Planetary Rings

Rings are swarms of orbiting particles

• Orbits have to be very circular

• Elliptical orbits will result in collisions, destroying the ring

Rings Have to be Very Flat

• Inclined orbits will result in collisions and destruction of the ring

Rings have to be Equatorial• Tilted orbits precess; the plane of the orbit

rotates due to the planet’s equatorial bulge, other satellites, and the Sun’s gravity

A tilted ring won’t stay flat very long

• Particle orbits will precess at different rates

• Collisions will soon destroy the ring

Rings, Gaps, and Resonances• When two objects return to the same

relative positions regularly, they are said to be in resonance

• Some resonances are stable. Mercury’s 3:2 resonance between its rotation and its orbit is an example.

• Pluto’s period is 3/2 that of Neptune’s. This resonance keeps the two planets from ever colliding even though their orbits cross.

Rings, Gaps, and Resonances

• Some resonances create gaps. There are no asteroids with one-half Jupiter’s period because repeated pulls by Jupiter eventually would change the asteroid’s orbit.

• Gaps in Saturn’s rings are due to resonances with Saturn’s satellites.

Shepherd Moons

Shepherd Moons help maintain the sharp edges of rings

A Shepherd Moon Outside a Ring Tends to Drag Stray

Particles Back, Causing Them to Fall Back Into the Ring

Outer Shepherd Moon

A Shepherd Moon Inside a Ring Tends to Speed Stray Particles Up, Causing Them to Rise Back Into

the Ring

Inner Shepherd Moon

Rings Are Probably Short-Lived

• Gravity of the Sun and planet’s satellites constantly disturbs orbits of particles

• Collisions probably happen frequently

• Particles should drift out of the ring

• Rings around outer planets have probably come and gone repeatedly during history of Solar System