Planetary rings are flat, disc-shaped rings of dust and other small particles in orbit around a planet.  The most well-known planetary rings are those around Saturn, but Jupiter, Uranus, and Neptune also have rings.

NASA Voyager 2 - Rings of Uranus

There are three known ways that planetary rings are formed:  First, it is believed to be left-over matter from the original protoplanetary planetary disc which couldn’t coalesce into moons; from the debris of a moon that was destroyed by an impact; or the debris from a moon that was (or is in the process of being) disrupted by the tidal stresses of its parent planet.

NASA/JPL Cornell University - Jupiter's rings

As we know from Saturn’s rings, many contain shepherd moons and tiny moonlets which contribute to the stable composition of the rings.  While rings are believed to be a fairly short-term phenomenon, Saturn’s rings are believed to be quite ancient.

NASA/ESA Hubble - The star of the show, Saturn in natural colors

While Saturn’s rings have been known since the 1600′s, the rings around Jupiter, Uranus and Neptune are modern discoveries.  Scientists will be watching New Horizons on its pass of Pluto for the presence of a ring system.  None of the inner planets are known to have a ring system.  It is believed that the force of the solar wind makes it impossible for a ring system to form that close to the Sun.  While there may have been debris in orbit around the inner planets, the force of the solar wind pushed the particles away.

NASA/JPL Cassini - Saturn ring mosaic... the enlargement on this one is incredible

Saturn’s rings have awed and delighted us for centuries.  The beauty and mystery of the rings have kept us looking up, and because of the rings Saturn is often times described as the most beautiful planet in the solar system.  The rings around Jupiter, Uranus, and Neptune have increased our awe of these giants, too.

Studying planetary rings has given us a deeper understanding of the development of our solar system.  However they are formed, they are a wondrous sight.  Mysterious, beautiful, and magnificent; an awesome spectacle that continues to delight.