The sun, the nine major planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto) and their natural satellites, the asteroids, the comets, and meteoroids. Over 99% of the mass of the system is concentrated in the sun. The solar system as a whole moves in an approximately circular orbit about the centre of the Galaxy, taking about 2.2 × 108 years to complete its orbit.
SOLAR SYSTEM
The solar system is dominated by a star, the sun. The sun's mass makes up more than 99% of the solar system's mass, and it is the gravitational attraction of the sun that holds the planets, asteroids, comets, and meteoroids in their orbits around it. Only the planetary satellites (such as the earth's moon) owe more allegiance to their parent planets, although the satellites too have to accompany the planets in their endless journey around the sun. The solar system itself also moves, rotating about the centre of the Milky Way galaxy about once every 2.2 × 108 years.
The solar system had its origin in a cloud of interstellar dust and gas that condensed around the proto-sun. The planets continued to grow by accretion, and by about 4.6 billion years ago the earth had formed. The inner, or terrestrial, planets – Mercury, Venus, earth, and Mars – are comparatively small. They are composed of rock and metal, with the metallic part forming a dense central core. Only the earth and Mars have natural satellites, and even these may be captured asteroids.
The outer, or giant, planets – Jupiter, Saturn, Uranus, and Neptune – have a rock-metal core surrounded by layers of solid, liquid, and gaseous hydrogen and helium. They are much farther from the sun and therefore much cooler than the inner planets, and have been able to retain gases of low density. They have many satellites; Jupiter has at least 16, Saturn 18, Uranus 17, and Neptune 8. Pluto, the outermost planet, is small with one huge satellite. It is thought that the planet and its moon are captured asteroids or satellites, escaped from a giant planet.
The space between the orbits of Mars and Jupiter is occupied by thousands of asteroids, or minor planets. Most of these are small, often irregularly shaped chunks of rock, with perhaps only 150 of them more than 100 km across. Even smaller are hundreds of thousands of meteoroids, some no larger than grains of dust. Those that enter the earth's atmosphere and burn up as trails of light (shooting stars) are termed meteors. The largest ones that reach the ground are called meteorites.
The last important members of the solar system are comets. Often described as ‘dirty snowballs’, they consist of a nucleus of dust and ice a kilometre or two across, surrounded by a gaseous coma and with a tail that appears as the comet nears the sun. A dusty tail behind the comet is the source of many meteoroids.
| Planet | Equatorial diameter (km) | Mean distance from sun (106 km) | Sidereal period |
| Mercury | 4879.4 | 57.91 | 86.70 days |
| Venus | 12 103.6 | 108.21 | 221.46 days |
| Earth | 12 756.3 | 149.6 | 0.999 years |
| Mars | 6794 | 227.94 | 677.0 days |
| Jupiter | 142 985 | 778.41 | 11.86 years |
| Saturn | 120 536 | 1426.72 | 29.42 years |
| Uranus | 51 118 | 2870.97 | 83.75 years |
| Neptune | 49 528 | 4498.25 | 163.72 years |
| Pluto | 2390 | 5906.38 | 248.02 years |
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