Dust made up of similar stuff as the Earth has been found in and around a handful of dead stars. The dust, which was left behind when the stars chewed up errant asteroids, suggests terrestrial planets may be common.

The Asteroid Belt

Six white dwarfs, the burned-out embers of Sun-like stars, showed heavy elements, or metals, in their atmospheres. That is unusual because white dwarfs contain about as much mass as the Sun squeezed into bodies the size of the Earth, giving them surface gravities 10,000 times stronger than the Sun’s. That should cause heavy elements to sink towards their centers - and out of sight.

In addition, the six stars also shine more brightly than expected in infrared light. That suggests the stars are surrounded by dust, which glows at infrared wavelengths. The dusty debris is thought to be the remains of asteroids that once orbited the white dwarfs but were gravitationally torn apart when they wandered too close to the stars.

Michael Jura of the University of California, Los Angeles, and colleagues measured the infrared light from these stars using NASA’s Spitzer Space Telescope.

Spitzer Space Telescope

Earth-like composition..

The team found the dust contains a glassy silicate material similar to olivine, which is common on Earth and has also been seen on the Moon and Mars.

OLIVINE (Magnesium Iron Silicate)

The dust also seems to have no carbon, consistent with Earth’s composition, which has little carbon compared to the Sun. The results were presented on Monday at the American Astronomical Society meeting in Long Beach, California.

Two previously studied white dwarfs have dust of a similar composition, bringing the tally of such stellar gluttons up to eight. “What was once kind of a freak is now a systematic pattern,” Jura said.

Rocky Planets..

Since asteroids form in the same way as planets, by bulking up through collisions between smaller rocky objects, they have a similar composition to their larger brethren. That suggests terrestrial planets might have once existed in these systems. “This strengthens suspicions that Earth-like planets are common,” Jura said.

Ring around white dwarf

Many white dwarfs may host rocky discs, but they may be impossible to detect because asteroids were not jostled out of position and sent careening towards the star, leaving traces of their existence in the star’s atmosphere and in surrounding dust.

And even when an asteroid has plunged into a star, the evidence of its violent end does not last long. Single asteroids no larger than 200 kilometres across could explain the dust around each of the newly studied white dwarfs, and their remains could be gobbled up and ‘digested’ - sinking to the star’s centre - in as little as 10,000 years. “In a way it’s amazing that any dust at all survives,” Jura told New Scientist.

Asteroid Defense..?

Experts say there are an estimated 1,100 known objects that are 1 kilometer (about a half-mile) or wider across — large enough to not only take out a sizable European country but threaten the entire world.

“The goal is to discover these killer asteroids before they discover us,” said Nick Kaiser of the University of Hawaii’s Institute for Astronomy, whose Pan-STARRS program will train four powerful digital cameras on the heavens to watch for would-be intruders.

NASA’s Spaceguard Survey, which already has identified 800 of the larger objects and has 103 on an impact risk watchlist, wants to find 90 percent by the end of 2008.

The U.S. Congress has asked the space agency for a plan to comb the cosmos for faint objects as small as 140 meters (153 yards) across and log their position, speed and course by 2020.

Astronomers will have their work cut out for them: Experts say there are about 100,000 such objects hidden among the haze of stars, and as many as 1 million half that size.

One known as the Tunguska object slammed into remote central Siberia in 1908, unleashing energy equivalent to a 15-megaton nuclear bomb and wiped out 60 million trees over an 830-square-mile area. Had it hit a populated area, the loss of life would have been staggering.

Giovanni Valsecchi of Italy’s National Institute of Astrophysics said the ultimate aim is a permanent warning system like those that now monitor the Pacific for tsunamis and keep tabs on volcanoes and earthquake zones.

The idea: Give the world enough lead time to come up with a workable response to a confirmed threat, such as sending up a rocket to deflect an Earth-bound object or a spacecraft to nudge it into a harmless orbit.

“Right now, unfortunately, there are no ‘asteroid busters’ or hotlines. Who ya gonna call?” said Andrea Milani Comparetti, a professor of mathematics at the University of Pisa.

The IAU offered some reassurance Thursday about 99942 Apophis, a smallish asteroid that will come within just 18,000 miles of Earth when it whizzes by in 2029. That’s closer than many commercial satellites, and 220,000 miles nearer than the moon.

Last year, scientists were concerned Apophis could come even closer in another fly-by in 2036, with a 1-in-5,500 chance of striking Earth with enough energy to wipe out New York City and its suburbs.

Asteroid Defense?