Alien Worlds May Be in Our
Backyard, scientists say
Posted Dec, 1 2004
Courtesy, the Harvard-Smithsonian Center for Astrophysics
and World Science staff
Our solar system may
have adopted some of its distant small planets or other objects from another
solar system, scientists say. They have calculated that the sun might have
traded some of its planets with another star during a close encounter between
two solar systems more than 4 billion years ago.
researchers reached this conclusion while working to explain the mystery object
Sedna, a world almost as large as the planet Pluto but located much farther from
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
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 into orbits like Sedna’s if they
start out beyond the orbit of Neptune, in the Kuiper Belt. The Kuiper belt is a
region beyond Neptune in which at least 70,000 small objects, including tiny
Scott Kenyon of the Smithsonian Astrophysical Observatory in Cambridge, Mass.,
and physicist Benjamin Bromley of the University of Utah performed computer
simulations to show how this stellar fly-by could have occurred. Their results
are to be published in the Dec. 2 issue of the research journal Nature.
The fly-by must have met two key requirements, say Bromley and Kenyon. 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.
Kenyon and Bromley suggest that the near-collision occurred when our Sun between
about 30 million years and 200 million years old. The fly-by distance might have
been between 150 and 200 A.U., they say.
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., the
only the detection of objects with orbits tilted more than 40 degrees with
respect to the major Solar System planets would strongly suggest these are
alient worlds in our backyard, the researchers say. Kenyon and Bromley say they
plan to estimate the sky density of captured
“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 Bromley.
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, say the researchers. Thus 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 Charles Lada of the Harvard-Smithsonian
Center for Astrophysics. “The denser the cluster, the more likely the chance
for an encounter between member stars.” The new research is “an important
piece of evidence that the Sun formed in near proximity to other stars,” he
Searching for Adopted Worlds
Kenyon and Bromley’s simulations indicate that thousands of alien Kuiper Belt
Objects were stripped from the passing star. However, none have 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 tilted orbit.
objects so that they can make a survey to find such adopted worlds.
“In principle, large telescopes like the MMT Telescope [a joint observatory of
the Smithsonian and the University of Arizona] can find them if they’re
numerous enough,” says Kenyon. The calculations were conducted at the at the
Jet Propulsion Laboratory, Pasadena, Calif.