New orbital clue reveals how hot Jupiters really formed

Determining which of these two routes a specific hot Jupiter followed has been challenging. High-eccentricity migration can tilt a planet's orbital axis compared with the rotational axis of its star, creating a detectable misalignment. Yet tidal forces near the star can gradually erase that misalignment over time. Because an aligned orbit could result from either process, astronomers have lacked a dependable way to identify planets that formed through disk migration.

A New Strategy Based on Migration Timescales

To overcome this problem, 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, introduced a new method that focuses on the length of time required for high-eccentricity migration to occur.

In this migration scenario, a planet first follows a highly stretched path before its orbit becomes circular again as it repeatedly swings close to its star. The amount of time needed for this circularization depends on several factors, including the planet's mass, orbital characteristics, and tidal forces. For a hot Jupiter to have formed through high-eccentricity migration, this circularization time must be shorter than the age of its planetary system. After calculating circularization times for more than 500 known hot Jupiters, the researchers found about 30 planets that did not meet this requirement. These planets have circular orbits even though their calculated circularization times exceed the ages of their systems.

Evidence Supporting Disk Migration

The hot Jupiters in this group also match other expectations for planets that traveled inward within a disk. Their orbits show no signs of misalignment, suggesting that their movement toward the star was smooth rather than heavily influenced by disruptive gravitational interactions. A number of these planets are also part of multi-planet systems, a configuration that high-eccentricity migration typically disrupts, since that process can scatter or eject neighboring planets.

Looking Ahead to What These Planets Can Reveal

Finding planets that retain clear evidence of how they migrated is essential for piecing together the history of planetary systems. Future studies of their atmospheres and elemental compositions may pinpoint the regions of the disk where they originally formed, offering deeper insight into the origins and evolution of hot Jupiters.