Life In The Habitable Zone Not Likely Around Cooling Stars
November 21, 2012

Planets Orbiting Cooling Stars Are Poor Candidates For Life

April Flowers for - Your Universe Online

New planets are discovered by astronomers in strange places, such as orbiting around a brown or white dwarf star. This leads scientists to wonder if they might support life because although neither brown nor white dwarfs are stars like our sun, they both glow and so could be orbited by planets with the right ingredients for life.

Although no terrestrial, or earth-like, planets have been confirmed orbiting white or brown dwarf stars, there is no reason to assume they don't exist. New research from the University of Washington and Leibniz Institute for Astrophysics, Potsdam, hints that planets orbiting white or brown dwarfs will prove poor candidates for life, however.

A white dwarf star is the hot core of a dead star, while a brown dwarf star is a failed star. Neither of these objects is massive enough to start nuclear burning as the sun does. In theory, however, they can both be bright enough to support a habitable zone. The habitable zone is a swath of space just right for an orbiting planet's surface water to be in liquid form, thus giving life a chance.

If a planet's orbit keeps it just at the inner edge of that habitable zone it starts to become a runaway greenhouse, like Venus. The planet becomes so warm that the heat removes the planet's surface water — and all chance of life.

One important difference between our sun and white or brown dwarfs is that over time, the dwarf stars slowly cool and become less luminous. As this happens, their habitable zone shrinks inward. This suggests that any planet found currently in the center of the habitable zone must have been near the zone's deadly inner edge at some earlier time.

Such a planet, even if found in the habitable zone today, would "face a difficult path to habitability,” Barnes said. The planet probably lost the means to host life long before it became a habitable zone resident.

“These planets, if we find them today in a current habitable zone, previously had to have gone through a phase which sterilized them forever,” Barnes said. Heller added, “So, even if they are located in the habitable zone today, they are dead.”

The findings of this study were recently published in the journal Astrobiology.