Explaining
Eccentricities
Summary (Apr 15, 2005): When astronomers
discovered that the planets around Upsilon Andromedae had very
strange orbits, they weren't sure what could have caused it.
Researchers from Berkeley and Northwestern have developed a
simulation that shows how an additional planet could have
given the other planets the orbital kick they needed to
explain their current eccentricities. If a similar planet had
passed through our own Solar System early on, all our planets
could be in wildly different orbits around the Sun.
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Explaining Eccentricities
based on UC Berkeley
report
 |
The Terestrial Planet Finder will
search for Earth-like planets orbiting 250 of the
closest stars.
Credit:
NASA |
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.
 |
Estimates suggest that up to a quarter of all stars
have planets.
Credit: NASA/ STScI/
ESA |
"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.
 |
Artist concept of star system,
HD70642.
Credit:John Rowe
animation |
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.
 |
| HD 28185 b was the first exoplanet
discovered with a circular orbit within its star's
habitable zone. Credit: STScI Digitized Sky
Survey |
"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."
Related Web Pages
Extrasolar
Planets Encyclopedia
Planet Quest
(JPL)
Kepler
Mission Darwin Mission Space Interferometry
Mission Voyager:
Beyond the Great Beyond Fire
and Ice Beyond
Pluto: Ice Planet Note:
New
Planets
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Friday, April 15, 2005
