# Smallest Body Possible to Have Rings

by 3point14rat
Tags: body, rings, smallest
 P: 3 What is the lower mass limit for a body to have rings? I'm thinking that Chariklo, at 155 miles in diameter is pretty small. Gravity would be so weak you'd have to hold on just to not 'jump' off with every step. Cheers, 3point14rat
 Homework Sci Advisor HW Helper Thanks P: 12,764 Welcome to PF; To figure that out you need to know the conditions required for rings to form. What are they? http://wordpress.mrreid.org/2013/04/...anetary-rings/
 P: 33 Awesome link Simon, very easy to understand. Thank you.
 P: 3 Smallest Body Possible to Have Rings So, there is no minimum size an object can have, below which it cannot support rings? The Roche Limit sounds like it's the closest an object can orbit without breaking up, not the minimum size the primary object must be to have the gravity necessary to support rings. I'm asking because the discovery of Centaur Chariklo doesn't seem to have raised much skepticism over the fact that such a tiny object has rings. I'm just a guy who read an article on it and immediately thought that there's no way something that small could hold onto anything, let alone a couple of rings. Is my ignorance showing?
Mentor
P: 11,819
 So, there is no minimum size an object can have, below which it cannot support rings?
"Support" is always a matter of time for rings, but there is no fundamental limit that says "this object is too small, it cannot have a ring".
 The Roche Limit sounds like it's the closest an object can orbit without breaking up, not the minimum size the primary object must be to have the gravity necessary to support rings.
Right - as long as internal stiffness of the objects is negligible. This is a good assumptions for large objects, it can be wrong for small objects.
 I'm just a guy who read an article on it and immediately thought that there's no way something that small could hold onto anything, let alone a couple of rings.
You can make two billard balls orbit each other in space if you like. Why not? They just have to move very slowly (and don't be too close to other massive objects).
 Homework Sci Advisor HW Helper Thanks P: 12,764 What he said. I'd add that the radius of the small object may be a limiting factor. Though I haven't done the math for a body whose radius is bigger than it's Roche limit. When you hear "rings" you tend to think "Saturn" ... but there is no need for rings to be so spectacular or even visible. Rings are generally be difficult to detect unless very big and/or they have lots of ice in them. These ones were detected by stellar occultation with spectrographic data in support. There's a decent reference: http://www.nature.com/nature/journal...ture13155.html ... there's plenty there but the bulk is (sadly) behind a paywall. Phil Plait (aka Bad Astronomy) has a nice discussion on the findings: http://www.slate.com/blogs/bad_astro...ve_a_ring.html But basically you have not seen skepticism concerning rings about such a small object because it is scientifically plausible with well known mechanisms. The main iffy bit is that there are two well-defined rings and Phil talks about that (link above).
 P: 3 I read Phil's article and the data really does demonstrate rings. That's awesome. I'm still very suprised that there are rings on such a small object and that it may even have a tiny moon that's herding the rings. These situations must have a very limited life span, so we're very fortunate to be around when it's discovered. So glad I came on here to see what smart people had to say on the subject. Thanks a heap Cheers, 3point14rat
 Sci Advisor P: 2,510 Last I heard, astronomers were still looking for the mechanism by which these rings might have been formed. This centaur's gravity is too weak for the usual method of tidal forces tearing a satellite apart. I have been thinking that this centaur orbits at a distance from the Sun at which ice (water ice) does not sublimate. But what about ammonia or methane or CO2?
Mentor
P: 11,819
 Quote by Simon Bridge What he said. I'd add that the radius of the small object may be a limiting factor. Though I haven't done the math for a body whose radius is bigger than it's Roche limit.
This just depends on the density ratio, as
$$d = 1.26\; R\left( \frac {\rho_M} {\rho_m} \right)^{\frac{1}{3}}$$
where d is the Roche limit and R is the radius of the primary object and the densities are the averages over the whole objects. Roche limit and radius scale in the same way.

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