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How small can a planet with rings be? It doesn't have to be inhabitable, but it must be a planet (not a dwarf planet).

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  • $\begingroup$ What types of rings are you looking for? Something spectacular like the Saturnian ring system, or is something like the rings around 10199 Chariklo aceptable? Something in between? $\endgroup$ – notovny Oct 21 '20 at 17:35
  • $\begingroup$ I don't really know. The rings should be at least visible from a decent distance, I guess. $\endgroup$ – The_CIA Oct 22 '20 at 18:35
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I agree with the answers posted so far (the gist being that any object which is a planet can have rings), but I also wanted to talk about real-life examples of low-mass bodies with rings:

  • Chariklo, a minor planet $\sim$230 km across, has two rings approximately 400 km in radius.
  • Chiron, another minor planet $\sim$200 km across, has a ring system about 325 km in radius.
  • Haumea, a dwarf planet $\sim$1000 km across and weighing in at $4\times10^{21}$ kg (0.07% the mass of Earth), has a ring about 2300 km in radius.
  • Rhea a moon of Saturn roughly half the mass of Haumea, was thought to have rings between 1000 and 2000 km in radius, but the observations have been disputed.

This strongly suggests that any body that can be considered a planet could have rings, assuming it tidally disrupts material in a close orbit near the Roche limit. It also suggests that the radius of such a a ring system would reasonably be about 3-4 times the radius of the planet, which fits in nicely with some reasonable assumptions about the Roche limit.

Certainly, it would be difficult to form a ring system because it's difficult for low-mass planets dies to capture moons or other small bodies. The giant planets in the Solar System have substantial moon and ring systems, whereas the four terrestrial planets currently sport only three moons between them. However, it's clearly not impossible.

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From the definition of planet

A planet is an astronomical body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and – acconding to the International Astronomical Union but not all planetary scientists – has cleared its neighbouring region of planetesimals.

The minimal sized body which can be classified as planet is also the minimal sized planet which can have rings.

Rings are made by small particles orbiting the body.

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Any planet can have rings (If it has things that can collide or fall below Roche limit in orbit around it) but IIRC the size of the planet, and the exact composition of the rings will determine for how long. An Earth-sized planet wouldn't be able to hold on a ring system if it had one for more than a few thousand years, and the rings of Saturn should be gone in a hundred million years.

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