Spinning stars slow as they grow older, and this rotational rate could be key to detecting life on planets around alien stars.

Our own Sun rotates around its axis once every 25 days, and astronomers calculate that two billion years ago, our stellar companion rotated once every 18 days. For the majority of the history of life on Earth, lifeforms remained exceedingly simple. Billions of years of evolution were required before intelligent life developed on our home planet. Such a long time line would likely need to take place on other planets as well, meaning older planets are more likely than younger ones to support intelligent life.

Gyrochronology, a term based on the Ancient Greek words for spin (gyro), time (chronos), and study (logos), is the measure of spin rates of stars, in an effort to measure the age of the stellar body. Astrobiologists believe intelligent life is most likely to be found on alien worlds at least as old as the Earth.

"Our goal is to construct a clock that can measure accurate and precise ages of stars from their spins. We've taken another significant step forward in building that clock," Soren Meibom of the Harvard-Smithsonian Center for Astrophysics (CfA), said.

Spin rates of stars are dependent on several characteristics of the bodies, including mass, with heavier stars rotating faster than less-massive bodies. The correlation between mass, rotational rate, and stellar age is precise enough that if astronomers know two of the values, they can calculate the third.

"We have found that the relationship between mass, rotation rate and age is now defined well enough by observations that we can obtain the ages of individual stars to within 10 percent," Sydney Barnes from the Leibniz Institute for Astrophysics in Germany, said.

Stars are so distant that detail cannot be resolved on their surfaces, even using the most powerful telescopes in the world. Astronomers measure the rotational rates of stars by measuring their brightness over time. As sunspot-like features move across the surface of the star as seen from Earth, light from the stellar body dims, before the dark patch rotates around the target once more. These changes can alter the luminescence of the star by less than one percent over the course of several days, making observations challenging.

Astronomers studied 30 stars, with masses between 80 and 140 percent as massive as the Sun, exhibiting rotational periods between four and 23 days.

Barnes first presented the idea of gyrochronology in 2003, based on his earlier research.

The role of gyrochronology in the search for alien life will be detailed at the 225th annual meeting of the American Astronomical Society, being held in Seattle, Washington through January 8.

A Spin-down Clock for Cool Stars from Observations of a 2.5-billion-year-old Cluster was published in the journal Nature.