Extreme water loss and abiotic O2 buildup on planets throughout the habitable zones of M dwarfs

We show that terrestrial planets in the habitable zones of M dwarfs older than *1 Gyr could have been in runaway greenhouses for several hundred million years following their formation due to the star’s extended premain sequence phase, provided they form with abundant surface water. Such prolonged runaway greenhouses can lead to planetary evolution divergent from that of Earth. During this early runaway phase, photolysis of water vapor and hydrogen/oxygen escape to space can lead to the loss of several Earth oceans of water from planets throughout the habitable zone, regardless of whether the escape is energy-limited or diffusion-limited. We find that the amount of water lost scales with the planet mass, since the diffusion-limited hydrogen escape flux is proportional to the planet surface gravity. In addition to undergoing potential desiccation, planets with inefficient oxygen sinks at the surface may build up hundreds to thousands of bar of abiotically produced O2, resulting in potential false positives for life.

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