Planet
A planet is a large object orbiting a star. Traditionally there are 9 planets orbiting our star, the sun. A redefinition of the term 'planet' now excludes the smallest and outermost planet, Pluto, from inclusion so now only 8 planets are counted. The largest planet is Jupiter, the 5th planet form the sun and the smallest is Mercury, the closest to the sun. Our own planet, Earth, is the 3rd planet out from the sun.
| Name | Distance from the sun | Diameter | Mass | Date of Discovery |
|---|---|---|---|---|
| Mercury | 57,910,000 km (0.38 AU) | 4,880 km | 3.30e23 kg | a long time ago |
| Venus | 108,200,000 km (0.72 AU) | 12,103.6 km | 4.869e24 kg | a long time ago |
| Earth | 149,600,000 km (1.00 AU) | 12,756.3 km | 5.972e24 kg | a long time ago |
| Mars | 227,940,000 km (1.52 AU) | 6,794 km | 6.4219e23 kg | a long time ago |
| Jupiter | 778,330,000 km (5.20 AU) | 142,984 km | 1.900e27 kg | a long time ago |
| Saturn | 1,429,400,000 km (9.54 AU) | 120,536 km (equatorial) | 5.68e26 kg | a long time ago |
| Uranus | 2,870,990,000 km (19.218 AU) | 51,118 km (equatorial) | 8.683e25 kg | 1781 |
| Neptune | 4,504,000,000 km (30.06 AU) | 49,532 km | 1.0247e26 kg | 1846 |
| Pluto | 5,913,520,000 km (39.5 AU) | 2274 km | 1.27e22 kg | 1930 |
The planets can be subdivided into two main groups. The inner 4 terrestrial planets have a rocky composition. They are relatively small in size. The outer 4 planets are known as Gas Giants and have a thick gaseous atmospheres. Pluto, the furthest out of the traditional list of planets, is now classified as a Dwarf Planet.
Evidence has been gathered indicating the potential existence of planets outside of our solar system, and orbiting other starts. These planets are far beyond the range of best telescopes, so we cannot observe them directly. An alternative method has been used to identify which stars may have planets. The gravitational pull of an orbiting planet causes the star to wobble. By measuring the wobble, astronomers are able to identify stars with planets and even estimate the size of the planet.
Planets are believed to form form a the disk of dust that surrounds a newly formed star. Gravitation forces pull the dust into lumps. The lumps gradually collide and join to make larger lumps or asteroids. These join further to make larger and larger planets until the planet has cleared it's orbit of objects.
In ancient times, the planets were thought to orbit the Earth rather than the sun. There were 7 planets known to the Romans and Greeks. The first was the Sun - Sol, then the Moon - Luna, Mercury, Venus, Mars, Jupiter, and Saturn. These were the only planets visible to the naked eye. The were distinct form other objects in the sky since they moved position relative to the other stars. The Romans attributed a deity to each of the plants. The tradition of using Clasical deities for planet names continued with the discovery of Uranus, Neptune and Pluto. The Romans designated a day of the week to each of their gods so the names for the 7 days share an origin with the planet names. Sunday is literally the day of the Sun. Monday the moon and so on. Later, some of the English day names were replaced with Norse gods instead of Roman. The Roman names can still be found in some other languages. For example Mercredi in French is Mercury day.
The orbits of the planets also has a bearing in Astrology. It is believed the the position of the planets within various constellations of stars can be an indicator of events on Earth. By carefully measuring the orbits and positions of the planets and calculating their future paths, it is believed that one can predict future events.