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In double-sunned worlds, black trees?

April 20, 2011
Courtesy of the Royal Astronomical Society
and World Science staff

A sky with two suns is a fa­vor­ite im­age for sci­ence fic­tion films, but how would a dou­ble star sys­tem af­fect life on an or­bit­ing plan­et?

The trees, for one, might be black or gray, ac­cord­ing to Jack O’Malley-James of the Un­ivers­ity of St. An­drews in the U.K., who has stud­ied the is­sue. He pre­sented find­ings at the Roy­al As­tro­nom­i­cal So­ci­ety’s an­nu­al meet­ing in Llan­dud­no, Wales, on April 19.

Artist's con­cep­tion (Cour­te­sy RAS)


Pho­to­syn­the­sis—the con­ver­sion of sun­light in­to en­er­gy—by plants is the founda­t­ion of life on Earth. It pro­vides en­er­gy to plants and, by ex­ten­sion, to the an­i­mals that eat them, and the an­i­mals that eat them, and so on. 

Plants here evolved to be green, most­ly, be­cause this col­or al­lows them to draw en­er­gy most ef­fi­ciently from the sun­light. But the ex­act leaf col­or really de­pends largely on the type of sun, in par­tic­u­lar on the dis­tri­bu­tion of col­ors with­in its light. 

With mul­ti­ple light sources, O’Malley-James said, life may have adapted to use all suns, or dif­fer­ent forms may de­vel­op that use one spe­cif­ic sun. This may be the more likely op­tion for plan­ets on which parts of the sur­face are il­lu­mi­nated by only one sun for long pe­ri­ods of time.

Black leaf col­or would ab­sorb all vis­i­ble light, max­i­miz­ing the en­er­gy in­take to ex­ploit dif­fer­ent light sources, O’Malley-James said.

“If a plan­et were found in a sys­tem with two or more stars, there would po­ten­tially be mul­ti­ple sources of en­er­gy avail­a­ble to drive pho­to­syn­the­sis. The tem­per­a­ture of a star de­ter­mines its col­or and, hence, the col­or of light used for pho­to­syn­the­sis. De­pend­ing on the col­ors of their star-light, plants would evolve very dif­fer­ently,” said O’Malley-James.

He is work­ing on a doc­tor­ate to as­sess the po­ten­tial for pho­to­syn­thetic life in mul­ti­-star sys­tems with dif­fer­ent com­bina­t­ions of Sun-like stars and red dwarf stars. Many Sun-like stars are known to host plan­ets. But red dwarfs are the most com­mon type of star in our gal­axy, of­ten found in mul­ti­-star sys­tems, and old and sta­ble enough for life to have evolved, as­tro­no­mers say.

Over a quar­ter of Sun-like stars and half of red dwarfs are found in mul­ti­-star sys­tems. In sim­ula­t­ions by O’Malley-James and col­leagues, the Earth-like plan­ets ei­ther or­bit two stars close to­geth­er or or­bit one of two widely sep­a­rat­ed stars. The team has al­so looked at com­bina­t­ions of these sce­nar­i­os, with two close stars and one more dis­tant star.

“Our sim­ula­t­ions sug­gest that plan­ets in mul­ti­-star sys­tems may host ex­ot­ic forms of the more fa­mil­iar plants we see on Earth. Plants with dim red dwarf suns for ex­am­ple, may ap­pear black to our eyes, ab­sorbing across the en­tire vis­i­ble wave­length [col­or] range in or­der to use as much of the avail­a­ble light as pos­si­ble. They may al­so be able to use in­fra­red or ul­tra­vi­o­let radia­t­ion to drive pho­to­syn­the­sis,” he went on.

“For plan­ets or­bit­ing two stars like our own, harm­ful radia­t­ion from in­tense stel­lar flares could lead to plants that de­vel­op their own UV-blocking sun screens, or pho­to­syn­the­tic mi­croor­gan­isms that can move in re­sponse to a sud­den flare,” he added.


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A sky with two suns is a favorite image for science fiction films, but how would a double star system affect life on an orbiting planet? The trees, for one, might be black or gray, according to Jack O’Malley-James of the University of St. Andrews in the U.K., who studied the issue. He presented his findings at the Royal Astronomical Society’s annual meeting in Llandudno, Wales, on April 19. Photosynthesis—the conversion of sunlight into energy—by plants is the foundation of life on Earth. It provides energy to plants and, by extension, to the animals that eat them, and the animals that eat them, and so on. Plants here evolved to be green, mostly, because this color allows them to draw energy most efficiently from the sunlight. But the exact leaf color really depends on the type of sun, in particular on the distribution of colors within its light. With multiple light sources, O’Malley-James said, life may have adapted to use all suns, or different forms may develop that use one specific sun. This may be the more likely option for planets on which parts of the surface are illuminated by only one sun for long periods of time. Black leaf color would absorb all visible light, maximizing the energy intake to exploit different light sources, O’Malley-James said. “If a planet were found in a system with two or more stars, there would potentially be multiple sources of energy available to drive photosynthesis. The temperature of a star determines its color and, hence, the color of light used for photosynthesis. Depending on the colors of their star-light, plants would evolve very differently,” said O’Malley-James. He is working on a doctorate to assess the potential for photosynthetic life in multi-star systems with different combinations of Sun-like stars and red dwarf stars. Many sun-like stars are known to host planets. But red dwarfs are the most common type of star in our galaxy, often found in multi-star systems, and old and stable enough for life to have evolved, astronomers say. Over a quarter of sun-like stars and half of red dwarfs are found in multi-star systems. In simulations by O’Malley-James and colleagues, the Earth-like planets either orbit two stars close together or orbit one of two widely separated stars. The team has also looked at combinations of these scenarios, with two close stars and one more distant star. “Our simulations suggest that planets in multi-star systems may host exotic forms of the more familiar plants we see on Earth. Plants with dim red dwarf suns for example, may appear black to our eyes, absorbing across the entire visible wavelength [color] range in order to use as much of the available light as possible. They may also be able to use infrared or ultraviolet radiation to drive photosynthesis,” he went on. “For planets orbiting two stars like our own, harmful radiation from intense stellar flares could lead to plants that develop their own UV-blocking sun screens, or photosynthesizing microorganisms that can move in response to a sudden flare,” he added.