Wayward Planet Knocks Extrasolar Planet For a Loop

http://www.berkeley.edu/news/media/r...3_planet.shtml

Wayward planet knocks extrasolar planets for a loop
By Robert Sanders
UC Berkeley Press Release
13 April 2005

BERKELEY - The peculiar orbits of three planets looping around a
faraway
star can be explained only if an unseen fourth planet blundered through
and knocked them out of their circular orbits, according to a new study
by researchers at the University of California, Berkeley, and
Northwestern University.

The conclusion is based on computer extrapolations from 13 years of
observations of planet motions around the star Upsilon Andromedae. It
suggests that the non-circular and often highly elliptical orbits of
many of the extrasolar planets discovered to date may be the result of
planets scattering off one another. In such a scenario, the perturbing
planet could be shot out of the system entirely or could be kicked into
a far-off orbit, leaving the inner planets with eccentric orbits.

"This is probably one of the two or three extrasolar systems that have
the best observations and tightest constraints, and it tells a unique
story," said Eric Ford, a Miller postdoctoral fellow at UC Berkeley.
"Our explanation is that the outer planet's original orbit was
circular,
but it got this sudden kick that permanently changed its orbit to being
highly eccentric. To provide that kick, we've hypothesized that there
was an additional planet that we don't see now. We believe we now
understand how this system works."

If such a planet had caromed through our solar system early in its
history, the researchers noted, the inner planets might not now have
such nicely circular orbits, and, based on current assumptions about
the
origins of life, Earth's climate might have fluctuated too much for
life
to have arisen.

"While the planets in our solar system remain stable for billions of
years, that wasn't the case for the planets orbiting Upsilon
Andromedae," Ford said. "While those planets might have formed
similarly
to Jupiter and Saturn, their current orbits were sculpted by a late
phase of chaotic and violent interactions."

According to Ford's colleague, Frederic A. Rasio, associate professor
of
physics and astronomy at Northwestern, "Our results show that a simple
mechanism, often called 'planet-planet scattering' - a sort of
slingshot
effect due to the sudden gravitational pull between two planets when
they come very near each other - must be responsible for the highly
eccentric orbits observed in the Upsilon Andromedae system. We believe
planet-planet scattering occurred frequently in extrasolar planetary
systems, not just this one, resulting from strong instabilities. So,
while planetary systems around other stars may be common, the kinds of
systems that could support life, which, like our solar system,
presumably must remain stable over very long time scales, may not be so
common."

The computer simulations are reported in the April 14 issue of the
journal Nature by Ford, Rasio and Verene Lystad, an undergraduate
student majoring in physics at Northwestern. Ford was a student of
Rasio's at the Massachusetts Institute of Technology before pursuing
graduate studies at Princeton University and arriving at UC Berkeley in
2004.

The planetary system around Upsilon Andromedae is one of the most
studied of the 160-some systems with planets discovered so far outside
our own solar system. The inner planet, a "hot Jupiter" so close to the
star that its orbit is only a few days, was discovered in 1996 by UC
Berkeley's Geoff Marcy and his planet-hunting team. The two outer
planets, with elongated orbits that perturb each other strongly, were
discovered in 1999. These three, huge, Jupiter-like planets around
Upsilon Andromedae comprised the first extrasolar multi-planet system
discovered by Doppler spectroscopy.

Because of the unusual nature of the planetary orbits around Upsilon
Andromedae, Marcy and his team have studied it intensely, making nearly
500 observations - 10 times more than for most other extrasolar planets
that have been found. These observations, the wobbles in the star's
motion induced by the orbiting planets, allow a very precise charting
of
the planets' motions around the star.

"The observations are so precise that we can watch and predict what
will
happen for tens of thousands of years in the future," Ford said.

Today, while the innermost planet huddles close to the star, the two
outer planets orbit in egg-shaped orbits. Computer simulations of past
and future orbital changes showed, however, that the outer planets are
engaged in a repetitive dance that, once every 7,000 years, brings the
orbit of the middle planet to a circle.

"That property of returning to a very circular orbit is quite
remarkable
and generally doesn't happen," Ford said. "The natural explanation is
that they were once both in circular orbits, and one got a big kick
that
caused it to become eccentric. Then, the subsequent evolution caused
the
other planet to grow its eccentricity, but because of the conservation
of energy and angular momentum, it returns periodically to a very
nearly
circular orbit."

Previously, astronomers had proposed two possible scenarios for the
formation of Upsilon Andromedae's planet system, but the observational
data was not yet sufficient to distinguish the two models. Another
astronomer, Renu Malhotra at the University of Arizona, had previously
suggested that planet-planet scattering might have excited the
eccentricities in Upsilon Andromedae. But an alternative explanation
claimed that interactions among the planets and a gas disk surrounding
the star could also have produced such eccentric orbits. By combining
additional observational data with new computer models, Ford and his
colleagues were able to show that interactions with a gas disk would
not
have produced the observed orbits, but that interactions with another
planet would naturally produce them.

"The key distinguishing feature between those theories was that
interactions with an outer disk would cause the orbits to change very
slowly, and a strong interaction with a passing planet would cause the
orbits to change very quickly compared to the 7,000-year time scale for
the orbits to evolve," Ford said. "Because the two hypotheses make
different predictions for the evolution of the system, we can constrain
the history of the system based on the current planetary orbits."

Ford said that as the planets formed inside a disk of gas and dust, the
drag on the planets would have kept their orbits circular. Once the
dust
and gas dissipated, however, only an interaction with a passing planet
could have created the particular orbits of the two outer planets
observed today. Perhaps, he noted, the perturbing planet was knocked
into the inner planets by interactions with other planets far from the
central star.

However it started, the resulting chaotic interactions would have
created a very eccentric orbit for the third planet, which then also
gradually perturbed the second planet's orbit. Because the outer planet
dominates the system, over time it perturbed the middle planet's orbit
enough to deform it slowly into an eccentric orbit as well, which is
what is seen today, although every 7,000 years or so, the middle planet
returns gradually to a circular orbit.

"This is what makes the system so peculiar," said Rasio. "Ordinarily,
the gravitational coupling between two elliptic orbits would never make
one go back to a nearly perfect circle. A circle is very special."

"Originally the main objective of our research was to simulate the
Upsilon Andromedae planetary system, essentially in order to determine
whether the outer two planets lie in the same plane like the planets in
the solar system do," said Lystad, who started working with Rasio when
she was a sophomore and did many of the computer integrations as part
of
her senior thesis. "We were surprised to find that, for many of our
simulations, it was difficult to tell whether the planets were in the
same plane due to the fact that the middle planet's orbit periodically
became so very nearly circular. Once we noticed this strange behavior
was present in all of our simulations, we recognized it as an earmark
of
a system that had undergone planet-planet scattering. We realized there
was something much more interesting going on than anyone had found
before."

Understanding what happened during the formation and evolution of
Upsilon Andromedae and other extrasolar planetary systems has major
implications for our own solar system.

"Once you realize that most of the known extrasolar planets have highly
eccentric orbits (like the planets in Upsilon Andromedae), you begin to
wonder if there might be something special about our solar system,"
Ford
said. "Could violent planet-planet scattering be so common that few
planetary systems remain calm and habitable? Fortunately, astronomers -
led by Geoff Marcy, a professor of astronomy at UC Berkeley - are
diligently making the observations that will eventually answer this
exciting question."

The research was supported by the National Science Foundation and UC
Berkeley's Miller Institute for Basic Research.