Not So Cozy

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Science  11 May 2007:
Vol. 316, Issue 5826, pp. 799
DOI: 10.1126/science.316.5826.799a

Habitable Earth-like planets must form just close enough to their parent star for liquid water—and hence life—to exist on their surfaces. Any closer and surface water would be boiled off; any further and it would freeze. Moreover, stars must be at least as long-lived as the Sun for habitable planets to form around them. Red dwarfs offer possible suitable sites: They are both the most common type of star in the Milky Way and also, being smaller than the Sun, exceptionally long-lived. However, Lissauer argues that red dwarfs may not be so hospitable after all. Because red dwarfs are faint, their current habitable zones lie very close to the star. Billions of years ago, though, the star would have been much hotter, and so if a planet were already in place then, its volatiles would have evaporated quickly. Also, the debris left over from disks around such star systems is relatively confined, and so any planets would have been buffeted by collisions with many asteroids, causing water and volatiles to be lost. — JB

Astrophys. J. 660, L149 (2007)10.1111/j.1469-8137.2007.02103.x (2007).

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