PUBLISHED : Friday, 07 November, 2008, 12:00am
UPDATED : Friday, 07 November, 2008, 12:00am

More than 70 million kilometres away lies a mysterious planet where a day lasts longer than a year

Mercury, taker of souls

The smallest planet in our solar system is Mercury. It is also the planet closest to the Sun. If you were standing on Mercury, the Sun would look three times bigger than it does from Earth. Mercury has no moons.

Days and years

A year on Mercury is shorter than a day on Mercury. Mercury speeds around the sun in 88 Earth days. But the time from sunrise to sunrise on Mercury takes 176 Earth days. During the day, the Sun appears to go backwards at one point. To see what a day on Mercury looks like, go to

Little hot head

By day it is hotter than an oven, at 427 degrees Celsius. But at night it is colder than a freezer, at -168C.

Light as a feather

Because there is less gravity than on Earth, you would weigh less. To work out how much you would weigh on Mercury, multiply your weight by 0.38.

Centre of iron

Mercury is very like our moon. There are huge holes on Mercury's surface called craters (kray-ters). These are caused by asteroids hitting the planet's surface. Pictures of Mercury show it might once have had volcanoes. But its centre is a big ball of liquid iron. It is one of the four Earth-like planets because its surface is solid, while others are made of gas. Mercury has water in the form of ice.

Heavy hitter

Mercury is pretty heavy for its size. It is a little smaller than the moons Ganymede and Titan, but more than twice as heavy. Earth is the densest planet in the solar system, and little Mercury is the second-densest.

Taker of souls

Mercury is named after the Roman god Mercury. He would escort the souls of the dead to the underworld.

Because Mercury is so close to the Sun, we can only see it at dusk and dawn. This is why Greek astronomers gave it two different names. They called it Apollo in the morning and Hermes in the evening.

Earth visitors

In 1973 and 1974, Nasa's Mariner 10 visited Mercury and mapped under half its surface. This year Messenger has visited Mercury and is studying the planet.