Why do only the gas giants in our solar system have rings? Wouldn’t the conditions that caused them also apply to the smaller planets, but of course on a smaller scale? Or is there a threshold, below which rings can’t form?
Virtually all planetary rings in our system are formed within the Roche limit of the planet, Saturn’s E-Ring being the notable exception. Within the Roche radius particulates are pulled apart by tidal differences that exceed their gravitational attraction, making it difficult to form bodies beyond a certain size, hence promoting ring formation. For smaller planets the Roche radius for a modest body is very small (for a strictly inelastic body held together only by gravity it is actually well inside the Earth), and drag from the upper atmosphere tends to slow material, causing it to fall planetward. Discrete rings (or rather, areas in the ring body that are discretely denser or lighter), such as seen most dramatically in Saturn, are the result of orbital resonance between inner moons. As only giant-size planets are able to support a system of stable moons, we can assume that large and complex ring systems will be found only around such planets. Although there is no reason that rings can’t form around smaller planets, they’re less likely to be stable and probably end up clumping together or being ejected.
In the case of the Earth, the very strong tidal competition between Earth and Luna serves to make a stable ring formation impossible; material will either tend toward one of the Lagrange libration points or be wiggled back and forth and ultimately ejected.
Stranger
Oh, is THAT what happened??? :eek:
