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Materials for “Earths” common in universe, researchers say

Jan. 5, 2009
Courtesy Jet Propulsion Laboratory
and World Science staff

The ma­te­ri­als that make up Earth could be com­mon in the uni­verse, sug­gest­ing rocky plan­ets like ours could be, too, as­tro­no­mers say.

The claims come from a study with NASA’s Spitzer Space Tel­e­scope of six dead “white dwarf” stars lit­tered with the re­mains of shred­ded as­ter­oids. 

NA­SA's Spitzer Space Tel­e­scope. The in­fra­red tel­e­scope was launched in 2003 in­to an or­bit like Earth's that takes it around the Sun. Above, an artist's di­a­gram shows the or­bit. (Cred­it: NA­SA/JPL-Caltech)


The debris con­sists of dust si­m­i­lar to what one might find in our So­lar Sys­tem “if you ground up our as­ter­oids and rocky plan­ets,” said Mi­chael Ju­ra of the Uni­ver­s­ity of Cal­i­for­nia, Los An­ge­les. He pre­sented the find­ings to­day at the Amer­i­can As­tro­nom­i­cal So­ci­e­ty meet­ing in Long Beach, Ca­lif. 

“This tells us that the stars have as­ter­oids like ours—and there­fore could al­so have rocky plan­ets,” added Ju­ra, lead au­thor of a pa­per on the find­ings ac­cept­ed for pub­lica­t­ion in the As­tro­nom­i­cal Jour­nal.

As­ter­oids and plan­ets form out of dusty ma­te­ri­al that swirls around young stars, as­tro­no­mers be­lieve. The dust sticks to­geth­er, form­ing clumps and even­tu­ally full-grown plan­ets. As­ter­oids are the left­o­ver de­bris. When a star like our sun nears the end of its life, it puffs up in­to a red gi­ant that con­sumes its in­ner­most plan­ets, while jostling the or­bits of re­main­ing as­ter­oids and out­er plan­ets. As the star con­tin­ues to die, it blows off its out­er lay­ers and shrinks down in­to a ske­l­e­ton of its form­er self—a white dwarf.

Some­times, a jos­tled as­ter­oid wan­ders too close to a white dwarf and meets its demise—the gra­vity of the white dwarf shreds the as­ter­oid to pieces. A si­m­i­lar thing hap­pened to Com­et Shoe­maker Levy 9 when Jupiter’s gra­vity tore it up, be­fore the com­et ul­ti­mately smashed in­to the plan­et in 1994.

Spitzer ob­served shred­ded as­ter­oid pieces around white dwarfs with its in­fra­red spec­tro­graph, an in­stru­ment that breaks light apart in­to its con­stit­u­ent col­ors. This re­veals the im­print of chem­i­cals sur­round­ing the ob­served ob­ject. Pre­vi­ously, Spitzer an­a­lyzed the as­ter­oid dust around two so-called pol­lut­ed white dwarfs; the new ob­serva­t­ions br­ing the to­tal to eight.

“Now, we’ve got a big­ger sam­ple of these pol­lut­ed white dwarfs, so we know these types of events are not ex­tremely rare,” said Ju­ra.

In all eight sys­tems ob­served, Spitzer found that the dust con­tains a glassy sil­i­cate min­er­al si­m­i­lar to ol­i­vine and com­monly found on Earth. “This is one clue that the rocky ma­te­ri­al around these stars has evolved very much like our own,” said Ju­ra.

A sin­gle as­ter­oid is thought to have bro­ken apart with­in the last mil­lion years or so in each of the eight white-dwarf sys­tems. The big­gest of the bunch is esti­mated to have once been 200 km (124 miles) wide, a bit larg­er than Los An­ge­les Coun­ty.

Ju­ra said the real pow­er of ob­serving these white dwarf sys­tems is still to come. When an as­ter­oid “bites the dust” around a dead star, it breaks in­to very ti­ny pieces. As­ter­oid dust around liv­ing stars, by con­trast, is made of larg­er par­t­i­cles. By con­tin­u­ing to use spec­tro­graphs to an­a­lyze the vis­i­ble light from this fi­ne dust, as­tro­no­mers will be able to see ex­quis­ite de­tails—in­clud­ing in­forma­t­ion about what el­e­ments are pre­s­ent and in what abun­dance. 

This will re­veal much more about how oth­er star sys­tems sort and pro­cess their plan­etary ma­te­ri­als, Jura explained. “It’s as if the white dwarfs sep­a­rate the dust apart for us,” he said.


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The materials that make up Earth could be common in the universe, suggesting rocky planets like ours could be, too, astronomers say. The claims come from a study with NASA’s Spitzer Space Telescope of six dead “white dwarf” stars littered with the remains of shredded asteroids. The chewed-up asteroids consist of dust similar to what one might find in our Solar System “if you ground up our asteroids and rocky planets,” said Michael Jura of the University of California, Los Angeles. He presented the findings today at the American Astronomical Society meeting in Long Beach, Calif. “This tells us that the stars have asteroids like ours—and therefore could also have rocky planets,” added Jura, lead author of a paper on the findings accepted for publication in the Astronomical Journal. Asteroids and planets form out of dusty material that swirls around young stars, astronomers believe. The dust sticks together, forming clumps and eventually full-grown planets. Asteroids are the leftover debris. When a star like our sun nears the end of its life, it puffs up into a red giant that consumes its innermost planets, while jostling the orbits of remaining asteroids and outer planets. As the star continues to die, it blows off its outer layers and shrinks down into a skeleton of its former self—a white dwarf. Sometimes, a jostled asteroid wanders too close to a white dwarf and meets its demise—the gravity of the white dwarf shreds the asteroid to pieces. A similar thing happened to Comet Shoemaker Levy 9 when Jupiter’s gravity tore it up, before the comet ultimately smashed into the planet in 1994. Spitzer observed shredded asteroid pieces around white dwarfs with its infrared spectrograph, an instrument that breaks light apart into its constituent colors. This reveals the imprint of chemicals surrounding the observed object. Previously, Spitzer analyzed the asteroid dust around two so-called polluted white dwarfs; the new observations bring the total to eight. “Now, we’ve got a bigger sample of these polluted white dwarfs, so we know these types of events are not extremely rare,” said Jura. In all eight systems observed, Spitzer found that the dust contains a glassy silicate mineral similar to olivine and commonly found on Earth. “This is one clue that the rocky material around these stars has evolved very much like our own,” said Jura. A single asteroid is thought to have broken apart within the last million years or so in each of the eight white-dwarf systems. The biggest of the bunch was once about 200 kilometers (124 miles) in diameter, a bit larger than Los Angeles County. Jura said the real power of observing these white dwarf systems is still to come. When an asteroid “bites the dust” around a dead star, it breaks into very tiny pieces. Asteroid dust around living stars, by contrast, is made of larger particles. By continuing to use spectrographs to analyze the visible light from this fine dust, astronomers will be able to see exquisite details—including information about what elements are present and in what abundance. This will reveal much more about how other star systems sort and process their planetary materials. “It’s as if the white dwarfs separate the dust apart for us,” said Jura.