A star's rotation slows with time, research shows, giving scientists a clock to understand its age.

THE GIST

Stars slow down as they age due to angular momentum, the same phenomenon that causes a spinning ice skater to slow down if he stretches out his arms.

The speed of rotation is a good measure of a star's age -- and the ages of any of its planets.

Homing in on Earth-like planets means finding places that are about the same age as Earth.

In an attempt to home in on habitable worlds beyond Earth, scientists have developed a new technique to approximate a planet's age.

"We know from studying our own planet that if a star and its planet are about 1 billion years old, only the most primitive microbial life can exist," astronomer Soren Meibom with the Harvard-Smithsonian Center for Astrophysics, wrote in an email to Discovery News.

A key strategy for finding ET then, is to is to focus on planets that not only are Earth-like in size and locale, but also in age.

"A star and its planets form together and share the same age, so if we can determine the age of a star, we also have the ages of its planets," Meibom told reporters at the American Astronomical Society meeting in Boston on Monday. "The question of age is highly relevant to the question of life."

NASA's Kepler Space Telescope is providing scientists a tool to do just that.

Meiborm heads a team that determines stars' spin rates, which vary depending on how old a star is. Young, sun-like stars spin fast. They also have lots of sunspots that trigger variations in brightness as the star spins around. Older stars spin slower and have fewer and smaller spots so there is less fluctuation in their brightness.

The Kepler telescope, which is conducting a survey of extrasolar planets, is proving very adept at finding stars' spin rates, even amongst the galaxy's older stellar residents.

"Kepler is unique in the sense that it is opening this window on measuring the fluctuation in brightness in older stars," said lead Kepler scientist William Borucki, with NASA's Ames Research Center in California.

Meibom and colleagues are using this data to put together a "clock" of sorts to learn the ages of stars, and hence their planets.

His team is studying star clusters in Kepler's field of view, including a cluster that is 1 billion years old, another that is 2.5 billion years old and even a nine-billion-year group, one of the oldest clusters in our galaxy.

"When we have measured this relationship between stellar spin period and age, we can go back and look at all the planet host stars, measure their spin periods, use the relationship to determine their ages," Meibom said.

The research is published in May 20 issue of The Astrophysical Journal Letters.