures to earlier phases of planetary system formation and evolution. Future work that measures atmospheric composition and elemental abundance ratios in these planets may reveal where in the protoplanetary disk they formed, placing tighter constraints on the origin and migration mechanisms of hot Jupiters.
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