The Solar System is the Sun and the objects that orbit the Sun. These are a planetary system (of eight planets) and various secondary bodies: dwarf planets and other small objects that orbit the Sun directly, as well as satellites (moons) that orbit many planets and smaller objects. The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant molecular cloud. The vast majority of the system’s mass is in the Sun, with most of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, called the gas giants, are substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are composed largely of substances with relatively high melting points (compared with hydrogen and helium), called ices, such as water, ammonia and methane, and are often referred to separately as “ice giants”. All planets have almost circular orbits that lie within a nearly flat disc called the ecliptic plane.
The Solar System also contains regions populated by smaller objects. The asteroid belt, which lies between Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune’s orbit lie the Kuiper belt and scattered disc, linked populations of trans-Neptunian objects composed mostly of ices. Within these populations are several dozen to more than ten thousand objects that may be large enough to have been rounded by their own gravity. Such objects are referred to as dwarf planets. Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to these two regions, various other small-body populations including comets, centaurs and interplanetary dust freely travel between regions. Six of the planets, at least three of the dwarf planets, and many of the smaller bodies are orbited by natural satellites, usually termed “moons” after Earth’s Moon. Each of the outer planets is encircled by planetary rings of dust and other small objects.
The solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which extends out to the edge of the scattered disc. The Oort cloud, which is believed to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of interstellar wind. The Solar System is located within one of the outer arms of the Milky Way, which contains about 200 billion stars.
Distances and scales
The distance from the Earth to the Sun is 1 astronomical unit (150,000,000 km). For comparison, the radius of the Sun is .0047 AU (700,000 km). Thus, the Sun occupies 0.00001% (10−5 %) of the volume of a sphere with a radius the size of the Earth’s orbit, while the Earth’s volume is roughly one million (106) times smaller than that of the Sun. Jupiter, the largest planet, is 5.2 astronomical units (780,000,000 km) from the Sun and has a radius of 71,000 km (0.00047 AU), while the most distant planet, Neptune, is 30 AU (4.5×109 km) from the Sun.
With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearer object to the Sun. For example, Venus is approximately 0.33 AU farther out from the Sun than Mercury, while Saturn is 4.3 AU out from Jupiter, and Neptune lies 10.5 AU out from Uranus. Attempts have been made to determine a relationship between these orbital distances (for example, the Titius–Bode law), but no such theory has been accepted. The images at the beginning of this section show the orbits of the various constituents of the Solar System on different scales.
RELATIVE POSITIONS OF THE VARIOUS PLANETS
Viewed from above the Solar System the five planets, Mercury, Venus, Mars, Saturn and Jupiter, would appear in these relative positions on May 3 2002. To understand why the planets change their relative positions from night to night, think of them orbiting the Sun at different speeds. At present they are all on the far side of the Sun. In the evening, their orbits are carrying them upwards, while the Earth is moving downwards relative to the Sun. Mercury and Venus orbit faster than the Earth. As a result they make progress upwards in the evening sky. (After May 3 Mercury will reverse when it comes towards the near side of the Sun.) Mars, Jupiter and Saturn travel more slowly than Earth so the effect is to bring them down in the evening sky, until they eventually pass behind the Sun.
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