SEMINAR: OI Seminar Series - Prof John Raven

Prof John Raven from The University of Dundee

Title: Brown Dwarfs and Black Smokers

Abstract: Brown Dwarfs are ‘failed stars’ which are too small to support nuclear fusion as an energy source; they have just gravitational and a little radioactive heating, and so continually cool after their formation but can emit radiation for billions of years. Their surface temperatures are 600 – 2500 K. Black Smokers are submerged hydrothermal vents emitting water at about 600 K. As photosynthetic energy sources the Brown Dwarfs and Black Smokers have a wavelength of maximum photon emission of black-body radiation which is well into the infrared at 1470-6120 nm, whereas algae and plants on Earth use 400-700 nm radiation. Considerations of maintaining liquid water on a planet orbiting a Brown Dwarf mean that more photons are incident on unit area of the planet than is the case for Earth, but each Brown Dwarf photon corresponds to less energy. With an upper wavelength limit for productive photochemistry of 1,100 nm and significant absorption by water in the 700-1,100 nm range, any Earth-like planets hosting photosynthesis that are orbiting Brown Dwarfs are unlikely to have primary productivities of more than a percent or two of that on Earth. Green photosynthetic bacteria have been found near Black Smokers; the radiation from the Black Smoker below 1,100 nm may just be sufficient to support slow growth of these organisms, which may have radiation supplements from sonoluminescence, chemiluminescence and bioluminescence.