1. Worlds amongst the stars
Exoplanets – planets outside our Solar System – were first proposed in the 16th Century. Galileo realised our Sun was a star like any other and Copernicus showed how our Solar System’s planets orbit around it. So it seemed fair to assume that planets existed elsewhere in the galaxy, orbiting stars of their own. But it would take hundreds of years to develop telescopes powerful enough to find out whether they did.
The first exoplanet was detected in 1995 and thousands more have been discovered since. At first, we could only see gas giants, similar to Jupiter. Now instruments are sensitive enough to detect planets tantalisingly similar to Earth. We don’t know whether different planets could be home to alien life, very different to us. But for us, we'd need a planet similar to Earth to have any chance of living there.
But we may never find a suitable second home. Find out why.
2. CLICKABLE: Looking for a Goldilocks planet
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Astronomers look for planets that are just like Earth using the ‘Goldilocks’ principle – they have to be just the right type; have the right temperature and atmosphere and orbit the right sort of star.
4. WATCH: Where in the galaxy are they?
Discover what an exoplanet is and where we've been looking for them by watching the animation.
5. Could we ever get there?
Unfortunately, it will be a long time before we can visit these places. Which of these factors do you think will present the greatest obstacle?