In 1990 Voyager took a photograph of Earth that was striking precisely because it showed so little. The spacecraft was 6 billion kilometres away at the time and the image it sent back was memorably described by Carl Sagan as a "pale blue dot". Imagine, then, how pale such a dot would be if the planet in the picture were 113,000 billion kilometres away. Yet this is the distance to the nearest confirmed exopla-net-a planet orbiting a star other than the sun. That gives some idea of the task faced by those who study these bodies. Only in the most special of circumstances can they actually see their quarry. Mostly, they have to work with indirect measurements, like watching for slight dips in the intensity of a star's light when a planet passes in front of it, a phenomenon known as a transit.
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