Again, it depends on how dense the rings are. Most rings aren't that dense or that broad. Even Saturn's rings aren't anywhere near opaque; it's easy to find pictures of them backlit, like this famous composite:
http://www.sydneyobservatory.com.au/blog...e-institute.jpg
So they wouldn't so much eclipse the light from the primary star as diffuse it like thin clouds. (Remember, rings are only a few meters thick and made mainly of small particles; they'd block a lot less sunlight than a good cumulus cloudbank would.) Which would certainly have an effect on surface insolation, true, but not a catastrophic one. Indeed, if the planet were close enough to get more insolation than Earth does, then the rings might actually enhance its habitability by cooling it to a more comfortable temperature.
As for the breakup, I'm not so sure it'd be a problem. We probably tend to think of something like Earth's Moon as the default, but Luna is actually exceptionally big as moons go; if it didn't orbit Earth, it would qualify as a dwarf planet easily. Something like a captured asteroid would result in a much smaller ring with less debris to worry about, and if it were something like a captured comet, it would be mostly ice particles that would evaporate in the atmosphere. Of course, such rings would be a lot smaller and less spectacular than Saturn's. Saturn only has such elaborate rings because it has so very many moons to break up and form them.
Anyway, assuming an Earthlike atmosphere, an incoming body would have to be rocky, hundreds of meters in diameter, and tens of millions of tons in mass. Nothing in Saturn's rings is anywhere remotely near that size -- that would be a full-on asteroid. Anything that big would be broken up by the same tidal stresses that create the ring in the first place. We're not talking about a moon exploding and sending pieces flying in all directions; we're talking about an orbiting body having its parts pulled into different orbits so that it breaks apart gradually over thousands of years. Anything that did get knocked onto a surface-impact trajectory by a collision would probably be way too small to reach the surface before burning up.
So as long as the planet in question has an Earthlike atmosphere, I don't think the formation of a ring or system of rings would have a catastrophic effect on the planet. I think I suggested earlier that it would, but I hadn't thought it through.
http://www.sydneyobservatory.com.au/blog...e-institute.jpg
So they wouldn't so much eclipse the light from the primary star as diffuse it like thin clouds. (Remember, rings are only a few meters thick and made mainly of small particles; they'd block a lot less sunlight than a good cumulus cloudbank would.) Which would certainly have an effect on surface insolation, true, but not a catastrophic one. Indeed, if the planet were close enough to get more insolation than Earth does, then the rings might actually enhance its habitability by cooling it to a more comfortable temperature.
As for the breakup, I'm not so sure it'd be a problem. We probably tend to think of something like Earth's Moon as the default, but Luna is actually exceptionally big as moons go; if it didn't orbit Earth, it would qualify as a dwarf planet easily. Something like a captured asteroid would result in a much smaller ring with less debris to worry about, and if it were something like a captured comet, it would be mostly ice particles that would evaporate in the atmosphere. Of course, such rings would be a lot smaller and less spectacular than Saturn's. Saturn only has such elaborate rings because it has so very many moons to break up and form them.
Anyway, assuming an Earthlike atmosphere, an incoming body would have to be rocky, hundreds of meters in diameter, and tens of millions of tons in mass. Nothing in Saturn's rings is anywhere remotely near that size -- that would be a full-on asteroid. Anything that big would be broken up by the same tidal stresses that create the ring in the first place. We're not talking about a moon exploding and sending pieces flying in all directions; we're talking about an orbiting body having its parts pulled into different orbits so that it breaks apart gradually over thousands of years. Anything that did get knocked onto a surface-impact trajectory by a collision would probably be way too small to reach the surface before burning up.
So as long as the planet in question has an Earthlike atmosphere, I don't think the formation of a ring or system of rings would have a catastrophic effect on the planet. I think I suggested earlier that it would, but I hadn't thought it through.
