1For centuries, people wondered whether the stars in the night sky had planets of their own.
2Since the nineteen nineties, astronomers have discovered thousands of planets orbiting distant suns beyond our solar system.
3Many of these worlds are huge balls of gas that circle their stars in just a few days.
4Others are rocky planets lying in the temperate zone where liquid water could exist on the surface.
5Scientists hunt for these planets by watching stars dim slightly when a planet passes in front.
6By studying the starlight that filters through a planet's air, they can identify the gases it contains.
7Traces of oxygen or methane might suggest that some form of life is at work there.
8So far, no signal from another civilisation has been detected, despite decades of careful listening.
9Radio telescopes across the globe scan the sky for patterns that nature alone cannot easily produce.
10The silence does not prove we are alone, because the galaxy is unimaginably vast and old.
11Perhaps the first message from the stars is already travelling towards us through the darkness.