Finding life on exoplanets may be more difficult than people thought, said Feng Tian, a professor at the Center for Earth System Science at Tsinghua University in Beijing, China. The report is being presented today to the American Astronomical Society Division for Planetary Sciences meeting in Denver, CO. The result is of special interest because it may shed light on how and where life could be identified on exoplanets.
Current efforts to find exoplanets with the potential to harbor life (habitable planets) and exoplanets with life (inhabited planets) focus on smaller stars than the Sun, because these so called M dwarfs or red dwarfs make up more than 75% of stars in the solar neighborhood. Therefore it may be possible to find habitable planets around these small stars with the current level of technology. Thus searching for habitable planets around M dwarfs is considered the fast track to find a second Earth. High levels of atmospheric oxygen are considered the most promising indicator for life on exoplanets.
However recent observations, using the Hubble Space Telescope, of several planet-hosting M dwarfs show that the ultraviolet (UV) properties of these small stars are quite different from those of the Sun (France et al. 2013). Using the observed UV spectrum of the M dwarf star GJ 876, Feng Tian and his US and Argentina colleagues (Kevin France and Jeffrey Linsky from University of Colorado at Boulder, Pablo J. D. Mauas and Mariela C. Vieytes from the Instituto de Astronomia y Fisica del Espacio, Buenos Aires, Argentina) have shown (Tian et al. 2013) that the atmospheres of a hypothetical habitable planet around GJ 876 could build up significant levels of oxygen even in the absence of life. "In this case the atmosphere of a lifeless planet can be close to that of the Earth's 2.2 billion years ago, after the so called Great Oxidation Event in Earth's geological history," said Feng Tian.
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