Planetary ring

Planetary Ring

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Planetary ring

A planetary ring is a ring of cosmic dust

Cosmic dust

Cosmic dust is a type of dust composed of particles in space which are a few molecules to 0.1 mm in size. Cosmic dust can be further distinguished by its astronomical location; for example: intergalactic dust, interstellar dust , interplanetary dust and circumplanetary dust ....
and other small particles orbit

ORBit

ORBit is a Common Object Request Broker Architecture 2.4 compliant Object Request Broker . It features mature C , C++ and Python bindings, and less developed bindings for Perl, Lisp , Pascal , Ruby , and Tcl....
ing around a planet

Planet

A planet , as 2006 definition of planet by the International Astronomical Union , is a celestial body orbiting a star or Stellar evolution#Stellar remnants that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared the neighbourhood of planetesimals....
in a flat disc-shaped region. The most spectacular planetary rings known are those around

Rings of Saturn

Saturn has the most extensive planetary ring system of any planet in the Solar System. The rings of Saturn consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about Saturn....
Saturn

Saturn

Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn, along with Jupiter, Uranus and Neptune, is classified as a gas giant....
, but the other three gas giant

Gas giant

A gas giant is a large planet that is not primarily composed of Rock or other solid matter. There are four gas giants in our Solar System: Jupiter, Saturn, Uranus, and Neptune....
s of the solar system

Solar System

The Solar System consists of the Sun and those Astronomical object bound to it by gravity: the eight planets and five dwarf planets, their 173 known Natural satellite, and billions of Small Solar System body....
(Jupiter

Jupiter

Jupiter is the fifth planet from the Sun and the Solar system by size planet within the Solar System. It is two and a half times as massive as all of the other planets in our Solar System combined....
, Uranus

Uranus

Uranus is the seventh planet from the Sun and the third-largest and fourth most massive planet in the Solar System. It is named after the ancient Greek deity of the sky Uranus the father of Kronos and grandfather of Zeus ....
and Neptune

NEPTUNE

=Overview=The project, along with sister project, VENUS, offers a unique approach to ocean science. Traditionally, ocean scientists have relied on infrequent ship cruises or space-based satellites to carry out their research....
) possess ring systems of their own.

Recent reports have suggested that the Saturnian moon Rhea

Rhea (moon)

'Rhea' is the second-largest natural satellite of Saturn and the List of natural satellites by diameter in the Solar System. It was discovered in 1672 by Giovanni Domenico Cassini....
may have its own tenuous ring system

Rings of Rhea

The Saturnian moon Rhea may have a tenuous planetary ring consisting of three narrow, relatively dense bands within a particulate disk. This would be the first discovery of rings around a natural satellite....
, which would make it the only moon known to possess a ring system.

Overview
There are three ways that planetary rings (the rings around planets) have been proposed to have formed: from material of the protoplanetary disk

Protoplanetary disk

A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star or Herbig Ae/Be stars....
that was within the Roche limit

Roche limit

The Roche limit , sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction....
of the planet and thus could not coalesce to form moons; from the debris of a moon

Natural satellite

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Encyclopedia

A planetary ring is a ring of cosmic dust

Cosmic dust

ORBit

Planet

Rings of Saturn

Saturn

Gas giant

Solar System

Jupiter

Uranus

NEPTUNE

Rhea (moon)

Rings of Rhea

Overview

There are three ways that planetary rings (the rings around planets) have been proposed to have formed: from material of the protoplanetary disk

Protoplanetary disk

A protoplanetary disk is a rotating circumstellar disk of dense gas surrounding a young newly formed star, a T Tauri star or Herbig Ae/Be stars....
that was within the Roche limit

Roche limit

Natural satellite

A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify non-artificial satellites...
that was disrupted by a large impact; or from the debris of a moon that was disrupted by tidal stresses when it passed within the planet's Roche limit. Most rings are thought to be unstable and to dissipate over the course of tens or hundreds of millions of years, but it appears that Saturn's rings might be quite old, dating to the early days of the Solar system.

The composition of ring particles varies; they may be silicate or icy dust. Larger rocks and boulders may also be present, and in 2007 tidal effects from eight 'moonlets' only a few hundred meters across were detected within Saturn's rings.

Sometimes rings will have "shepherd" moons, small moons that orbit near the outer edges of rings or within gaps in the rings. The gravity of shepherd moons serves to maintain a sharply defined edge to the ring; material that drifts closer to the shepherd moon's orbit is either deflected back into the body of the ring, ejected from the system, or accreted onto the moon itself.

Several of Jupiter's small innermost moons, namely Metis

Metis (moon)

Metis , also known as , is the Jupiter's inner moons moon of Jupiter. It was discovered in 1979 in images taken by Voyager 1, and was named in 1983 after the first wife of Zeus, Metis ....
and Adrastea

Adrastea (moon)

Adrastea , also known as , is the second by distance, and the smallest of the four Inner satellites of Jupiter. It was discovered in Voyager 2 probe photographs taken in 1979, making it the first natural satellite to be discovered from images taken by an interplanetary spacecraft, rather than through telescopic photography....
, are within Jupiter's ring system and are also within Jupiter's Roche limit

Roche limit

The Roche limit , sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction....
. It is possible that these rings are composed of material that is being pulled off of these two bodies by Jupiter's tidal force

Tidal force

The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force exerted on one body by a second body is not constant across its diameter....
s, possibly facilitated by impacts of ring material on their surfaces. A moon inside the Roche limit is held together only by its mechanical strength rather than by its gravity, and so loose material on its surface would simply "fall off" to join the rings.

Neptune's rings are very unusual in that they first appeared to be composed of incomplete arcs in Earth-based observations, but Voyager 2

Voyager 2

The spacecraft is an Unmanned space mission interplanetary space probe launched on August 20, 1977. Identical in form to its sister Voyager program craft Voyager 1, Voyager 2 followed a slower trajectory that allowed it to be kept in the ecliptic so that it could be sent to Uranus and Neptune by means of gravity assist during...
's images showed them to be complete rings with bright clumps. It is thought that the gravitational influence of the shepherd moon Galatea

Galatea (moon)

'Galatea' , also known as 'Neptune VI', is the fourth closest inner satellite of Neptune . It is named after Galatea , one of the Nereids of Greek Mythology....
and possibly other as-yet undiscovered shepherd moons are responsible for this clumpiness.

Pluto is not known to have any ring systems. However, some astronomers think that the New Horizons

New Horizons

New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon , Nix , and Hydra ....
probe might find a ring system when it visits in 2015.

It is also predicted that Phobos

Phobos (moon)

'Phobos' is the larger and closer of Mars ' two small natural satellites, the other being Deimos . It is named after the Greek mythology Phobos , a son of Ares ....
, a moon of Mars, will break up and form into a planetary ring in about 50 million years due to its low orbit.

After the impact of Theia

Giant impact hypothesis

The giant impact hypothesis is the now-dominant scientific hypothesis for the formation of the Moon, which is thought to have formed as a result of a collision between the young Earth and a Mars-sized body that is sometimes called Theia ....
and before the coalescence of the Moon

Moon

The Moon is Earth's only natural satellite and the List of natural satellites by diameter satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is km, about thirty times the diameter of the Earth....
, it is generally assumed that the Earth had a ring system.

Planetary ring

Overview

Visual comparison

See also

External links