Research press release

M型矮星系 - 湿り過ぎか乾き過ぎか、あるいはちょうど良いか?ブックマーク

Nature Geoscience

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


Feng TianとShigeru Idaは数値モデルを用いて、M型矮星誕生後、1億年間にわたり徐々に光度が低くなる(これによって居住可能域は内側に移動する)ことによって、地球に似た質量の惑星の水分量が両極端になる、すなわち砂漠惑星か海洋惑星になることを示した。砂漠惑星は、よく知られているように恒星の放射により水が失われ生命を維持することが不可能であり、他方海洋惑星には気候の安定に必要な陸地がなくやはり居住不可能と考えられる。


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 | 英語の原文

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