fluence of other planets in our solar system. However,
gravity of a passing star can pull objects beyond the orbit of Neptune,
the Kuiper Belt, into orbits like Sedna's. Kenyon and Bromley have
detailed computer simulations to show how this stellar fly-by likely
The fly-by must have met two key requirements. First, the star must have
stayed far enough away that it did not disrupt Neptune's nearly circular
orbit. Second, the encounter must have happened late enough in our solar
system's history that Sedna-like objects had time to form within the
Kenyon and Bromley suggest that the near-collision occurred when our Sun
at least 30 million years old, and probably no more than 200 million
old. A fly-by distance of 150-200 A.U. would be close enough to disrupt
outer Kuiper Belt without affecting the inner planets.
According to the simulations, the passing star's gravity would sweep
the outer solar system beyond about 50 A.U., even as our Sun's gravity
pulled some of the alien planetoids into its grasp. The model explains
the orbit of Sedna and the observed sharp outer edge of our Kuiper Belt,
where few objects reside beyond 50 A.U.
"A close fly-by from another star solves two mysteries at once. It
both the orbit of Sedna and the outer edge of the Kuiper Belt," says
A Crowded Birthplace
But where did such a star come from, and where did it go? Since the
happened more than 4 billion years ago, any suspects have long since
the Sun's neighborhood. There is no practical way to find the culprit
The visitor's origin may seem equally mystifying because the Sun
lives in a sparse region of the Milky Way. Our closest neighbor is a
4 light-years away, and stellar close encounters are correspondingly
However, a near-collision would be much more likely for a young Sun if
were born in a dense star cluster, as recent evidence suggests.
"We believe that 90 percent of all stars form in clusters with a few
to a few thousand members," says astronomer Charles Lada
(Harvard-Smithsonian Center for Astrophysics). "The denser the cluster,
more likely the chance for an encounter between member stars."
"This work is an important piece of evidence that the Sun formed in near
proximity to other stars," he adds.
Searching for Adopted Worlds
Kenyon and Bromley's simulations indicate that thousands or possibly
millions of alien Kuiper Belt Objects were stripped from the passing
However, none have yet been positively identified. Sedna is probably
homegrown, not captured. Among the known Kuiper Belt Objects, an icy
dubbed 2000 CR105 is the best candidate for capture given its unusually
elliptical and highly inclined orbit. But only the detection of objects
orbits inclined more than 40 degrees from the plane of the solar system
clinch the case for the presence of extrasolar planets in our backyard.
Kenyon and Bromley's next goal is to estimate the sky density of
objects so that they can make a survey to find such adopted worlds.
"In principle, large telescopes like the MMT Telescope [a joint
Smithsonian/University of Arizona observatory] can find them if they're
numerous enough," says Kenyon.
The calculations reported here were made using about 3,000 cpu-days of
computer time at the supercomputing center at the Jet Propulsion
Laboratory, Pasadena, Calif.
Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.
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