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