I have a space question. I know about the Roche limit but is there an upper limit to how big a ring can get compared to its planet?

Asked by Dan

Of course there are limits, the Universe is all about limits! But they’re not as strict as one would think.

We found an object in 2012, J1407b, with a ring system 200 million kilometres in diameter. If the rings were centred on Earth they would cover the inner solar system all the way to the orbit of Mercury on the opposite side of the Sun. The media quickly dubbed the object Super-Saturn, and the Ring system seems to have the same characteristics: a proportional mass, resonance gaps, shepherd moons. But the system is very young (16 million years, a baby in cosmic term) and the object is not exactly a planet, rather a brown dwarf. So, what we see as a Ring system could be the beginning of a system of moons, rather than a stable ring.

But what about spatial limits? Depending where the material for the ring is coming from, the limit could be given by the Roche limit or simply by the position of the first Lagrangian point. The Roche limit, as mention by Dan, is the distance from an object that a satellite would be disintegrated at. If the material within the Roche limit it would not be able to coalesce back into a moon and the ring configuration would become the only option. This rule has two notably ring exceptions: the E-ring and the Phoebe-Ring of Saturn. They are both beyond the Roche radius of the planet. Phoebe is between 50 and 300 Saturn Radii (and Phoebe also has a retrograde orbit and it’s inclined with respect to the equator), so their stability depends on the position of the Lagrangian point, and by the escape velocity of the specific orbit. As long as the dust and gas are attracted by the planet, they will form a ring.

If we can imagine a rogue planet, kicked out of its original star system, it could be able in its solitary travel among the stars to acquire a ring system whose size would be determined only by the interstellar medium influences, rather than by the mass of the planet.