Main Sequence Lifetime:

1 Trillion Years

Habitable Zone:

Known Companion:

UV Ceti

Dayside of UV Ceti b
The dim sun will never set.

Stars of type M are the weakest of all main sequence stars. Sometimes coming with one tenth the mass of our sun and 10.000 times less brightness, they can hardly be called suns at all. But this dim stars comprise more than 70% of the stellar population of the galaxy, so they are certainly worth a closer look. M-type stars have an extremely low energy output and will be stable for eternity - from 50 billion to 50 trillion years, depending on their actual mass. When all other stars in the galaxy will long have ceased to exist, there will still be plenty of Red Dwarfs around.

Red Dwarfs are so weak in brightness, that any ecosphere will be narrow and very close to the parent star, usually between 0,04 and 0,2 AU. This proximity will lock a planet's rotation - there would be one side of eternal daylight, and one of eternal darkness. Would this allow for a stable climate? The conventional answer is that under such conditions, the gases of the atmosphere would freeze out on the night side, turning the entire planet in a Mars-like desert.

According to a recent publication, climate would remain stable. As soon as the atmosphere contains at least 150 mbar of carbon dioxide, heat from the star will be trapped on the dayside via greenhouse effect and transported to the nightside, effectively preventing the freezing of the atmosphere. A 1 bar atmosphere of carbon dioxide and nitrogen would keep the dayside at an average temperature of +20°C, while the nightside would be cooler by only 40°C. Warm air would move in two directions along the equator to the nightside, while strong winds from the nightside would bring cold air back across the polar regions.

Side of eternal darkness on UV Ceti b

Even if Red Dwarfs may produce a comfortable climate for life, the light emitted by the star would pose more stringent problems. Red Dwarfs emit most of their energy in the infrared region of the spectrum. Very few energy is emitted in wavelenghts suitable for photosynthesis, so green plants like here on Earth could hardly survive. But there would be other sources of energy on a Red Dwarf planet. So close to its star, the planet would feel the full force of tidal friction, creating a very active interior. Most of our model planet would show intense volcanism, very similar to conditions found in Iceland or Yellowstone National Park. In the absence of effective photosynthesis, organisms on such a world may use geothermal energy to live upon. Such forms of live would thrive even on the planet's dark side.

This would solve yet another problem inherent to Red Dwarfs - the problem of stellar eruptions. UV Ceti is a typical flare star, sometimes showing a double or threefold increase in brightness within a few minutes. The atmosphere of our model planet would buffer this considerable increase in heat, even if these flares should last for months. And if the dominant forms of life do not directly depend upon the sun, they won't be effected seriously.

Some day we might learn that Red Dwarfs are home to most of the habitable worlds within the galaxy, inhabited by the strangest and hardiest of all creatures imaginable. And in a universe that is dying inevitably, Red Dwarfs might be the last refuge for advanced civilizations.

JOSHI M. , HABERLE R. & REYNOLDS T. ,1997: Simulations of the Atmospheres of Synchronously Rotating Terrestrial Planets Orbiting M Dwarfs: Conditions for Atmospheric Collapse and the Implications for Habitability. Icarus, 129, 450