When a Star Steals a Planet, the Moons Go Too

A rogue planet drifting through interstellar space might not be traveling alone. New simulations of nearly 34,000 encounters between a passing star and a planetary system tracked what happens to any moons caught in the process. In plenty of cases, they go too.

Leiden Observatory astronomers Yannick Badoux and Simon Portegies Zwart ran the simulations and posted the results as a preprint, not yet peer-reviewed. Everything comes down to the Hill radius, the zone where a planet's own pull beats its star's, essentially the leash keeping a moon in place. Stay within about 40 percent of that leash and a moon rides out the ejection with its planet. Past the halfway point, the two separate and drift off alone.

Our own moons would be fine. Io circles well inside one percent of Jupiter's Hill radius, so if a star yanked Jupiter loose tomorrow, Jupiter's moons, Io, Europa, Ganymede, Callisto, would make the same trip. The researchers also noticed that a surviving moon's final orbit records how rough the eviction was: tidy and round if it stayed close, stretched and tilted if it barely held on.

The team checked the idea against something real: a microlensing detection called MOA-2011-BLG-262L that might be a rogue world towing a moon lighter than Earth, though other readings of the data remain possible. If that holds up, the planet likely started about 5.2 astronomical units out, right where Jupiter orbits the sun.

One wrinkle: some moons, Europa included, stay warm without sunlight, their planet's gravity flexing them enough to keep an ocean liquid under the ice. Send the whole Jupiter system into the dark and that flexing never stops. Somewhere past every star, a planet may be dragging a moon through the void with a hidden ocean still running. No sun required.

Read the full story at Universe Today, July 31, 2026

Hot Take: Every popular picture of a rogue planet shows it alone, and that turns out to be the least likely version of the story. Staying close to your planet, it turns out, is a good survival strategy at every scale.