Orange Dwarfs Are the Best Suns for Life

Author:  Phuongmai Truong
Institution:  UC Berkeley

The sun in our solar system does a great job nourishing life on Earth, but a new study shows that it might not be the best candidate for the job. Edward Guinan and collaborators at Villanova University in Pennsylvania compared different types of stars in probability for hosting a planet that can sustain carbon-based life. The group published their result in August at the International Astronomical Union meeting in Rio de Janeiro and in an online publication at on November 10, 2009. The study may provide future direction in searching for extraterrestrial life.

The researchers used archived data from the now-inoperative International Ultraviolet Explorer satellite to examine ultraviolet (UV) and X-ray emissions of different stars and compute the damage they cause to DNA structures. Each star has a habitable zone, an area around the star with suitable temperature for orbiting planets to have liquid water. The Earth's sun is a mid-level star whose brightness and size are average. The stars bigger than our sun produce intense near-UV radiation which can contribute 5 times more DNA damage than sun-like stars. UV radiation is also more energetic than visible light, making it easier to penetrate the planet's atmosphere.

What about stars smaller and dimmer than the Sun? There are two main classes of such stars: red dwarfs (M dwarfs) and orange dwarfs (K dwarfs). These stars produce less UV radiation, and are thus more life-friendly than the Sun. However, since small stars rotate faster than the sun, their magnetic fields are stronger. The high surface magnetic fields of the M dwarfs carry energy onto the stars' outer layers and produce violent stellar flares. These flares cause spikes in UV and X-ray emission and can potentially strip away a planet's atmosphere over time. Moreover, since an M dwarf is so dim, its orbiting planet must be quite close to the star in order to receive enough heat to sustain life. This close distance often results in the planet being tidally locked: one side of the planet always faces the star. The rotational motion of a tidally locked planet is relatively too slow to maintain a magnetic field strong enough to protect the planet from the star's flares.

Conclusively, the orange dwarfs, or K dwarfs, with lower UV radiation rate than sun-like stars and far less magnetic dynamo than the M dwarfs, can provide the best habitable zones for planets. Guinan's team also argue that the K dwarf stars live five to six times longer than the sun, giving biological evolution more time. In our galaxy, K-type stars make up about 15 percent of the stellar population and are therefore much more abundant than sun-like stars.

The result of the study certainly does not exclude life in any solar system, but it can help astronomers narrow their search for extraterrestrial life.