Planets

What Is A Planet?

Planets are among the many worlds and smaller objects that orbit the Sun. The formal definition of planet, as voted on by the International Astronomical Union in 2006, is as follows:

A planet is a celestial body that

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and

(c) has cleared the neighbourhood around its orbit.

Under this definition, Pluto is NOT a planet, but has been deemed a dwarf planet because it has not yet cleared its orbit. This definition is under discussion, particularly by members of the planetary science community, and it may yet be further refined.

Planet Facts

There are 8 planets in our solar system, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. With the exception of Neptune and Uranus the other 6 planets can be seen unaided and all 8 are visible with a small telescope or binoculars.

The order of the planets from closest to the Sun outwards is; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and finally Neptune.

The largest planet in the solar system is Jupiter, followed by Saturn, Uranus, Neptune, Earth, Venus, Mars with the smallest being Mercury.

The table below shows the size of the planet, how far it is from the Sun and how long it takes to complete a single orbit.

Name Diameter Distance from the Sun Orbit Period

Mercury 4,879 km 57,909,227 km (0.39 AU) 88 days

Venus 12,104 km 108,209,475 km (0.73 AU) 225 days

Earth 12,742 km 149,598,262 km (1 AU) 365.24 days

Mars 6,779 km 227,943,824 km (1.38 AU) 1.9 years

Jupiter 139,822 km 778,340,821 km (5.20 AU) 11.9 years

Saturn 116,464 km 1,426,666,422 km (9.58 AU) 29.5 years

Uranus 50,724 km 2,870,658,186 km (19.22 AU) 84.0 years

Neptune 49,244 km 4,498,396,441 km (30.10 AU) 164.8 years

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