Researchers at the University of Tokyo have developed a timing-based approach to distinguish how hot Jupiters migrated inward to their stars. By analyzing orbital circularization timescales, they identified about 30 such planets that likely moved peacefully through protoplanetary disks rather than via violent scattering. This finding provides clearer evidence of formation processes for these massive exoplanets.
The discovery of the first confirmed exoplanet in 1995 revealed a hot Jupiter: a gas giant similar in mass to Jupiter but orbiting its star in just a few days. Unlike Jupiter in our Solar System, which resides far from the Sun, these planets are thought to have formed at greater distances before migrating inward. Two primary theories explain this movement: high-eccentricity migration, involving gravitational tugs from other bodies that elongate orbits before tidal forces near the star circularize them; and disk migration, a smoother process where planets spiral inward while embedded in the protoplanetary disk surrounding a young star.
Distinguishing between these paths has proven difficult. High-eccentricity migration can misalign a planet's orbit with its star's rotation, but tidal effects often realign it over time, mimicking disk migration outcomes. To address this, PhD student Yugo Kawai and Assistant Professor Akihiko Fukui, along with colleagues Noriharu Watanabe, Sho Fukazawa, and Norio Narita from the University of Tokyo's Graduate School of Arts and Sciences, devised a method focusing on circularization timescales.
In high-eccentricity scenarios, a planet's highly eccentric orbit circularizes through repeated close approaches to the star, a process influenced by the planet's mass, orbital traits, and tidal interactions. For this migration to explain a hot Jupiter's current circular orbit, the circularization must complete within the system's age. The team calculated these times for over 500 known hot Jupiters and found roughly 30 where the required time exceeded their systems' ages, ruling out high-eccentricity migration.
These candidates align with disk migration signatures: their orbits show no misalignment, indicating undisturbed paths, and many reside in multi-planet systems, which violent migration would likely disrupt by ejecting companions. This evidence supports primordial alignment and a preference for nearby planetary neighbors, hinting at runaway migration dynamics in some cases.
Such identifications are crucial for reconstructing planetary system histories. Future atmospheric and compositional analyses could reveal the disk regions where these hot Jupiters originated, enhancing understanding of exoplanet evolution. The study appears in The Astronomical Journal (2025, volume 170, issue 6, article 299).