Gliese 667 is one of two multiple star systems known to host planets below 10 Earth masses. (Courtesy: ESO/L Calçada)
By Tushna Commissariat
If you have thought about planets with two or more suns ever since you saw the dual suns of Tatooine in the first Star Wars film, looks like you are on the same wavelength as some astrobiologists. Jack O’Malley-James, a PhD student at the University of St Andrews, Scotland, has been studying what kind of habitats would exist on Earth-like planets orbiting binary or multiple star systems. He shared his results with peers at the RAS National Astronomy Meeting in Llandudno, Wales on Tuesday 19th April.
O’Malley-James and his team have been running simulations for planets that would orbit multiple star systems and trying to understand the kind of vegetation that might flourish there, depending on the type of stars in the system. Energy via photosynthesis is the foundation for majority of life on Earth, and so it is natural to look for the possibility of photosynthetic processes occurring elsewhere.
With different types of stars occurring in the same system, there would be different spectral sources of light shining on the same planet. Because of this plants may evolve that photosynthesize all types of light, or different plants may choose specific spectral types. The latter would seem more plausible for plants exposed to one particular star for long periods, say the researchers.
Their simulations suggest that planets in multi-star systems may host exotic forms of the 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 range in order to use as much of the available light as possible,” says O’Malley-James. He also believes the plants may be able to use infrared or ultraviolet radiation to drive photosynthesis.
The team simulated combinations of G-type stars (yellow stars like our Sun) and M-type stars (red-dwarf stars), with a planet identical to Earth, in a stable orbit around the system, within its habitable “Goldilocks zone”. This was because Sun-like stars are known to host exoplanets and red dwarfs are the most common type of star in our galaxy, often found in multi-star systems, and are old and stable enough for life to have evolved.
While the binary systems were not exact copies of any particular observed systems, plenty of M-G star binary systems exist within our own galaxy. O’Malley-James calculated the maximum amount of light per unit area- referred to as the “peak photon flux density” from each of the stars as seen on the planets for each set of simulations. This was compared to the peak photon flux density on Earth to determine whether Earth-like photosynthesis would occur.
Factors like star separation were taken into consideration, to give the best possible scenario for photosynthesis. “We kept the stars as close to the planet as we could, so that there would be a useful photon flux from each one [star] on the planet’s surface while still maintaining a stable planetary orbit and a habitable surface temperature,” says O’Malley James.