Dead Star Debris Reveals Earth-Like Traces

April 24, 2009 -- Scientists picking through the burnt-out remains of sun-like stars have a message for the Kepler team hunting for other Earths: Expect to find systems with terrestrial bodies like ours about 1 to 3 percent of the time.

So concludes a three-year study of white dwarf stars, which are the dead bodies of stars that were at one time about the same size as our sun. About 95 to 99 percent of the stars in our galaxy end up as white dwarfs.

"We're finally able to put a statistical limit on how frequent rocky Earth-like systems were around progenitor stars," University of Leicester astronomer Jay Farihi told Discovery News.

Farihi and colleagues used NASA's infrared Spitzer Space Telescope and other data to inventory the number of white dwarf stars that have a faint ring of debris, similar to the rings circling Saturn and other planets. They also previously found telltale chemical traces of rocky bodies in the atmospheres of otherwise pristine white dwarfs.

"We see both the debris ring and the pollution in the star, so it's really a complete picture," Farihi said.

White dwarfs have strong gravitational grips -- about 1 million times the pull of Earth's gravity -- so heavy elements normally would be pulled inside the stars, leaving only hydrogen and helium molecules external.

But when astronomers looked at some white dwarfs, they found the fingerprints of iron, calcium and magnesium, which are believed to be the chemical remains of rocky worlds that have been recently absorbed by the star.

"If you dropped it in today, it'd sink away rapidly -- it should sink away -- so what we're seeing here has to be very recent events," Farihi said.

The team followed up that discovery with additional observations of debris rings around 14 white dwarf stars. The rings presumably are the remains of rocky planets similar to Mercury, Venus, Earth and Mars, as well as asteroids.

"That's really cool," said San Francisco State University astronomer Adrienne Cool, who studies white dwarfs in distant globular star clusters. "I have heard only once or twice a mention of this type of thing."

The stars studied by the Spitzer team are located about 100 to 150 light-years from Earth. Cool looks at white dwarf in globular star clusters 7,000 light years-away, too far for looking for signs of planetary bodies. She uses information about white dwarfs to learn about stellar evolution.

NASA last month launched a telescope named Kepler to hunt for Earth-like planets around some 100,000 target stars.

"They can prove or disprove our findings," said Farihi, who presented his research at the European Week of Astronomy and Space Science conference at the U.K.'s University of Hertfordshire this week.