read of one planet off in the seventh dimension that got
used as a ball in a game of intergalactic bar
— Ford Prefect in
The Hitchhiker's Guide to the
areening planets wreaking havoc
may seem like the stuff of science fiction, but
according to a study by researchers at the University of
California, Berkeley, and Northwestern University, it
really can happen.
A puzzling aspect of many
extrasolar planets is the unusually out-of-round orbits
they follow around their stars. Planets in our solar
system follow nearly circular orbits, as expected if
they formed by accretion of material in the same disk of
debris that gave birth to the Sun.
In the new
study, scientists examined 13 years of observations of
three planets orbiting the sunlike star Upsilon
Andromedae (Ups And) 44 light-years away. "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," says Eric Ford, the Berkeley
astronomer who led the study.
interactions with a fourth planet, he says, warped the
orbit of what astronomers now see as the outermost
world, designated Ups And d. This planet weighs in at 10
Jupiter masses and now orbits 3.75 times farther from
its star than Earth does the Sun.
interactions shot the trouble-making fourth planet
entirely out of the system, but that wasn't the end of
the story. Ups And d's perturbed orbit excited periodic
changes in the orbit of Ups And c, the next planet in.
Ups And c is a 1.9 Jupiter-mass planet that circles the
star at about 80 percent of Earth's distance from our
Sun. The team reported its results in today's issue of
the journal Nature
"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 says. "While those planets might have
formed similarly to Jupiter and Saturn, their current
orbits were sculpted by a late phase of chaotic and
refer to this caroming-world scenario as "planet-planet
scattering," which team member Frederic Rasio of
Northwestern University describes as "a sort of
slingshot effect due to the sudden gravitational pull
between two planets when they come very near each
"We believe planet-planet scattering
occurred frequently in extrasolar planetary systems,"
Rasio says. "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 planetary system around
Upsilon Andromedae is one of the most studied so far.
The inner planet, Ups And b, is a "hot Jupiter" so close
to the star that it takes only 4.6 days to complete an
orbit. It was discovered in 1996 by Berkeley's Geoff
Marcy and his planet-hunting team. When the two outer
planets were discovered in 1999, Ups And became the
first known multiplanet extrasolar system around a
Upsilon Andromedae's unusual nature
prompted unusual attention. Marcy and his team have made
nearly 500 observations. Wobbles in the star's motion
induced by the orbiting planets allow astronomers to
infer the motions of the planets. These observations are
so precise, says Ford, "[W]e can watch and predict what
will happen for tens of thousands of years in the
Astronomers didn't have enough
observational data to distinguish between two different
models accounting for the Ups And system. Renu Malhotra
at the University of Arizona first suggested
planet-planet scattering might have excited the orbital
eccentricities in Upsilon Andromedae's planetary system.
But an alternative explanation claimed interactions
among the planets and the disk in which they formed also
could have produced elongated orbits.
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," explains Ford.
The orbital cycle of
Ups And c actually gives the planet a circular orbit
every 6,700 years. "That property … is quite remarkable
and generally doesn't happen."
"This is what
makes the system so peculiar," Rasio says. "Ordinarily,
the gravitational coupling between two elliptic orbits
would never make one go back to a nearly perfect circle.
A circle is very special."
The fact that Ups And
c's orbit regularly becomes circular suggests that it
began that way. And the 6,700-year cycle of orbital
change hints that whatever altered the system did so
Initially, the team ran computer
simulations in order to determine if Ups And's outermost
planets orbited in the same plane, something made more
challenging by the middle planet's periodic circular
orbits. The odd behavior was present in all of the
simulations. Eventually, the astronomers realized it was
the hallmark of a system that had undergone
"Once you realize that
most of the known extrasolar planets have highly
eccentric orbits, you begin to wonder if there might be
something special about our solar system," says Ford.
"Could violent planet-planet scattering be so common
that few planetary systems remain calm and
Stay tuned for the