If a tree casts a shadow in the woods, can anyone see it from light-years away?
Considering that the best extrasolar planet pictures to date are dots in deep-space exposures, this question may sound ludicrous.
But a pair of scientists thinks that detecting alien forests might be doable, at least in theory.
Call it the search for Extra-Arboreal life.
We’re never going to see treetops on worlds resembling such memorable sci-fi destinations as the forest moons Yavin, Endor, or Pandora. That would require a telescope mirror the size of the sun’s diameter.
But, writing in the journal Astrobiology, Christopher Dougherty of the University of Oxford and Adam Wolf of Princeton University predict that a forest planet would have a different look because of shadowing by tall trees.
I’ve previously written about finding inhabited planets by their unusual atmospheric chemistry. I’ve insisted we could never know about the presence of anything bigger than a microbe without actually traveling there and exploring an extraterrestrial Serengeti.
Finding convincing evidence of macro-life via telescopic observations would be extraordinary. The idea was toyed with a century ago that large animal migrations on Mars created the Red Planet’s reported cobweb pattern of “canals.” Also, that Venus’ ashen light might be the glow of lightning-sparked forest fires.
Dougherty and Wolf start out by hypothesizing an interplanetary convergent evolution that commonly leads to forests on habitable terrestrial planets. For photosynthetic organisms, competition for light and the need to transport water and nutrients would frequently lead to tree-like structures towering above the surface, they say. This is beautifully illustrated in the Discovery Channel program, “Alien Planet.”
Seen from afar, the forest canopy would present a fractal topology that would markedly change in appearance depending on the angle of illumination by the parent star as the planet orbits it, the researchers explain.
They modeled how light would be reflected from four different types of terrestrial planets: dry and lifeless like our moon, a lifeless ocean planet, a water planet covered only with microbial life, and a water planet with trees.
They found that the forest planet peaks sharply in brightness when it is on the opposite side of the star as seen from Earth. We’d essentially be viewing the planet at its noontime lighting, and the tree canopy would reflect infrared and visible light into space (the proportions depending on the details of the alien world’s photosynthetic processes). When the planet is on either side of the star, we’d be seeing the effect of long tree shadows cast by the early morning or late evening light.
The other planet models do not have the same unique rate of change in reflectivity during their orbits as the forest world does.
The models have to factor in the ever-changing influence of clouds on a planet’s reflectively. This effect vastly complicates how the planet looks from day to day. In running their models, the researchers folded in a decade’s worth of Earth observations from the International Satellite Cloud Climatology dataset (1986-1996). They found that what helps detangle the data is that trees backscatter light (like snowflakes illuminated by a camera flash), while clouds forward scatter light (like seeing fog in front of an oncoming car’s headlights.)
This means that trees rise steeply in reflectivity as the planet approaches a noontime position, but clouds reflect much less.
Several weaknesses complicate the “exo-arboreal” search strategy. The team models a planet with no axial tilt, and in an edge-on orbit as seen from Earth. Therefore, the researchers do not take into account how astronomers could interpret the effect of seasonal variability. How does the planet look when the leaves fall off the trees in the autumn?
If the planet were tilted, the view should dramatically change as northern and southern poles are alternately tipped toward us during the orbit. This would complicate the planet’s variability, especially if continents are distributed asymmetrically between the northern and southern hemispheres.
What’s more, suppose the planet has a very exotic topology that mimics the shadow play of trees? For example, imagine if a significant fraction of the planet is covered with limestone pillars like Utah’s Brice Canyon? Or perhaps the planet has a dried up seafloor that left behind tall limestone spires? Or maybe the world is covered in skyscrapers like the Star Wars megapolis planet Coruscant.
The observations the researchers are proposing could only be accomplished with a large space telescope capable of isolating a planet’s feeble glow from its star. Sensitive photometers would frequently measure the planet’s brightness. The useful range would be within 20 light-years of Earth. Many years’ worth of data would need to be collected to sort out the various factors influencing the planet’s fluctuating brightness patterns.
If we pursue the convergent evolution idea, then arboreal creatures would likely evolve on a forest planet. Like us, they would require stereoscopic vision, hands, feet and enough brainpower to navigate the woods in three dimensions. Would forests help set the stage for the emergence of intelligent creatures? Would their inhabitants resemble us to the point of having a pair of close-set eyes, and multiple limbs aligned along bilateral symmetry?
Maybe some of the Star Trek humanoids really do exist out there. Or at least Rocky the Flying Squirrel.
Photo Credits: NASA, D.Deering, C. Dougherty, A. Wolf, U.S. National Parks Service