What Happens When a Planet Gets Kicked Out of Its Solar System? Its Moon Might Escape With It

Imagine a planet suddenly being thrown out of its solar system and sent into the cold darkness between the stars. What happens to its moons?

For a long time, scientists assumed that a moon would probably be lost during such a violent event. After all, planets can be ejected from their systems when another planet gives them a powerful gravitational kick, or when a passing star disturbs the entire planetary system.

But new research suggests something surprising: many moons could survive the chaos and travel through interstellar space with their planet.

The study, conducted by astronomers Yannick Badoux and Simon Portegies Zwart at Leiden Observatory, explores what happens to moons when their planets are thrown out of their original star systems. The researchers simulated nearly 34,000 different stellar encounters, examining different distances, directions and angles of encounters and tracking what happened to both planets and their moons.

Their findings reveal that a moon's distance from its planet is one of the most important factors determining whether it survives.

The gravitational leash around a planet

To understand the result, we first need to understand something called the Hill radius.

Every planet has a region around it where its own gravity is strong enough to dominate over the gravitational influence of its star. This region is known as the planet's Hill sphere, and it effectively determines how far away a moon can orbit while remaining gravitationally attached to its planet.

You can imagine it like an invisible gravitational leash.

A moon close to its planet has a strong connection and is difficult to separate. But as the moon moves farther away, the star's gravity becomes increasingly important.

According to the simulations, moons located within roughly 40% of their planet's Hill radius have a good chance of remaining attached when the planet is ejected.

Beyond that distance, however, the situation becomes increasingly unstable.

By around half of the Hill radius, a moon is very unlikely to remain with its planet during a violent ejection. The planet can be thrown into interstellar space while the moon is left behind, potentially becoming a free-floating world of its own.

Our Solar System's major moons could survive

This result is especially interesting when we look at Jupiter.

Jupiter has four famous large moons: Io, Europa, Ganymede and Callisto. Although their distances from Jupiter vary considerably, all four orbit relatively close compared with Jupiter's enormous Hill sphere.

Io, for example, orbits at less than 1% of Jupiter's Hill radius.

That means that if a passing star somehow disturbed the Solar System strongly enough to eject Jupiter, its major moons would probably not simply be left behind.

Instead, Jupiter could potentially leave the Solar System carrying its moons with it.

Imagine Jupiter becoming a rogue planet, wandering through the Milky Way without the Sun. Its moons could continue orbiting it, even though the entire planetary system they once belonged to had disappeared.

The same principle could apply to other giant planets with relatively close-in moons.

Not every moon would escape unchanged

Survival, however, does not mean that nothing happens to the moon.

The researchers found that the encounter can leave behind clues in a moon's orbit.

Moons that remain relatively close to their planet tend to preserve nearly circular and orderly orbits.

But moons that are only barely held by their planet can experience much stronger disturbances. Their orbits can become stretched, tilted or otherwise distorted.

In other words, a moon's present-day orbit could potentially contain information about the violent event that separated its planet from its original star.

Even more interestingly, the researchers found that in nearly 90% of ejection events, the distance between the planet and moon does not change dramatically.

That means the planet-moon system can remain surprisingly intact even while being completely removed from its original solar system.

A rogue planet may carry its own history

This opens an intriguing possibility for astronomers.

If scientists eventually discover a rogue planet with a moon, the moon's orbit might tell them something about where that system came from and how it was ejected.

For example, a relatively circular and stable moon orbit could indicate that the moon remained safely inside the planet's gravitational zone during the ejection.

A highly stretched or tilted orbit could suggest that the system experienced a much more violent gravitational encounter.

The researchers even applied their calculations to a possible real-world candidate known as MOA-2011-BLG-262L.

This object was detected through a technique called gravitational microlensing and may potentially be a rogue planet accompanied by a moon smaller than Earth.

However, scientists still cannot be certain about the system's true nature. The available observations allow different interpretations.

If it really is a rogue planet with a moon, the simulations suggest that the system may originally have formed around 5.2 astronomical units from its star.

That distance is remarkably close to Jupiter's average distance from the Sun.

Could a rogue moon still be warm?

Perhaps the most fascinating part of this story involves something much deeper than orbital mechanics.

A moon does not necessarily need sunlight to generate internal heat.

Gravitational interactions between a planet and its moon can continuously stretch and squeeze the moon. This process, known as tidal heating, converts orbital energy into heat inside the moon.

Jupiter's moon Europa is a famous example.

Europa's powerful gravitational interactions with Jupiter help generate heat inside the moon, contributing to conditions that allow a vast ocean of liquid water to exist beneath its icy surface.

Now imagine that Jupiter and its moons were suddenly expelled from the Solar System.

The Sun's light would become irrelevant, but Jupiter's gravity would still be there.

The tidal interactions between Jupiter and its moons could continue.

That raises an extraordinary possibility: a rogue planet could travel through the darkness of interstellar space while carrying a moon that still has internal heat and potentially a subsurface ocean.

Such worlds would be extremely cold on their surfaces, but their interiors could remain surprisingly active.

Could life exist without a star?

This leads to one of the biggest questions.

If a moon has liquid water, chemical energy and a stable source of internal heat, could life survive without a nearby star?

We don't yet know.

Earth's surface life depends heavily on sunlight, but scientists studying environments deep beneath Earth's oceans have discovered ecosystems that rely on chemical energy rather than direct sunlight.

A subsurface ocean on a rogue moon could therefore provide an interesting environment for life.

This does not mean scientists have discovered extraterrestrial life on rogue moons. There is currently no evidence for that.

But the possibility is scientifically fascinating.

A planet drifting alone between the stars may look like a frozen, lifeless world. Yet beneath the surface of a moon orbiting that planet, conditions could potentially remain much more interesting.

The galaxy may contain wandering planetary families

Rogue planets were once considered unusual possibilities, but astronomers now know that planetary systems can be disrupted and worlds can be thrown into interstellar space.

The new simulations suggest that these planets do not necessarily travel alone.

Some may carry their moons with them.

Instead of imagining rogue planets as isolated worlds wandering through the galaxy, we may need to picture some of them as entire planetary families traveling together through interstellar darkness.

A giant planet, its moons and perhaps hidden subsurface oceans could continue their journey for millions or even billions of years without ever orbiting another star.

The universe may therefore contain worlds that lost their suns but didn't lose their families.

And somewhere in the darkness between the stars, there could be a rogue Jupiter carrying moons that have survived the journey—and perhaps worlds beneath their ice that are still warm.

Reference: Yannick Badoux, Simon Portegies Zwart, "Planet-moon ejections in close stellar encounters", A&A, 2026. https://arxiv.org/abs/2607.16402