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“Forests” detectable even in distant solar systems, scientists suggest

Dec. 4, 2010
Special to World Science  

“Tree-like” life forms might be de­tect­a­ble even from across the vast spaces that sep­a­rate us from plan­ets in oth­er so­lar sys­tems, two sci­en­tists pro­pose.

But any test of the claim will have to wait un­til hu­mans are able to pho­to­graph Earth-like plan­ets out­side our so­lar sys­tem, an achieve­ment that NASA sci­en­tists say they’re work­ing to­ward.

As­sum­ing this does be­come pos­si­ble, past re­search has al­ready sug­gested ways to in­di­rectly de­tect liv­ing things on such plan­ets. For ex­am­ple, hordes of breath­ing crea­tures can lead an at­mos­phere to have dis­tinc­tive char­ac­ter­is­tics; these could be de­tect­a­ble in the light that at­mos­phere sends our way. Liq­uid oceans, with their ready po­ten­tial to host life, might give away their pres­ence through a dis­tinc­tive “glint.”

An aerial view of pine forest in the Unit­ed States' Yel­low­stone Na­tion­al Park. (Cour­tesy Jim Peaco, U.S. Nat'l Park Svc.)


Plant-like or­gan­isms would probably sus­tain them­selves by tak­ing in light from the host star, sci­en­tists say. Cer­tain col­ors of light would ab­sorbed more than oth­ers, lead­ing the light re­flected by the “plants” to have dis­tinc­tive, and measura­ble, char­ac­ter­is­tics. 

But this meth­od does­n’t re­veal the struc­ture of the plant, and thus can’t tell apart, say, trees and al­gae.

Now, re­search­ers Chris­to­pher E. Dough­ty and Ad­am Wolf of the Car­ne­gie In­sti­tu­tion in Stan­ford, Cal­i­for­nia, pro­pose an ad­di­tion­al tech­nique that could re­veal wheth­er plants are “tree-like” in struc­ture.

For plants on Earth, “com­pe­ti­tion for light and the need to trans­port wa­ter and nu­tri­ents has led to a tree-like body plan char­ac­terized by hi­er­ar­chi­c branch­ing net­works,” the pair wrote, set­ting forth their pro­pos­al in the Dec. 1 is­sue of the re­search jour­nal As­tro­bi­ol­o­gy.

These tree-like forms cast shad­ows, they not­ed. The large-scale pat­tern of shad­ows would lead the light re­flected off the vegeta­t­ion to take on spe­cif­ic bright­ness and co­lor char­ac­ter­is­tics. For a view­er on a dis­tant plan­et, Dough­ty and Wolf wrote, these char­ac­ter­is­tics would de­pend on the an­gles at which the view­er, the plan­et and its sun lie with re­spect to each other; but these characteristics would change in a pre­dicta­ble way over time, pro­duc­ing de­tect­able pat­tern.

“The pres­ence of tree-like struc­tures is clearly distinguisha­ble” from, say, flat ground with the same col­or, wrote the re­search­ers, who de­vel­oped a com­put­er sim­ula­t­ion to work out this spe­cial pat­tern of re­flect­ance char­ac­ter­is­tics.

If this vegeta­t­ion were wide­spread enough, it would af­fect the re­flect­ance prop­er­ties of the whole plan­et, they added. Clouds would be a prob­lem, they not­ed, but could po­ten­tially be tak­en in­to ac­count.

Most in­ter­est­ing­ly, they added, the re­flect­ance pat­tern due to wide­spread tree-like struc­tures is dis­tinc­tive enough that it can be ob­served even if a plan­et is barely vis­i­ble, ap­pear­ing as a sin­gle point of light in our tele­scopes. The changes in this dot’s bright­ness and col­or at dif­fer­ent “sun/view ge­ome­tries,” they ex­plained, should be the givea­way.

To date, only a hand­ful of plan­ets out­side our so­lar sys­tem have been de­tected through ac­tu­al im­ag­ing of their sur­faces, and all of these are be­lieved to be gas­e­ous plan­ets at least a few times the size of Ju­pi­ter. De­tect­ing smaller, Earth-like plan­ets by pho­to­graphing their sur­faces is be­yond the reach of cur­rent tele­scopes. This could change, though. NASA has an­nounced that it is stu­dy­ing a mis­sion dubbed the Ter­res­tri­al Plan­et Find­er, a pair of com­ple­men­tary space-based ob­ser­va­to­ry de­signed to cap­ture the dim light from dis­tant Earth-like plan­ets.


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“Tree-like” life forms might be detectable even from across the vast spaces that separate us from planets in other solar systems, two scientists propose. But any test of the claim will have to wait until humans are able to photograph Earth-like planets outside our solar system, an achievement that NASA scientists say they’re working toward. Assuming this does become possible, past research has already suggested a variety of ways to indirectly detect living things on such planets. For example, hordes of breathing creatures can lead an atmosphere to have distinctive characteristics that would be detectable in their reflected light. Liquid oceans, with their ready potential to host life, might give away their presence through a distinctive “glint.” As far as plant-like organisms, these would probably sustain themselves by taking in light from the host star, scientists say. Certain colors of light would more easily absorbed than others, leading the light reflected by the “plants” to have distinctive, and measurable, characteristics. But this method doesn’t reveal the structure of the plant, and thus can’t tell apart, say, trees and algae. Now, researchers Christopher E. Doughty and Adam Wolf of the Carnegie Institution in Stanford, California, propose an additional technique that could reveal whether plants are “tree-like” in structure. For terrestrial plants, “competition for light and the need to transport water and nutrients has led to a tree-like body plan characterized by hierarchical branching networks,” the pair wrote, setting forth their proposal in the Dec. 1 issue of the research journal Astrobiology. These tree-like forms cast shadows, they noted. The large-scale pattern of shadows would lead the light reflected off the vegetation to take on specific characteristics. For a viewer on a distant planet, Doughty and Wolf wrote, these characteristics would depend on the angles at which the viewer and the planet’s own sun lie, but they would change in a predictable way over time. “The presence of tree-like structures is clearly distinguishable” from, say, flat ground with the same color, wrote the researchers, who developed a computer simulation to work out this special pattern of reflectance characteristics. If this vegetation were widespread enough, it would affect the reflectance properties of the whole planet, they added. Clouds would be a problem, they noted, but could potentially be taken into account. Most interestingly, they noted, the reflectance pattern due to widespread tree-like structures is distinctive enough that it can be observed even if a planet is barely visible, appearing as a single point of light in our telescopes. The changes in this dot’s brightness and color at different “sun/view geometries,” they explained, should be the giveaway. To date, only a handful of planets outside our solar system have been detected through actual imaging of their surfaces, and all of these are believed to be gaseous planets at least a few times the size of Jupiter. Detecting smaller, Earth-like planets by photographing their surfaces is beyond the reach of current telescopes. NASA has announced that it is studying a mission dubbed the Terrestrial Planet Finder, a pair of complementary space-based observatories designed to capture the dim light from distant Earth-like planets.