PLANET HUNTING About 170 planets have been found outside the Solar System Some astronomers think there may be planets around up to half the stars in our Galaxy alone - that's 50 billion possible planetary systems

The habitable zone For an alien planet to house life it must be: Close enough to its sun to receive enough energy Not so close to its sun that it's too hot for liquid water to exist This region around a star is known as the 'habitable zone'.

Until the 1990s, the only planets that we knew of were the ones that orbited the Sun in our Solar System.

Then in 1995, two astronomers, Michel Mayor and Didier Queloz, made a momentous discovery. They detected a new planet orbiting a distant star called 51 Pegasi.

The discovery sent shock waves through the scientific community and the public press. Our Solar System was no longer alone. New worlds were out there waiting to be discovered.

How to find a new planet

The Sun supports a family of planets around it. So it makes sense to look for new planets around stars that are similar to the Sun. There are over a quarter of a billion stars similar in weight and composition to the Sun in our Galaxy alone.

Discover more about the Sun and its family of planets

The problem is how to detect new 'extrasolar planets' around these stars. Unfortunately, you can't spot them by looking through a telescope as they're too far away. The only way to identify extrasolar planets is to look for the effect they have on their parent star.

Spotting the cosmic clues

An orbiting planet will affect its parent star in two ways:

• The movement of the star
  Stars revolve slowly around the centre of their galaxy in a smooth, elliptical orbit.
  
  But if a star has planets around it, their combined gravity will yank the star out of its smooth orbit. This makes it wobble slightly as it cruises around its galactic path.
  
  By measuring the size of the star's wobble, astronomers can work out the size of its planet, and how far it is from its parent star.
  
  
• Light from the star
  Normally, stars shine with a constant brightness. However, if a planet passes between you and a star, then the star dims as it passes by.
  
  Measuring the changing brightness of a star can tell us whether there are planets orbiting around it.

Find out all about stars

Artist's impression of an extrasolar planet

What have we found so far?

So far, all of the worlds that have been discovered are gas giants like the planet Jupiter. We have yet to find any smaller, rocky planets like Earth.

But that doesn't mean there aren't any new planets like Earth out there. It's much easier to spot larger planets than smaller ones, so it's no surprise that we've found them first.

Over the next 20 years, a flotilla of new spacecraft will be launched to hunt for new Earth-like planets. The discovery of habitable worlds could be just around the corner.