Planets! A Short history of our celestial neighbours

Photo: Corbis

The term 'Planet' comes from the Greek word for 'Wanderer', and a planet can be defined as a celestial body that orbits a star, has sufficient mass for it's gravity to overcome rigid body forces so that it assumes a round shape, has cleared the area surrounding it's orbit.

The major planets of the solar system comprise of Mercury, Venus, Earth, Mars, Jupiter and Saturn. Their distance from the sun is respective of their order. These planets are quite visible to the naked eye and because of this fact they have been known to the human race for over a 1000 years, although because they are visible it's difficult to establish the first discoverers.

The first recorded telescope was invented by Hans Lippershey in 1608, a year later Galileo made a telescope with 3x magnification, which it later improved on to develop a telescope with 30x magnification. With this device, now known as the Galilean telescope, he first discovered the moons of Jupiter in 1610.

Uranus was originally believed to be a rogue star until 1781, when the British astronomer William Herschel'scareful tracking proved that it orbited the sun, complying the planetary definition. Slight variations in Uranus's orbit hinted that is was being disturbed by another celestial body. This prediction was proved correct in 1846 by French astronomer and celestial mechanic Urbain Jean Joseph Le Verrier.

Finally in 1930, the existence of Pluto was confirmed by Clyde Tombaugh at the Lowell Observatory in the United States. Pluto orbited the Sun for another 75 years as a planet before it was dethroned in 2005 by the International Astronomical Union, being reassigned as a dwarf planet.

The same year Pluto was dethroned another celestial body was discovered, that is larger than Pluto, by Michael Brown of Caltech. Caltech named the planet Eris. The discovery created a divide in astronomical community, with those for calling Eris the 10th planet of the solar system and those who argue it isn't a true planet.

Some interesting statistics:

Celestial Body	Distance to Sun	Orbit in days	Diameter
Mercury	0.39 AU	87.96 days	4,878 km
Venus	0.723 AU	224.68 days	12,104 km
Earth	1 AU	365.26 days	12,576 km
Mars	1.524 AU	686.98 days	6,787 km
Jupiter	5.203 AU	4332.7 days	142,796 km
Saturn	9.539 AU	10759.9 days	120,660 km
Uranus	19.18 AU	30707.4 days	51,117 km
Neptune	30.06 AU	164.81 years	48,600 km
Pluto	39.53 AU	247.7 years	2,274 km

AU stands for Astronomical Unit and is the average distance from the Earth to the Sun.