At least one in every 100 white dwarf stars may be orbited by asteroids and rocky planets, new observations from NASA's Spitzer Space Telescope suggest. The finding could mean that these now dead stars once hosted solar systems similar to our own.
Also, the work could help scientists determine whether other rocky, Earth-like planets are orbiting around other stars.
White dwarf stars are the compact, hot remnants left behind when stars like our own sun reach the end of their lives.
As a middleweight star like our sun ages, it eventually swells into a red giant phase. Stars of this mass aren't heavy enough to end their lives in the spectacular explosions known as supernovas; instead, they gradually expel their atmospheres until they shrivel into the hot, dense dead stars called white dwarfs.
The atmospheres of white dwarfs usually consist entirely of hydrogen and helium, but sometimes heavier elements such as calcium or magnesium are detected contaminating the stellar material.
Data from Spitzer suggest that at least 1 to 3 percent of white dwarf stars are contaminated in this way.
Scientists think that the out-of-place elements come from a gradual rain of orbiting dust onto the sun. The dust emits infrared radiation which Spitzer detects.
The dust is entirely contained within what is called the Roche limit of the star, or close enough that any object larger than a few kilometers would be ripped apart by gravitational tides. (This is the same phenomenon that produced Saturn's rings.) Because of the location of the dust, scientists think that the dust may originate from rocky bodies such as asteroids (also known as minor planets) that were torn apart in this way.
This could mean that as many as 5 million white dwarfs in our own Milky Way are surrounded by orbiting asteroids.
For the asteroids to get within the Roche limit to be pulled apart at the seams, they must be perturbed from an orbit farther out from their star — the asteroids could be nudged by as yet unseen planets.
Because the white dwarfs descend from main sequence stars like the sun, the team's work, presented at the European Week of Astronomy and Space Science conference in England, implies that at least 1 to 3 percent of main sequence stars have terrestrial planets around them.
"In the quest for Earth-like planets, we have now identified numerous systems which are excellent candidates to harbor them," said study team member Jay Farihi of the University of Leicester in England. "Where they persist as white dwarfs, any terrestrial planets will not be habitable, but may have been sites where life developed during a previous epoch."
The composition of the crushed asteroids can be measured by detecting the heavy elements present in white dwarfs, so scientists hope to learn more about the solar systems that may once have circled around the star.
"With high quality optical and ultraviolet observations (for example, the Hubble Space Telescope), we should be able to measure up to two dozen different elements in debris-polluted white dwarfs," Farihi said. "We can then address the question, "Are there rocky extrasolar planets we find similar to the terrestrial planets of our own solar system?"
The hunt for other Earth-like worlds is the primary mission of the Kepler spacecraft, which recently sent back its first images.
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