NAI astrobiologists from the Virtual Planetary Laboratory at the University of Washington, graduate student Rodrigo Luger and professor Rory Barnes, have shown that many terrestrial planets in the habitable zones of low mass (M dwarf) stars could have experienced extreme stellar heating for up to 1 billion years after planet formation. This could lead to oceanic evaporation and atmospheric oxygenation. The study has been accepted for publication inAstrobiology.
Unlike the Sun, which formed over a span of a few tens of millions of years, M dwarfs can take several hundreds of millions of years to fully contract from a giant molecular cloud into a hydrogen-fusing star. Since terrestrial planets around these stars probably form within the first ten million years, these planets will be around while the star is still contracting, during which time its luminosity can be up to two orders of magnitude greater than the steady, post-contraction luminosity.
The extreme stellar flux can trigger a runaway greenhouse on these planets, a positive feedback process that can lead to the complete evaporation of their oceans. When all a planet’s surface water is in its atmosphere, the intense X-ray and ultraviolet radiation from these stars can break apart water molecules, leading to the escape of the lighter hydrogen atoms to space and the accumulation of the heavier oxygen atoms in the atmosphere. This process can result in the partial or complete loss of a planet’s surface water to space, and, if enough water escapes, it could also lead to the buildup of massive oxygen atmospheres.
Many terrestrial planets in the habitable zones of M dwarfs could thus have been desiccated, potentially rendering them uninhabitable; they may have also built up large quantities of abiotic oxygen. As the habitable zone is the first place we look for life, and oxygen is often considered an important indicator of life, these new discoveries could change how we search for life on exoplanets.
