BALTIMORE - Of the more
than 130 planets found around distant stars, a large number have highly
elliptical orbits, crazy oblong shapes that have surprised theorists who try to
explain the configurations with near collisions or perturbing disks of gas.
An elliptic orbit is
characterized by the eccentricity, which is how much a planet's distance from
its star varies as it carves out a year. Most of the planets in our solar
system have relatively low eccentricities, less than about 5 percent (tiny
Pluto being a notable
exception and considered not really a planet by some astronomers).
By contrast, the average
eccentricity of extrasolar planets is about 25 percent. And these are not
Plutos. They are typically more massive than Jupiter.
"The eccentricities
are the most remarkable thing about these planets," said Geoff Marcy of
the University of California, Berkeley, during a meeting here last week at the Space
Telescope Science Institute (STScI). The conference was set up to celebrate 10
years of successful exoplanet hunting.
Some have eccentricities of
80 percent, which is as high as the crazy orbits of some comets in our solar
system.
Marcy and others detect
extrasolar planets - most often by a wobble they induce in their host stars.
This planet-star swing dance might seem to be more noticeable when the planets
orbit is highly eccentric, but Marcy said that is not true. As more and more planets are found, it appears that high
eccentricities are common, making our solar system more the exception than the
rule.
"This is surprising
because massive planets would form in nearly circular orbits, and interactions
with a gas disk would tend to keep the eccentricity low," said Phil
Armitage of the University of Colorado.
There are many theories for
generating - or driving - eccentricities, but the two main ideas are that the
planet interacts early on with gaps in the planet-formation disk or later on
with another planet. Both theories have drawbacks. Observations of young stars
with disks might help settle the issue.
Planet-planet
The standard planet
formation scenario involves rocky cores in a gas disk around a star. Those
cores that grow large enough will capture some of the gas to become a gas giant
like Jupiter or Saturn.
In most cases, it is
believed that the gas disk will circularize the orbit of a planet. But at some
point between one million and 10 million years after the star was born, the gas disk disappears - either accreted onto the star and
planets, or blown out into space. With no gas, the orbits would presumably be
free to de-circularize.
If there are multiple
planets in the system and two of them lie in nearby orbits, they can interact
with each other.
"For the extremely
high eccentricities, it is hard to imagine these being generated without planet
interactions," Armitage said.
The smaller mass planet
will often get tossed out into space, while the lager planet survives in a
highly elliptical orbit.
"Earth-sized planets
usually lose out in these interactions," said David Bennett of the
University of Notre Dame.
There are theories that our
solar system started out with more planets, but some were ejected through interactions.
It may be possible to
detect a free-floating planet - one that got kicked out of a stellar system by
a bigger bully. Previous searches for these
"orphans"
have come up empty. But Bennett, who looks for the gravitational magnification
of background stars by foreground planets, thought this microlensing technique
might get lucky and catch one of these stray planets.
About 15 percent of the
stars found with planets have more than one planet, according to Marcy, but the actual
percentage could be higher because some secondary planets might be currently
undetectable. Still, having multiple planets interact at close-in orbits -
where most have been found so far - could be a problem, said Armitage.
Another concern is that
lower mass planets should get a bigger eccentric "kick" from an
interaction. But this is not observed. In fact, the data seems to imply the
reverse - that more massive planets are more eccentric.
Planet-gap
An alternative mechanism
for generating eccentricities could occur while the gas disk is still around.
Once a planet eats up all the gas around its orbit, this gap in the disk could
pull the planet out of its circular orbit.
"If the gap gets wide
enough, you would expect the eccentricities to grow," Armitage explained.
Even if this is true, it
may be quite hard for a smaller Saturn-sized planet to etch out a big enough
gap to generate an eccentricity.
Because this mechanism
occurs early in a stellar system's history, observations of young stars with
disks could confirm whether planet-gap or planet-planet interactions are
relevant.
"If we could observe
planets still embedded in their disks, we would hopefully be able to say
whether eccentricities form early or late," Armitage said.
The Atacama Large Millimeter Array - projected for 2011 - is expected
to be able to peek into the dense environments around stars and hopefully see
planets in the formation process.
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