The shadows cast by alien trees should change the amount of light their host planet reflects as it orbits its star (Image: Edward Webb/Rex Features)

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.

Plants and some microbes on Earth reflect a large fraction of the near-infrared light that hits them, apparently because absorbing it would cause them to overheat during photosynthesis. So any exoplanet that showed a spike in the near-infrared light it reflected, called a “red edge“, might potentially host plant life.


This new technique could help distinguish between worlds with simple photosynthetic life, such as algae or bacterial mats, and those in which competition for light and the need to distribute water and nutrients drove the evolution of branching, tree-like life-forms.

Nancy Kiang of the NASA Goddard Institute for Space Studies in New York City says the proposal is interesting, but cautions that steep mountains could mimic the effect.

Wolf counters that an Earth-like planet exhibiting erosion and plate tectonics would probably boast few sharp geological features, noting that less than 1 per cent of Earth’s surface has a slope of 45 degrees or more.

Journal reference: Astrobiology, DOI: 10.1089/ast.2010.0495