Brief Answers to the Big Questions

by Stephen Hawking

Other stars are too far away for us to be able to see directly if they have planets going round them. However, there are two techniques that have enabled us to discover planets around other stars. The first is to look at the star and see if the amount of light coming from it is constant. If a planet moves in front of the star, the light from the star will be slightly obscured. The star will dim a little bit. If this happens regularly, it is because a planet’s orbit is taking it in front of the star repeatedly. A second method is to measure the position of the star accurately. If a planet is orbiting the star, it will induce a small wobble in the position of the star. This can be observed and again, if it is a regular wobble, then one deduces that it is due to a planet in orbit around the star. These methods were first applied about twenty years ago and by now a few thousand planets have been discovered orbiting distant stars. It is estimated that one star in five has an Earth-like planet orbiting it at a distance from the star to be compatible with life as we know it.