by Richard A. Lovett
in Timberline Lodge, Oregon
May 11, 2012
Odd orbits of remote objects
hint at unseen world, new calculations suggest.
Artist's conception of a small icy object beyond Pluto (file
Illustration courtesy G. Bacon, STScI/NASA
An as yet undiscovered planet might be orbiting at the dark fringes
of the solar system, according to new research.
Too far out to be easily spotted by telescopes, the potential unseen
planet appears to be making its presence felt by disturbing the
orbits of so-called
Kuiper belt objects, said Rodney Gomes, an
astronomer at the
National Observatory of Brazil in Rio de Janeiro.
Kuiper belt objects are small icy bodies - including some dwarf
planets - that lie beyond the orbit of Neptune:
Neptune - Click above image
Once considered the ninth planet in our system, the dwarf planet
Pluto, for example, is one of the largest Kuiper belt objects, at
about 1,400 miles (2,300 kilometers) wide. Dozens of the other
objects are hundreds of miles across, and more are being discovered
every year (see "Three
New 'Plutos'? Possible Dwarf Planets Found.")
What's intriguing, Gomes said, is that, according to his new
calculations, about a half dozen Kuiper belt objects - including the
remote body known as Sedna - are in strange orbits compared to where
they should be, based on existing solar system models (related "Pluto
Neighbor Gets Downsized".)
The objects' unexpected orbits have a few possible explanations,
said Gomes, who presented his findings Tuesday at a meeting of the
American Astronomical Society in Timberline Lodge, Oregon.
"But I think the easiest one is a
planetary-mass solar companion" - a planet that orbits very far
out from the sun but that's massive enough to be having
gravitational effects on Kuiper belt objects.
a Captured Rogue?
For the new work, Gomes analyzed the orbits of 92 Kuiper belt
objects, then compared his results to computer models of how the
bodies should be distributed, with and without an additional planet.
If there's no distant world, Gomes concludes, the models don't
produce the highly elongated orbits we see for six of the objects.
How big exactly the planetary body might be isn't clear, but there
are a lot of possibilities, Gomes added.
Based on his calculations, Gomes thinks a Neptune-size world, about
four times bigger than Earth, orbiting 140 billion miles (225
billion kilometers) away from the sun - about 1,500 times farther
than Earth - would do the trick.
But so would a Mars-size object - roughly half Earth's size - in a
highly elongated orbit that would occasionally bring the body
sweeping to within 5 billion miles (8 billion kilometers) of the
Gomes speculates that the mystery object could be a rogue planet
that was kicked out of its own star system and later captured by the
sun's gravity (see "'Nomad' Planets More Common Than Thought, May
Orbit Black Holes.")
Or the putative planet could have formed closer to our sun, only to
be cast outward by gravitational encounters with other planets.
However, actually finding such a world would be a challenge.
To begin with, the planet might be pretty dim.
simulations don't give astronomers any clue as to where to point
"it can be anywhere," he said.
No Smoking Gun
Other astronomers are intrigued but say they'll want a lot more
proof before they're willing to agree that the solar system - again
- has nine planets (see "Record
Nine-Planet Star System Discovered?.)
"Obviously, finding another planet
in the solar system is a big deal," said Rory Barnes, an
astronomer at the University of Washington. But, he added, "I
don't think he really has any evidence that suggests it is out
Instead, he added, Gomes,
"has laid out a way to determine how
such a planet could sculpt parts of our solar system. So while,
yes, the evidence doesn't exist yet, I thought the bigger point
was that he showed us that there are ways to find that
Douglas Hamilton, an astronomer from the
University of Maryland, agrees that the new findings are far from
"What he showed in his probability
arguments is that it's slightly more likely. He doesn't have a
smoking gun yet."
And Hal Levison, an astronomer at the
Southwest Research Institute in Boulder, Colorado, says he isn't
sure what to make of Gomes's finding.
"It seems surprising to me that a
[solar] companion as small as Neptune could have the effect he
sees," Levison said.
But "I know Rodney, and I'm sure he did the calculations right."
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