M dwarf systems - Too wet, too dry, or just right?

Efforts to identify nearby habitable planets should be refocused on G dwarf systems instead of systems around M dwarf stars, according a study published online in Nature Geoscience. Earth-like planets in the habitable zone of M dwarf stars were believed to be more plentiful and easier to detect than those around larger, Sun-like G dwarfs. However, this study reports that a prolonged decline in luminosity early in the evolution of M dwarf stars may render Earth-like planets in these systems uninhabitable.

Feng Tian and Shigeru Ida used numerical models to show that the gradual dimming of M dwarf stars over the first 100 million years of their life - which results in an inward migration of the habitable zone - leads to two extremes in the wetness of planets with Earth-like mass: desert planets and ocean planets. Desert planets have too little water to support life as we know it due to water loss from stellar radiation; whereas ocean planets lack the land needed for a stable climate and are also considered uninhabitable.

According to the simulation, planets with Earth-like water content are rare around M dwarfs, but may occur 10 to 100 times more frequently around G dwarf stars because water loss does not affect planets in the habitable zones of these stars. The authors suggest that future searches for Earth-like planets over the next decade, by TESS, Plato and others, should focus on stars within 30 light years that have a similar mass to the Sun, and not the smaller M dwarfs.

doi: 10.1038/ngeo2372

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