Tue, 02 Dec 2008 16:23 UTC
Comets that take longer than 200 years to orbit the Sun come from all directions in the sky. That has long led scientists to believe that they were nudged out of a diffuse halo of icy objects that surrounds the solar system - the Oort Cloud.
The objects probably formed from the same disc of material that gave rise to the planets but were scattered outwards by Jupiter and Saturn a few hundred million years after their birth.
The Oort Cloud is too dim to be seen by telescopes, but astronomers believe it has two components. Based on observations of long-period comets, an outer portion seems to extend from 20,000 to 200,000 astronomical units from the Sun (where 1 AU is the Earth-Sun distance).
Solar system models also predict the existence of an inner shell that stretches some 3000 to 20,000 AU from the Sun. But there is less evidence for this shell - most passing stars are too distant to jostle the inner halo and dislodge comets. Only a few recently spotted objects, such as the icy bodies 2006 SQ372 and Sedna, point to its existence.
Now, a new study suggests that the inner Oort Cloud might be detectable by looking at all-sky surveys of the cosmic microwave background - the first radiation emitted in the universe after the big bang.
The inner Oort Cloud objects are dense enough at their orbital distance to block a significant portion of the cosmic radiation. And since the icy debris is roughly -268 °Celsius, about twice as warm as the CMB, it should show up in maps of the radiation.
The inner cloud would not be detectable if it formed a perfect sphere, since that would leave the same imprint on the CMB in all directions. However, if a star passed close enough to the Sun to rearrange objects in the inner cloud, the distortions might be visible in the CMB, say Daniel Babich and Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.
"A star that passes there would actually kick the cloud," say Loeb. "In principle, that would leave a signature that is detectable on the CMB."
The researchers estimate that about five such stars could have passed within 2000 AU of the Sun during the solar system's nearly 5-billion-year lifetime. After each event, it might take a billion years or more for the gravitational pull of distant stars and the Milky Way to smooth out evidence of the star's passage.
Maps made with the European Space Agency's Planck telescope, set to launch in April 2009, might reveal the signal, which could be used to determine the inner Oort Cloud's distance, shape and the distribution of its icy bodies.
"It's an interesting way to look for some sign of the Oort Cloud, which is very hard to do from the Earth," says Renu Malhotra of the University of Arizona in Tucson.
Since it's unclear how aspherical the inner halo is even when it is not deformed by passing stars, the signal might actually be stronger than the authors estimate, Malhotra adds. While the most distant icy bodies would have been very vulnerable to gravitational tugs from passing stars, pushing them into a spherical shell that became the outer Oort Cloud, those closer to the Sun might have retained some of their original disc-like distribution.
In principle, Planck could detect the signal, says cosmologist Douglas Scott of the University of British Columbia in Vancouver, Canada.
But Scott says it's unclear how a signal from the Oort Cloud would compete with the intrinsic signals from the telescope, as well as radiation from the Milky Way and other solar system features, such as the Kuiper Belt, a ring of icy bodies beyond Neptune.
"What is really needed is a more careful simulation which includes at least the effect of the other likely signals," Scott told New Scientist.
Since the cloud is a relic of the early solar system, pinning down its structure could provide important clues about the formation of the giant planets, Loeb told New Scientist. For example, the size distribution of objects in the cloud could shed light on the structure of the dusty disc from which the planets formed.
Astronomers might also look for stars that dim when Oort Cloud objects pass between them and ground-based telescopes. The technique is tricky because the events are fleeting and detecting them requires a perfect alignment of the objects and telescopes.
Researchers are already on the lookout for such "occultations" of stars by Kuiper Belt Objects that are too small and dim to detect by other methods. Extending the search to inner Oort Cloud objects may be 10 to 20 years off, says Luke Dones of the Southwest Research Institute in Boulder, Colorado.
Still, finding the signature of the Oort Cloud in the CMB could prove even more challenging. "It's not something I think can be done in the near future," Dones told New Scientist. "I suspect these occultation events are going to find Oort Cloud objects sooner."
Journal reference: New Astronomy (vol 14, p 166)