in the making, or planets that have been destroyed, or possibly even a giant planet like Jupiter that, though unseen, orchestrates the chaos of collisions that created the debris. UCLA graduate student Christine Chen and her advisor, astronomy professor Michael Jura, announced the findings here today at the annual meeting of the American Astronomical Society.
"We believe we see either the remnants of planet formation or material that may become planets," Chen said.
The scientists have not actually seen any asteroids around Zeta Leporis, a young star twice as massive as the
and 60 to 70 light-years away. Instead they have studied the temperature and position of the star's swirling mass of debris, which they say shows evidence of chaotic collisions among rocks that creates the dust needed to sustain such a disk. It is a scenario similar to what is thought to have occurred during the birth of our solar system and, to a lesser extent, continues today.
Zeta Leporis, also called HR 1998, is between 50 million and 400 million years old, compared to our middle-aged Sun, which is about 4.5 billion years old. Along with some other young stars, it was found in the 1980s to have a ring of dusty debris. And in 1991 astronomers learned that this debris ring was unusually warm and close to its parent star, unlike other disks that are farther out, and hence colder.
This dust, given its known properties, should spiral into a star within 20,000 years, according to current theories of physics and star formation, scientists say. But this star is much older.
"This tells us that these dust grains that we observe now were not there when this star first formed, so they must be generated through some secondary process such as collisions between larger objects," Chen said. These presumed asteroids could be the size of small or large boulders, "which collide together, and grind down, and form micron-sized grains."
The new study, funded by NASA, used the Keck Observatory in Hawaii to examine how much light is reflected by the ring of debris, which absorbs visible light from the star and emits it in the infrared wavelength. Similar techniques allow scientists to estimate the composition of asteroids closer to home. Chen and Jura determined the debris around Zeta Leporis to be, on average, about 150 degrees Fahrenheit (65 degrees Celsius, or 340 Kelvins), and they estimate the mass of the material to be about 1,000 times what is found in the Asteroid Belt between Mars and Jupiter.
Chen then calculated that the ring of debris must be confined to region between 2.5 and 12.2 astronomical units from the star. One AU is the distance from Earth to the Sun, and the Asteroid Belt sits between Mars and Jupiter, 1.5 and 5.2 AU from the Sun, respectively.
Echoes of our solar system
Leading models of solar system formation hold that as the Sun gathered itself together out of a cloud of gas and dust, the leftovers settled into a vast disk that rotated around the newborn star and gradually flattened out.
In the early years of our solar system, dust grains collided and coalesced, and the seeds of
, comets and planets were formed. The gravity of some of the more distant protoplanets attracted gas, and Jupiter and the other gas giants developed. These giant planets swept much of the dust disk clean. Most of the rest of the debris spiraled in and was swallowed by the Sun or was driven out of the solar system. But collisions still generate some dust.Research reported in the journal Nature in 1999 showed that these dust disks tend to disappear when a star is about 400 million years old -- the upper end of the age estimate for Zeta Leporis. Previously, another star was found to have a gap in its ring of debris, hinting at planet formation.
Hinting at a planet
Dozens of planets have been found around other stars, but so far all are giant gaseous planets very close to their host stars, leaving open the question of how common solar systems like ours -- the habitable kind -- might be.
Mark Sykes, a Steward Observatory researcher who has studied how dust behaves in our Asteroid Belt, speculated that the debris disk around Zeta Leporis might have been caused by a Jupiter-sized planet that has so far gone undetected. Such a planet would have kicked asteroids that were in circular orbits into more elliptical ones.
Once that happens, collisions between two asteroids are no longer sideswipes between two objects on a similar path, but instead more like cars slamming together at an intersection.
Sykes said that the study therefore provides a possible detection method to be used in the ongoing hunt for extrasolar planets. "In a way it's a minor-planet and a major-planet detection system as well," he said.
Jura warned that while there could be a rocky planet, perhaps even one like Earth, embedded in the dust, it could be a decade before any detection techniques would be able to pick it out.
The researchers also plan to learn whether the potential asteroids around Zeta Leporis are made up of the same stuff as the asteroids in our solar system. If so, then the fledgling solar system around that star could serve as an even better window to the formation of our own.
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