Eccentric Worlds: Strange Orbits Puzzle Astronomers

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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