If a tree casts a shadow on a distant planet...

A few weeks ago I mused on the possibly very dark coloured flora of a distant planet. Today a report came in from New Scientist that the shadow cast by plants on distant planets would alter the amount of light reflected back to us.

Scientists from the universities of Oxford and Princton reckon one day we'll be able to see from Earth the shadows of trees in other galaxies. Well, sort of.

The New Scientists Rachel Courtland summarised the results of their paper from Astrobiology (DOI: 10.1089/ast.2010.0495 if you are into that kind of thing)

And this is how you see a tree shadow when it's a long, long, long...long way a way. When a planet is behind its star from our perspective on Earth the trees have little visible shadow, but elsewhere in the orbit the shadows are longer, from where we sit. We are asked to imagine this on our own moon.

When it is 'full' it's behind the sun, our star. If there were trees on the moon, they would be dead... Just kidding. Let's assume their is water and all the other things they need. Then, these trees would hardly cast a shadow on the other side of the sun but a big one on our side.

Researchers Christopher Doughty and Adam Wolf reckon telescopes will one day be able to pick up these subtle changes in brightness.

Photosynthetic things, like plants, reflect a lot of light (hence the suggestion in my earlier post that blackish plants would be more efficient than our Earthly green ones). It's noted that one reason for this reflection is to stop overheating - a problem for dark coloured plants and perhaps why they aren't common on our planet, or maybe any other.

Astrobiologists (scientists looking for life throughout the universe) look for planets with a lot of reflected (near-infrared) light. This spike is apparently called a 'red edge' and is thought to indicated plant life. The shadow effects might help distinguish planets dominated by algae (hurray!) from those with more complex, bigger life, like Planet Earth.

Nancy Kiang, who I mentioned last time, says 'the proposal is interesting, but cautions that steep mountains could mimic the effect). The shadow advocates say that even an apparently lumpy planet like Earth has less than 1 per cent of its surface with a slope of 45 degrees or more so the geological shadows won't rival the tree ones.

I have no idea whether this makes sense or not, but I love the idea that we can think about, and plan for, measuring tree shadows on distant planets.

Image: Tree shadows at Mount Annan Botanic Garden (photo by Jaime Plaza), possibly visible from far galaxies.