Computer simulations show a close encounter with
a passing star about 4 billion years ago may have given our solar system
its abrupt edge and put small, alien worlds into distant orbits around
our sun.

The study, which used a supercomputer at NASA’s Jet Propulsion
Laboratory in Pasadena, Calif., was published in the Dec. 2 issue of the
journal Nature by physicist Ben Bromley of the University of Utah and
astronomer Scott Kenyon of the Smithsonian Astrophysical Observatory in
Cambridge, Mass.

Bromley and Kenyon simulated what would have happened if our sun
and another star in our Milky Way galaxy had passed a relatively close
14 billion to 19 billion miles from each other a few hundred million
years after our solar system formed. At that time, our solar system was
a swirling “planetary disk” of gas, dust and rocks, with planets newly
formed from the smaller materials.

Imagine the encounter of two young solar systems by envisioning
two circular saw blades brushing past each other while spinning rapidly.
When they make contact, their outer edges are buzzed off by the other
saw. But in the case of planetary disks, colliding rocks at the edges of
the solar systems are pulverized into pebbles, causing particles to be
flung in all directions.

“Any objects way out in the planetary disk would be stirred up
greatly,” says Bromley, an associate professor of physics at the
University of Utah.

Bromley and Kenyon conclude the shearing motion and dueling
gravity of the passing stars could have done several things:

— Taken young planets formed with circular orbits in our solar
system and catapulted them into highly elongated orbits. That may explain
the existence of Sedna, a “planetoid” that orbits beyond Pluto and
measures between 600 and 1,000 miles wide.

— Created a sharp edge to the solar system by shearing off the
outer part of the Kuiper belt, a collection of small, rocky-and-icy
objects in space starting beyond Neptune’s orbit and ending abruptly
about 4.7 billion miles from the sun.

— Allowed our sun and solar system to capture a planet or smaller
object from the passing star’s solar system. Sedna might be an example.

Houston, We Have an Alien … Planet

Astronomers have been searching for years for extrasolar
planets, or planets in other solar systems. Few considered the
possibility that “the nearest extrasolar planet might be right here in
our solar system,” says Kenyon.

Computer simulations of a close encounter by two stars – a
stellar flyby – demonstrated there is a chance a planet could be
captured from another solar system. Bromley and Kenyon predicted
locations in our solar system where captured objects would be, based on
the angle and shape of their orbits. Finding captured objects in the
predicted locations would be “proof that a flyby occurred,” says
Bromley. He hopes astronomers will look more closely at sections of the
sky where he and Kenyon predict alien planets might be.

Between 30 and 50 astronomical units from the sun – that is, 2.8
billion to 4.7 billion miles from the sun – several Kuiper belt objects
larger than 600 miles in diameter are known to orbit the sun. Sedna,
discovered in 2003, is similar to these cold, rock-and-ice worlds, but
orbits 70 to 1,000 astronomical units from the sun. It has a
high-inclination orbit, which means it does not travel around the sun in
the same plane as the major planets. Sedna’s orbit also is highly
elliptical or elongated.

Bromley says Kuiper belt objects are influenced by Neptune’s
gravity, but Neptune alone is too far away to have launched Sedna on its
bizarre path, he says.

What caused Sedna’s elongated orbit? Answering this question was
a key goal of Bromley and Kenyon’s study. Their simulations show there
is a 5 percent to 10 percent chance Sedna formed within our solar system,
probably closer to Neptune or Pluto, and was later launched into its
current orbit when our solar system was “buzzed” by another.

“In order for a flyby [between two stars] to put Sedna on its
orbit, we need to have Sedna in place at the time of the flyby,” says

Bromley says it is possible Sedna is an alien planet, formed in
a solar system that later flew near our own. Bromley and Kenyon’s
simulations suggest that there is a 1 percent chance that Sedna is a
planet captured during a stellar flyby.

“There may be thousands of objects like Sedna near the edge of
our solar system,” Bromley says. “So there is an even greater chance
that some may be alien worlds captured from another solar system.”

The Kuiper belt ends abruptly at 50 astronomical units from the
sun and “there is no evidence that the hard edge of the Kuiper belt is
in any sense natural,” says Bromley.

If the edge of our solar system were unperturbed, scientists
would predict a gradual tapering of debris at increasing distances from
the sun. The computer simulations showed that a close encounter with
another solar system could explain why rocky, icy Kuiper belt objects
vanish abruptly at 50 astronomical units.

Does the solar system face another destructive encounter with a
neighboring star? Not according to Bromley, who says the chance of that
happening is “effectively nil” because the sun no longer is close to
other stars in a cluster as it once was.

Press release prepared by Jill Johnston West.

High-resolution images of the simulated encounter by two solar systems
may be found at:

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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