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Smaller Stars Offer Little Hope

By Jeff Zweerink - May 2, 2007
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The recent discovery of an Earth-sized planet around a nearby star generated a lot of excitement in the scientific community - and rightly so. For one thing, most exoplanets (planets outside the solar system) are gas giants like Jupiter and thus have no potential to sustain life. While an earth-sized planet clearly presents far more possibilities, one must ask the question whether that single criterion justifies the enthusiasm that scientists will find life.

The universally known answer is "no." Life also requires liquid water, so the Earth-like planet must always reside in a region known as the habitable zone. The planet mentioned above does reside within the habitable zone of an M-class star. The size and location of this zone depends on the type of star because, in general, larger stars emit more heat and smaller stars emit less heat. Thus, the habitable zone around an M-class star will be located closer to the star than around our G-class sun. As such, many consequences arise that militate against expecting life around these M stars as identified in the March 2007 issue of the journal Astrobiology.

  1. Because the habitable zone resides much closer to the star, any planet in that zone will rapidly become tidally locked with the star, leading to detrimental effects. The rotation rate dramatically slows to the point where the planet rotates only once for each orbit around the star.
  2. Such a slow rotation rate dramatically diminishes the planet's intrinsic magnetic field and will lead to a rapid end to plate tectonics.
  3. With a dramatically diminished magnetic field, the enhanced stellar activity of an M star will rapidly remove the atmosphere from any planet in the habitable zone.
  4. Without plate tectonics (and the carbonate-silicate cycle it drives), there is no process to generate and replenish the atmosphere on the planet.

With no atmosphere, plate tectonics or magnetic field (while adding a smaller habitable zone and the increased stellar activity) the excitement about finding life on Earth-sized planets orbiting M stars seems premature. Some authors suggest that larger planets, which would have larger intrinsic magnetic fields and greater gravitational pull, could overcome these difficulties. It may be that further research demonstrates some circumstances in which truly habitable planets could exist around M stars. However, using the past as a guide, RTB's creation model predicts that future research will further validate the uniqueness of Earth in its capacity to support life.


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  • Life on Other Planets
  • Extrasolar Planets
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