Living With A Red Dwarf

Artist's impression of the five-Earth mass planet, Gliese 581 c, found in the habitable zone around the red dwarf Gliese 581.  Credit: European Southern Observatory

Source and Credit:  Astrobiology Magazine (NASA)

Summary (Apr 09, 2009): Not astrobiologists' first choice, red dwarf stars have now gained acceptance as potential hosts for habitable planets. They may not be great to live by in the first couple billion years, but they eventually settle down into relatively pleasant stars.

Roughly three quarters of the stars in the galaxy are red dwarfs. Planet searches have typically passed over these tiny faint stars because they were thought to be unfriendly to potential life forms. However, this prejudice has softened. Preliminary results from a dedicated research program have shown that planets around red dwarfs could be habitable if they can maintain a magnetic field for a few billion years.

Red dwarfs - also called M dwarfs - are between 7 and 60 percent as massive as our sun. Their lower mass means they don’t burn as hot or as brightly, emitting less than 5 percent as much light as the sun. However, they have strong magnetic activity, which makes them relatively bright in X-rays and UV radiation and causes them to flare frequently.

To understand the environment around these common stars, the "Living with a Red Dwarf" program was started three years ago. It is piecing together observational data to provide a profile of how red dwarfs vary in brightness and magnetic activity as they age.

"This is the information that you would want to know to model the suitability for life on a nearby planet," says Ed Guinan of Villanova University, a scientist working with the program.

As habitability goes, red dwarfs were thought to be the bad roommates of the cosmos.

Because they are so faint, the habitable zone — the distance from a star where liquid water can exist — is in many cases closer than the orbital distance between Mercury and our sun. When a planet orbits a star this closely, the gravitational pull of the star may cause the planet to become tidally locked with the same side always facing the star (similar to the Moon's fixed gaze on the Earth).

The habitable zone (HZ) around a red (dM), orange (dK) and yellow (dG) dwarf star. The dotted pink circle is orbit which would have our Earth's temperature. Credit: Living with a Red Dwarf program.

Previously, scientists speculated that the dark side of a tidally locked planet would become so cold that it would freeze up the entire atmosphere, leaving even the sun-lit side with little air for breathing. But more recent models have shown that winds would distribute the heat sufficiently to avoid this atmospheric collapse.

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