Preparing the planet and moon...
Written by MySimulator Team ยท Reviewed by MySimulator Editorial Review
Last updated: 11 July 2026
This interactive Roche limit simulation lets you bring a moon toward a planet and watch differential tidal gravity stretch it, then tear it apart into a debris ring once it crosses the live-computed Roche limit โ the same process thought to have formed Saturn's rings.
Tidal force is differential gravity: the side of a moon facing its planet is pulled more strongly than the far side, stretching the body along the line to the planet. The Roche limit is the orbital distance at which this stretching tidal force exceeds the moon's own self-gravity holding it together, computed here as d = kยทRplanetยท(ฯplanet/ฯmoon)1/3, where k differs for rigid versus loosely bound "rubble-pile" bodies.
Drag the orbital distance slider toward the planet and watch the moon elongate as it nears the dashed Roche-limit circle. Adjust planet and moon density to see the limit itself move, and switch between a rigid body (resists disruption until closer in) and a rubble-pile body (breaks apart farther out) to compare their fates.
Saturn's rings lie almost entirely inside its Roche limit, supporting the idea that they formed from a shattered moon or captured icy body that strayed too close and was torn apart by tidal forces rather than gravitationally collapsing into a single satellite.
The Roche limit, named after French astronomer รdouard Roche, is the distance from a planet within which tidal forces alone โ without any other help โ would pull apart a moon held together solely by its own gravity. This simulation computes that limit live from the sliders you set: d = k ยท Rplanet ยท (ฯplanet / ฯmoon)1/3, using a coefficient of about 2.44 for a fluid or loosely bound "rubble-pile" body and about 1.26 for a rigid, solid body, since rigid bodies can withstand tidal stress that a loose rubble pile cannot.
As the orbital-distance slider approaches the Roche limit, the visualisation elongates the moon along the line to the planet, reflecting the real physics of tidal stretching. Push it inside the limit and, for a rubble-pile body especially, the moon comes apart into a spreading debris stream โ the same fate thought to have created the ring systems seen around Saturn, Jupiter, Uranus and Neptune when a satellite strayed too close.
Tidal force differential scales with planet mass and inversely with distance cubed; self-gravity holding a moon together scales with its own density, giving the density-ratio-dependent Roche formula.
Orbital distance drives the moon toward or away from disruption; planet and moon density shift where the Roche limit itself sits; body type changes how resistant the moon is to breakup.
Planetary rings are widely thought to be the debris of moons or captured bodies torn apart inside the Roche limit, or material that never coalesced into a moon in the first place.
The Roche limit is the minimum distance a moon can orbit a planet before differential tidal gravity โ the difference in pull between the near and far sides of the moon โ exceeds the moon's own self-gravity, causing it to be pulled apart.
A denser moon holds itself together more strongly with its own gravity, so it can survive at a closer distance to the planet. The Roche limit formula includes the ratio of planet density to moon density raised to the one-third power, so a less dense moon has a Roche limit farther out.
A rigid body has internal structural strength (like solid rock) that can resist tidal stress beyond pure gravity, so it survives closer to the Roche limit. A rubble-pile body is essentially a loose pile of debris held together only by gravity, with no structural strength, so it breaks apart farther from the planet.
Yes, it is one leading hypothesis. Saturn's main rings lie almost entirely within its Roche limit, consistent with the idea that they are the remains of a moon or captured icy body torn apart by tidal forces, or material that could never accrete into a moon there in the first place.
Small, sufficiently rigid bodies bound by material strength rather than gravity alone can persist somewhat inside the classical Roche limit, which strictly applies to bodies held together by self-gravity. This is why small rocky moons and asteroids can sometimes be found closer to their planet than the naive Roche calculation would suggest.