The first exoplanet ever discovered in 1995 was what we now call a "hot Jupiter", a planet as massive as Jupiter with an orbital period of just a few days. Today, hot Jupiters are thought to have formed far from their stars—similar to Jupiter in our Solar System—and later migrated inward. Two main mechanisms have been proposed for this migration: (1) high-eccentricity migration, in which a planet's orbit is disturbed by the gravity of other celestial bodies and subsequently circularized by tidal forces near the star; and (2) disk migration, in which the planet moves gradually inward within the protoplanetary disk.
However, it is not straightforward to distinguish the mechanism a particular hot Jupiter experienced from observations alone. In the case of high-eccentricity migration, the gravitational perturbations can tilt the planet's orbital axis relative to the star's rotational axis, resulting in a measurable misalignment. However, tidal forces can realign these axes over time, meaning that an aligned orbit does not necessarily imply disk migration. As a result, there has long been no reliable observational method to identify planets that formed through disk migration.
To address this challenge, a research group 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 observational method that takes advantage of the timescale of high-eccentricity migration itself.
In high-eccentricity migration, a planet's orbit becomes highly elongated before tidal forces circularize it as it passes close to the star. The timescale of this circularization depends on factors such as the planet's mass, orbital period, and tidal forcing. If a hot Jupiter indeed formed through high-eccentricity migration, its circularization time must be shorter than the system's age. However, by calculating the circularization times for over 500 known hot Jupiters, the researchers identified a group of about 30 planets for which this condition is not satisfied—they have circular orbits despite circularization times longer than their system ages.
Moreover, this group of hot Jupiters exhibits characteristics consistent with other predictions of disk migration, such as regarding the alignment of orbits and planet multiplicity. None of the planets in this subset show orbital misalignment, implying they migrated smoothly within the disk without strong perturbations. Additionally, several of these hot Jupiters belong to multi-planet systems. Such an arrangement is unlikely if they had formed via high-eccentricity migration, which tends to eject other planets.
Identifying planets that have preserved the memory of their migration is crucial for understanding planetary system evolution. Future observations of atmospheric compositions and elemental ratios in these planets are expected to reveal where within the disk they formed, shedding new light on the origins of hot Jupiters.
