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It sounds like a zen koan. If a tree on an alien world falls, would we notice? Christopher Doughty of the University of Oxford and Adam Wolf of Princeton University think we just might.

They say the shadows cast by trees would change the amount of light a planet reflects as it orbits its star. When the planet is behind its star as seen from Earth – as the moon is during its full phase – the trees would cast little visible shadow, while at other points in its orbit the shadows would grow longer from Earth's perspective. Future telescopes should be able to search for these changes in brightness, they say.

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