What Happens to a Moon When Its Planet Becomes a Rogue?

• Rogue planets are often accompanied by their moons when ejected from solar systems, contradicting the traditional view of them as solitary worlds.
• A moon's survival during ejection depends on its position within the planet's Hill radius.
• Moons orbiting within 40% of the Hill radius typically remain attached to their planets during stellar encounters.
• Tidal heating from planetary gravity can keep subsurface oceans liquid on moons even after they are ejected into the dark of interstellar space.

The Hill Radius and Survival

• The Hill radius defines the region where a planet's gravity dominates over its host star's gravity.
• Moons located closer to their planet (within 40% of the Hill radius) successfully exit the solar system with their parent planet.
• Beyond 50% of the Hill radius, moons are typically lost during the chaotic ejection process.

Orbit Evidence and History

• Researchers Yannick Badoux and Simon Portegies Zwart at Leiden Observatory conducted simulations involving 34,000 stellar encounters to track these dynamics.
• Surviving moons often retain circular orbits if they were close to the planet, while those at the edge of the Hill radius emerge with visible signs of distortion.
• Scientists can reverse-engineer the ejection history of rogue systems by analyzing the orbital characteristics of surviving moons, as 90% of ejections do not significantly alter the planet-moon spacing.

Implications for Habitability

• Even without sunlight, moons like Europa generate internal heat through tidal flexing.
• If a Jupiter-like planet is ejected into deep space, its moons could theoretically maintain liquid oceans and potentially support life, powered entirely by tidal forces rather than solar energy.