CfA Press Release
Release No.: 04-34|
For Release: Embargoed until 1:00 p.m. EST, December 1, 2004
Note to editors: Images and animations to accompany this release are
Alien Treasures In Our Backyard
Cambridge, MA--A hit TV program like "Antiques Roadshow" lures viewers
its universal appeal. Who wouldn't want to find secret riches in their
or basement? But rare paintings and heirloom jewelry aren't the only
valuable items waiting to be discovered. Cosmic treasures also lay hidden
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 -
"It's possible that some of the objects in our solar system actually
around another star," says astronomer Scott Kenyon (Smithsonian
How did these adopted worlds join our solar family? They arrived through
interstellar trade that took place more than 4 billion years ago when a
wayward star brushed past our solar system. According to calculations
by Kenyon and astronomer Benjamin Bromley (University of Utah) and
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
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
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
gravitational influence 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