Reference Terms
from Wikipedia, the free encyclopedia

Solar system

The Solar System consists of the Sun and the other celestial objects gravitationally bound to it: the eight planets, their 165 known moons, three dwarf planets (Ceres, Pluto, and Eris and their four known moons), and billions of small bodies.

This last category includes asteroids, Kuiper belt objects, comets, meteoroids, and interplanetary dust.

In broad terms, the charted regions of the Solar System consist of the Sun, four terrestrial inner planets, an asteroid belt composed of small rocky bodies, four gas giant outer planets, and a second belt, called the Kuiper belt, composed of icy objects.

Beyond the Kuiper belt lies the scattered disc, the heliopause, and ultimately the hypothetical Oort cloud.

In order of their distances from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Six of the eight planets are in turn orbited by natural satellites, usually termed "moons" after Earth's Moon, and each of the outer planets is encircled by planetary rings of dust and other particles.

All the planets except Earth are named after gods and goddesses from Greco-Roman mythology.

The three dwarf planets are Pluto, the largest known Kuiper belt object;

Ceres, the largest object in the asteroid belt; and Eris, which lies in the scattered disc.

The four inner or terrestrial planets have dense, rocky compositions, few or no moons, and no ring systems.

They are composed largely of minerals with high melting points, such as the silicates which form their solid crusts and semi-liquid mantles, and metals such as iron and nickel, which form their cores.

Three of the four inner planets (Venus, Earth and Mars) have substantial atmospheres; all have impact craters and tectonic surface features such as rift valleys and volcanoes.

The middle region of the Solar System is home to the gas giants and their planet-sized satellites.

Many short period comets, including the centaurs, also lie in this region.

It has no traditional name; it is occasionally referred to as the "outer Solar System", although recently that term has been more often applied to the region beyond Neptune.

The solid objects in this region are composed of a higher proportion of "ices" (water, ammonia, methane) than the rocky denizens of the inner Solar System.

The four outer planets, or gas giants (sometimes called Jovian planets), collectively make up 99 percent of the mass known to orbit the Sun.

Jupiter and Saturn's atmospheres are largely hydrogen and helium.

Uranus and Neptune's atmospheres have a higher percentage of “ices”, such as water, ammonia and methane.

Some astronomers suggest they belong in their own category, “ice giants.” All four gas giants have rings, although only Saturn's ring system is easily observed from Earth.

Note:   The above text is excerpted from the Wikipedia article "Solar system", which has been released under the GNU Free Documentation License.
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