Cambridge, MA- A hit TV program like “Antiques Roadshow” lures viewers with
its universal appeal. Who wouldn’t want to find secret riches in their
attic or basement? But rare paintings and heirloom jewelry aren’t the only
valuable items waiting to be discovered. Cosmic treasures also lay hidden
in the vast realm of outer space. Among the most highly prized of those
treasures are planets that formed around other stars.

Astronomers have just gained an important clue to guide their hunt for
extrasolar worlds. And that clue points to the unlikeliest of places – our
own backyard.

“It’s possible that some of the objects in our solar system actually formed
around another star,” says astronomer Scott Kenyon (Smithsonian
Astrophysical Observatory).

How did these adopted worlds join our solar family? They arrived through an
interstellar trade that took place more than 4 billion years ago when a
wayward star brushed past our solar system. According to calculations made
by Kenyon and astronomer Benjamin Bromley (University of Utah) and
published in the Dec. 2, 2004, Nature, the Sun’s gravity plucked
asteroid-sized objects from the visiting star. At the same time, the star
pulled material from the outer reaches of our solar system into its grasp.

“There may not have been an equal exchange, but there was certainly an
exchange,” says Bromley.

A Close Brush

Kenyon and Bromley reached this surprising conclusion while working to
explain the mystery object Sedna, a world almost as large as Pluto but
located much farther from the Sun. Sedna’s discovery in 2003 puzzled
astronomers because of its unusual orbit-a 10,000-year-long oval whose
closest approach to the Sun, 70 astronomical units, is well beyond the
orbit of Neptune. (One astronomical unit, abbreviated A.U., is the average
distance between the Earth and the Sun, or about 93 million miles.)

Understanding Sedna is a challenge because its orbit is far away from the
gravitational influence of other planets in our solar system. However, the
gravity of a passing star can pull objects beyond the orbit of Neptune, in
the Kuiper Belt, into orbits like Sedna’s. Kenyon and Bromley have
performed detailed computer simulations to show how this stellar fly-by
likely took place.

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 Kuiper

Kenyon and Bromley suggest that the near-collision occurred when our Sun
was at least 30 million years old, and probably no more than 200 million
years old. A fly-by distance of 150-200 A.U. would be close enough to
disrupt the outer Kuiper Belt without affecting the inner planets.

According to the simulations, the passing star’s gravity would sweep clear
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 both
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 explains
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 fly-by
happened more than 4 billion years ago, any suspects have long since
escaped the Sun’s neighborhood. There is no practical way to find the
culprit today.

The visitor’s origin may seem equally mystifying because the Sun currently
lives in a sparse region of the Milky Way. Our closest neighbor is a
distant 4 light-years away, and stellar close encounters are
correspondingly rare. However, a near-collision would be much more likely
for a young Sun if it 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
hundred to a few thousand members,” says astronomer Charles Lada
(Harvard-Smithsonian Center for Astrophysics). “The denser the cluster, the
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 star.
However, none have yet been positively identified. Sedna is probably
homegrown, not captured. Among the known Kuiper Belt Objects, an icy rock
dubbed 2000 CR105 is the best candidate for capture given its unusually
elliptical and highly inclined orbit. But only the detection of objects
with orbits inclined more than 40 degrees from the plane of the solar
system will clinch the case for the presence of extrasolar planets in our

Kenyon and Bromley’s next goal is to estimate the sky density of captured
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 superconducting 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.

Note to editors: Images and animations to accompany this release
are online: and

Astrophysics, abstract

From: Scott J. Kenyon [view email]
Date: Wed, 1 Dec 2004 19:29:08 GMT   (746kb)

Stellar encounters as the origin of distant solar system objects in
highly eccentric orbits

Scott J. Kenyon,
Benjamin C. Bromley
Comments: 9 pages, 3 figures
Journal-ref: Nature, Vol. 432, p. 598, 2004

The discovery of Sedna places new constraints on the origin and evolution of
our solar system. Here we investigate the possibility that a close encounter
with another star produced the observed edge of the Kuiper belt, at roughly 50
AU, and the highly elliptical orbit of Sedna. We show that a passing star
probably scattered Sedna from the Kuiper Belt into its observed orbit. The
likelihood that a planet at 60-80 AU can be scattered into Sedna’s orbit is
roughly 50%; this estimate depends critically on the geometry of the flyby.
Even more interesting, though, is the roughly 10% chance that Sedna was
captured from the outer disk of the passing star. Most captures have very high
inclination orbits; detection of these objects would confirm the presence of
extrasolar planets in our own Solar System.

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