MOON

Moons may bow to planets in terms of size, but in character they often outshine their stolid parents. The named moons of the solar system outnumber planets by more than 20 to 1, and they display a remarkable diversity. There are fully fledged worlds such as Titan, as complex as any planet. There are possible havens for life, such as the ice-crusted water world Europa. New mysteries surround even the smallest satellites, most recently the apparent flying saucers orbiting Saturn.

This year it will be four centuries since Galileo discovered Jupiter's four large satellites, at a stroke quintupling the number of moons then known to humanity.

1. IO
Pockmarked with sulphurous pits, bathed in intense radiation and shaken by constant volcanic eruptions, Io is the fiery hell of the solar system.

2. Iapetus
Even a cursory glance at Saturn's moon Iapetus reveals it to be an oddball. For one thing it is two-toned: one half is black, the other shining white.

3. Europa, Enceladus and Triton
The seemingly bleak icy surfaces of Europa, Enceladus and Triton are in fact among the most active landscapes in the solar system. They may even contain cosy habitats for living creatures.

4. Pan and Atlas
Pan and Atlas come straight from a 1950s B-movie. With a central bulge set inside a disc-like ridge, they bear an uncanny resemblance to your stereotypical flying saucers.

5. Nereid
An otherwise undistinguished satellite of Neptune, moderately lumpy and middling in size, Nereid travels on the most eccentric orbit of any moon in the solar system.

6. Titan
It is perhaps the strangest of all moons because it is so eerily familiar. The newly revealed face of Titan has the same weather-beaten features as Earth.

7. Earth's MOON
Scores of moons have been discovered within our solar system, yet Earth's companion still stands out as one of the most remarkable.

iF OUR solar system holds so many remarkable moons, then what strange satellite worlds might we find among the billions of planetary systems in the Milky Way? Perhaps there are temperate, habitable moons orbiting some giant exoplanets. We shouldn't expect to find them inhabited by intelligent life such as the furry Ewoks of Endor in Star Wars, but such moons may be among the most likely habitats for life in the universe.

On the face of it, detecting a moon around a planet orbiting a distant star seems like a spectacularly difficult task, but with a bit of luck today's technology may be able to do it. The best approach is to look for transits, in which an orbiting planet passes in front of its star, dimming the amount of light we detect on Earth. This method has already been used to find several planets, and it could indirectly reveal exomoons. As a moon orbits a planet, its gravity makes the planet move, speeding it up and slowing it down and so changing the timing and duration of transits.

The bigger the moon in relation to the planet, the bigger this effect. In one simulation, a planet with the mass of Neptune situated in the habitable zone of a star - not too hot, not too cold - was given a moon the size of Earth. This weighty moon would change the timing and duration of its planet's transits enough to be detectable by the Kepler planet-finding satellite, or even by ground-based telescopes. Such a large moon would also be able to hold onto a thick atmosphere, making it a prime spot for life.

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