Nowadays, the solar system consists of the Sun, eight planets and many other minor bodies. It used to have nine planets, but Pluto was recently demoted to the status of a dwarf planet.

There are quite a few mnemonics for remembering the order of the planets. My favourite is My Very Elderly Mother Just Served Us Nine Pizzas.

IAU

The four inner planets are relatively small, rocky and Earth-like.

Mercury is the first planet out from the sun and the smallest. The Sun long ago baked away the atmosphere. Its surface looks a lot like the moon but without the ‘seas’.

Venus is known as the morning or evening star, when it is the brightest object in the sky, near the horizon. It is nearly earth size. Its atmosphere is mainly carbon dioxide and the resulting greenhouse effect results in an average surface temperature of 460 degrees C.

Home sweet home and the third rock from the Sun. The Earth is the only place in the solar system where water exists as a liquid and, so far, the only place where life has been found.

Known as the Red Planet, it is coloured red by rust. Mars has two moons and a thin atmosphere. There are indications that water once flowed freely on the surface. Mars is currently being extensively explored with intension of sending a manned mission sometime. There has long been speculation of life on Mars.

Between Mars and Jupiter is a region filled with debris from the formation of the Solar System. This is known as the Asteroid Belt.

Further out lie the four gas giants.

Jupiter is the largest planet in the solar system. It is composed mostly of hydrogen and helium and is radiating more energy than falls on it. It has 16 moons and a Great Red Spot. It is about ten times the size of Earth.

Saturn is easily recognisable from its rings. These are made up of rocks ranging in size from hundreds of metres to small specks of dust. Like Jupiter, Saturn is composed mainly of hydrogen and helium and is about ten times the size of Earth.

It has at least 18 moons including Titan, another popular candidate for extra-terrestrial life.

Uranus is just bright enough to be seen with the naked eye from Earth. It has 17 moons and like the other gas giants is composed mainly of hydrogen and helium.

It is four times the diameter of the Earth and has a thin ring system which can not be seen from Earth.

Neptune has a small rocky core surrounded by methane, water and ammonia ice. The outer atmosphere is mainly hydrogen and helium, with a little methane. It is almost four times the size of the Earth, has eight moons and probably incomplete rings.

I do not care what ever anyone says, Pluto will always be a planet to me. But with only one fifth of the mass of the moon it is no longer classed as such. With a surface temperature of -400 degrees C it is one of the coldest places in the solar system. It has a elongated, elliptical orbit, which sometimes brings it inside the orbit of Neptune. Pluto has three moons.

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