Some planets seem to defy established rules. This is the case for hot Jupiters orbiting extremely close to their stars... in binary systems where two stars coexist. A new study proposes an explanation for this atypical configuration.
View of binary stars from a hot Jupiter-type planet
Credit: NASA/JPL-Caltech/T. Pyle
Long considered exceptions, these exoplanetary systems might actually reveal hidden mechanisms of planetary migration. Researchers rely on a known phenomenon: the von Zeipel-Lidov-Kozai (ZLK) mechanism, which could allow a planet to gradually approach its star under the gravitational influence of a neighboring star.
This slow but effective process might explain how some gas giants end up in close orbits, contradicting classical formation scenarios. The team's simulations, based on NASA and Gaia satellite data, have helped trace the evolution of these rare configurations.
The study also highlights a determining factor: the distance between the two stars in the system. Too close, they disrupt planetary formation. Too far, their influence becomes negligible. A subtle balance is required for the ZLK mechanism to act effectively.
What is the von Zeipel-Lidov-Kozai mechanism?
The ZLK mechanism describes how a celestial object's orbit can evolve under the gravitational effect of a third body. In binary systems, this interaction can disturb a planet's orbit until it migrates toward its star.
The effect only occurs if the planet's orbit is inclined relative to the orbital plane of the binary system. Over time, this inclination generates variations that, combined with other effects, alter the planet's trajectory and bring it into a close orbit.
This mechanism, long studied in stellar contexts, now proves relevant for explaining the presence of massive planets very close to their star - a scenario previously unexplained.
Why are these systems so rare?
The rarity of these systems stems from several factors. First, their formation requires a binary system with the right proportions. Second, the protoplanetary disk must allow for the emergence of a gas giant - a process already unlikely with a single star, and which becomes even more unstable with two.
Furthermore, not all configurations allow the ZLK mechanism to fully operate. The initial inclination, the bodies' masses, and the distance between the stars must be compatible with a slow but effective migration.
Despite their rarity, these systems represent a valuable opportunity to test theories. They remind us that the diversity of worlds in the Universe often exceeds our models, and that much remains to be discovered about the hidden dynamics of planetary formation.