How Binary Star Systems May Launch Rogue Jupiters

Over the past few decades, astronomers have found that space between the stars is far from empty, it is dotted with “free-floating planets,” or planets that drift alone without orbiting a star. These objects, known as FFPs, are detected using microlensing and infrared observations, but how they came to be is still debated. In this study, Aleksandra Ćalović and collaborators explore a new origin story for Jupiter-like free-floating planets (JFFPs): ejection from young, fragmenting discs that form binary stars.

Background: From Star-Birth to Planet Ejection

Traditionally, scientists think such planets either form like stars, by collapsing from molecular clouds, or form in a star’s disc and later get kicked out through gravitational interactions. Ćalović’s team focuses on this latter pathway, specifically in systems where a massive disc around a young star becomes unstable and splits into multiple clumps. Some of these clumps may grow into planets, while others become companion stars. The authors suggest that interactions between these growing bodies can fling out Jupiter-mass planets before the system even settles into its final binary configuration.

Methodology: Simulating Chaos in 3D

To test this idea, the researchers used 3D hydrodynamical simulations (with the Phantom code) to model massive self-gravitating discs, the kind that might exist around newborn stars. In each simulation, they embedded several Jupiter-mass planets at distances of tens of astronomical units (AU) and a more massive “secondary seed” farther out, representing a forming companion star. The key parameter was the mass of this secondary, which they varied between 5 and 50 times the mass of Jupiter. They then watched how gravitational forces and disc turbulence drove chaotic interactions over roughly 100,000 years of simulated evolution.

Results: The Fate of Gas Giants

Ćalović and colleagues found that as the secondary object migrates inward, it often destabilizes nearby planets. Close encounters between the secondary and Jupiter-like planets act as gravitational slingshots, flinging many of the planets completely out of the system. The simulations revealed that the probability of ejection, the “ejection fraction”, increases with the mass of the secondary: about two-thirds of planets are expelled when the companion exceeds roughly 0.05 times the mass of the primary star. These ejected planets typically escape at relatively low speeds (around 2 km/s), much slower than planets expelled from core-accretion systems, suggesting they would remain near their birth clusters for millions of years.

Survivors and Orbits

Not all planets are lost. Those that remain tend to occupy eccentric orbits, elongated paths around the primary star, with average eccentricities around 0.2. Planets forming closer to the star often survive, while those farther out, near the secondary’s orbit, are more likely to be expelled. These results align with observations showing that only a few percent of stars host wide-orbit gas giants, while free-floating Jupiters appear much more common.

Discussion: Implications for Planet Formation

This study suggests that disc fragmentation in binary systems could be a dominant source of Jupiter-like rogue planets. Unlike planets formed through the slower core accretion process, these JFFPs form early, within the first few hundred thousand years of a system’s life, and are expelled at gentle speeds, potentially carrying small circumplanetary discs with them. The authors note that this mechanism could naturally explain why so many free-floating Jupiters are observed in young stellar clusters.

Conclusion: A Binary’s Violent Birth and Its Planetary Casualties

Ćalović’s simulations reveal that the birth of binary stars may come at the expense of their first planets. The same gravitational chaos that forms a stellar companion also launches planetary bodies into interstellar space. These findings connect two major cosmic mysteries, the origin of binaries and the abundance of rogue planets, suggesting that they might be born together, only for gravity to tear them apart soon after.

Source: Ćalović