WASHINGTON, Aug. 16 (UPI) -- The planets orbiting the sun may have formed differently than those in other solar systems, which is why astronomers have not yet discovered systems that resemble Earth's planetary neighborhood.
Scientists have known for some time the local planets follow more circular orbits around the sun, while all the other planets detected around stars seem to have more elliptical orbits, said Mario Livio, senior astronomer at the Space Telescope Science Institute in Baltimore.
Those other systems also feature gas-giant planets that are located much closer to their parent star compared to the distance of Jupiter -- the largest planet -- from the sun.
New research -- by Livio and others, published in the Monthly Notices of the Royal Astronomical Society -- theorizes that Earth and its neighbors were formed via a different mechanism than planets in other discovered systems, while systems similar to this one remain difficult to detect with conventional observation methods.
Astronomers have been able to find about 100 extra-solar planets, as they are called, by detecting slight wobbles in the motion of stars caused by the gravitational pull of closeby, gas-giant planets, Livio explained.
If there are solar systems similar in structure to the sun's, astronomers believe, their gas giants are too far away from the star to exert a detectable tug.
"With the current techniques, we are not yet in a position to detect a system like the solar system, even if there are billions of those out there," Livio told United Press International. "The reason that we have not detected such systems so far -- and we've detected somewhat different systems -- may mean absolutely nothing other than the fact that our measurements are still not accurate enough."
This phenomenon, known as selection effects, may be the reason the solar system appears unique compared to all the other known systems, instead of some actual, physical difference, Livio explained.
"At present state, with the caveat of selection effects, the solar system does look different from all the other systems," Livio said. "This is in terms of the closest approach of the planet to the parent star, and the closest approach of Jupiter to the (sun) deviates significantly from those of all the other planetary systems."
Astronomers suggest there may be two different planetary formation processes that create two types of systems as opposed to previous speculation there was only one mechanism of planet formation.
In the first mechanism -- which may have created the solar system and others with giant planets far from the sun -- a disc made up of gas and dust surrounds a star to help form terrestrial planets, such as Earth and Mars, which are located closer in the solar system.
The dust particles from this disc begin to collect together under the influence of gravity. They form rocks that later coalesce into terrestrial planets, Livio explained. If terrestrial planets grow large enough, they begin to collect gas to form giant planets, he said.
In the second mechanism -- which the researchers theorize forms systems with giant planets closer to the star -- a gas disc somehow becomes gravitationally unstable, so it separates into large clumps, he said. Those clumps then collapse and form gas-giant planets in a more dynamic process that does not involve the slow and retracted method of dust collecting, Livio said.
It is likely terrestrial planets do not form under the second mechanism, however, Livio said, adding that scientists remain unsure.
"It is possible there are these two separate formation mechanisms, and the solar system may belong to the class that is not the class that has been observed so far, which have the giant planets close in," he said. "This is not to say ... the solar system is unique."
Livio said he was unsure why gas giants formed closer to the sun in the second mechanism, but he added it was more likely due to planet migration, where gravitational interaction by planets pushes them closer to the parent star.
"It looks like very interesting research, but I think it is misleading to think that astronomers active in the field all think that all the planets were made in one way," said Stephen P. Maran, assistant director of Space Sciences for Information and Outreach at NASA's Goddard Space Flight Center in Greenbelt, Md.
"I think many feel at least two mechanisms were involved in some way, so (the research) doesn't come as a surprise," he told UPI.
William Cochran, a senior research scientist at the McDonald Observatory at the University of Texas at Austin, agreed. The two formation theories have been around for the past 10 years and the paper tries to take a "provocative stance" on those arguments, he noted.
"As we look at the data we have on other systems, we cannot rule out that there will be a large number of planetary systems just like our own solar system," Cochran told UPI. "We just don't have the data yet to make definitive detections of these systems."
Other astronomers have found systems with debris discs surrounding a star that do not have giant planets close to the star, Livio said, but it will take several years to observe these types of systems to meet the required time baseline for observations to validate the preliminary findings.
"We will know within a few years whether this is all selection effects or not," Livio explained. "The orbital period of Jupiter is 12 years, so you need to observe these systems for this long in order to detect things like Jupiter."
The first extra-solar planetary system around a solar-type star was discovered in 1995, he added.
"Of the observed systems, there are likely no Earth-like planets," Martin Beer, a lead researcher at the University of Leicester in England, told UPI. "But that doesn't rule out that there are other systems out there that have Earth-like planets."
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Brad Amburn is an intern for UPI Science News. E-mail [email protected]
