A swath of space beyond Neptune is getting stranger all the time as astronomers find an ever-more diverse array of objects in various orbits and groupings.
A pair of discoveries this month along with a handful of others in 2005 have begun to reveal what some astronomers long suspected: The outer solar system contains a dizzying array of round worlds on countless odd trajectories around the sun, often with multiple satellite systems.
The problem is, current theories of the solar system's formation and evolution can't account for it all.
What is now called the Kuiper Belt was proposed in the 1940s by Irish economist and astronomer Kenneth Edgeworth and separately by American astronomer Gerard Kuiper in 1951. The first object out there was found in 1992. Now several worlds a third as massive as Pluto and larger are known to roam the solar system's outskirts, including one revealed this year that is at least as big as Pluto and considered by some to be the 10th planet.
Meanwhile, discoveries of binary setups in the Kuiper Belt have led experts to estimate that at least 10 percent of large Kuiper Belt Objects (KBOs) have moons.
The premier KBO is actually Pluto, considered by most astronomers to be a member of the region rather than a true planet (though many say its historical and cultural significance as a planet suggest it should have dual status). In October, Pluto was found to have two small satellites in addition to its large moon Charon.
A KBO known as 2003 EL61, which is about one-third as massive as Pluto, was found in January to have a moon. This month, researchers led by Mike Brown at Caltech announced it has a second small satellite. Many other KBOs could have multiple moons that can't yet be detected.
"It appears likely, now, that other [large] Kuiper Belt Objects ... might also have multiple satellite systems," Brown said.
On Dec. 13, another group said they'd found an object half the mass of Pluto orbiting twice as far from the Sun as Neptune. The object's path has them puzzled.
The faraway world is catalogued as 2004 XR 190 and known temporarily as Buffy. It was discovered as part of the Legacy Survey on the Canada France Hawaii Telescope.
"It was quite bright compared to the usual Kuiper Belt Objects we find," said the University of British Columbia's Lynne Allen, who was part of the international discovery team. "But what was more interesting was how far away it was."
Buffy never gets closer than 52 astronomical units (AU) from the Sun, or 52 times the distance from Earth to the Sun. Neptune is 30 AU from the Sun. Pluto ranges from 30 to 50 AU.
What makes Buffy special is its nearly circular path, which extends out to just 62 AU.
"To find the first known object with a nearly circular orbit beyond 50 AU is indeed intriguing," said Brian Marsden, who runs the Minor Planet Center where all of these objects are catalogued.
Most other known KBOs are on highly elliptical orbits and off-center orbits, typically coming to within 38 AU of the Sun and then soaring out beyond 50 AU. Theory suggests Neptune has acted on them like a gravitational slingshot to produce these eccentric paths.
A passing star?
Other than the KBOs that slice in and out of the zone, the space beyond 50 AU seems fairly empty, as though it represents the outer edge of the Kuiper Belt.
But then there is Sedna, which ranges from 76 to 900 AU.
In late 2004, Scott Kenyon of the Smithsonian Astrophysical Observatory and Benjamin Bromley from the University of Utah proposed a passing star long ago may have played a role in all this.
At a distance of 150-200 AU, the star could have stolen objects from the outer Kuiper Belt and lured Sedna into its present orbit without affecting Neptune or the inner planets, the researchers concluded through computer modeling.
Intriguingly, the model suggests some KBOs in our solar system (not including Sedna) are actually alien worlds that were captured from the passing star in a two-way swap of material.
"A close fly-by from another star solves two mysteries at once," Bromley said then. "It explains both the orbit of Sedna and the outer edge of the Kuiper Belt,"
The stellar flyby scenario does not appear to explain Buffy, however.
Buffy is too far out there to have been affected by Neptune, at least given Neptune's current position, say its discoverers. Yet Buffy's very extreme orbital tilt, which takes it 47 degrees above and below the main plane of the solar system, raises another question. All of the planets, asteroids, comets and KBOs are thought to have formed from a relatively flat disk that circled the newborn Sun 4.5 billion years ago. Most of them remained in that plane after formation, unless perturbed by something.
"If a star could have affected Buffy so strongly, it should also have disrupted much of the main Kuiper belt as well," write the researchers who found Buffy. "Since astronomers do not detect that strong disruption, a more complex theory is needed to explain Buffy's orbit."
Perhaps, they speculate, the answer lies in some as-yet unknown event early in the solar system's history. Maybe Neptune's orbit slowly grew bigger, as theorists have previously proposed, pushing some KBOs into tilted circular orbits by means that have yet to be modeled, they said.
Answers await technology that can detect fainter and more distant objects, so researchers can make a fuller inventory of the out solar system. That inventory is likely to include many objects in what is called the Oort Cloud, extending 1.5 light-years away and nearly half the distance to the Alpha Centauri star system.
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