Planets and Planetary Rings

A planet is an object which is not a star {i.e. no nuclear fusion takes place in it} and that orbits around a star and is mostly round because its own gravitational pull has shaped it into more or less a sphere. As per current scientific classification, the planets must satisfy three basic criteria as follows:

• A planet must be in hydrostatic equilibrium which implies a balance between the inward pull of gravity and the outward push of the supporting structure. Objects in this kind of equilibrium are almost always spherical or very close to it.
• A planet’s primary orbit must be around the Sun. That means objects like the Moon, Titan, or Ganymede, are not planets, even though they are round due to hydrostatic equilibrium, because their primary orbit is around a planet.
• A planet must have cleared out other, smaller objects in its orbital path, and thus must be by far the largest object in its orbital neighbourhood. Due to this condition, Pluto is not considered a planet {but a dwarf planet}, even though it meets the other two criteria; there are thousands of Plutinos in the orbital path of Pluto, and it crosses the orbit of Neptune, which is a much larger and more massive object.

As per the current system, there are eight planets in the solar system—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune—and a number of dwarf planets, including Pluto, Charon, Ceres, Eris etc.

Planetary Rings

A planetary ring is a system of huge numbers of small bodies—ranging in size from grains of sand to house-sized boulders—that orbit in a coherent ring-shaped pattern around a planet.

The most spectacular planetary rings in the solar system orbit around Saturn; they are more than 170,000 miles across, and are less than one mile thick.

Inner Planets/ Terrestrial Planets

The planets that are collectively thought of as belonging to the inner solar system are Mercury, Venus, Earth, and Mars. These four objects are called the terrestrial planets because they resemble one another (specifically, Earth) in their structure: a metallic core, surrounded by a rocky mantle and thin crust.

There are three moons in the terrestrial zone as well: Earth’s moon, and the two moons of Mars: Phobos and Deimos.

Outer Planets / Gas Giants

Gas giant planets are so named because they are much larger than the terrestrial planets and they have atmospheres so thick that the gas is a dominant part of the planets’ structure. Jupiter, Saturn, Uranus, and Neptune are all categorized as gas giants.

The gas giant zone is the part of the solar system roughly between the orbit of Jupiter and the orbit of Pluto. It contains the outer (gas giant) planets Jupiter, Saturn, Uranus, and Neptune. Each of the gas giant planets has a host of moons and rings or ringlets.