The tally of known exoplanets currently stands at 334. The list includes nearly three dozen systems that have two or more known planets. But in these multiplanet systems, how are the orbits aligned? Do the planets orbit in nearly the same plane, as planets do in our solar system and as you'd expect in a system spawned from a protoplanetary disk? Or do they orbit in different planes oriented every which way, as you might expect if the system went through a period of chaotic interactions? The elliptical orbits of most exoplanets discovered so far do hint at possible past chaos.

An artist's concept of Gliese 876 in Aquarius and its super-Jupiter, with two hypothetical moons.

Alan MacRobert

The method used to discover most of these systems, tracking the radial-velocity wobbles of their stars, reveals the orbits' periods, sizes, and shapes (circular or elliptical). But it doesn't reveal how an orbit is inclined to Earth’s line of sight or to the other planets in the system.

But in a groundbreaking study announced Wednesday at the American Astronomical Society meeting in Long Beach, California, Jacob Bean (Institute for Astrophysics Goettingen, Germany) presented an analysis that reveals the inclinations of the two outer planets in the three-planet system around the red-dwarf star Gliese 876, a 10th-magnitude speck 15 light-years away in Aquarius. The study shows that the two planets orbit in nearly the same plane, just like those in our solar system.

“This is the first measurement of co-planarity in a normal planetary system,” says Bean, noting that astronomers have previously determined the co-planarity of three planets around a pulsar.

The two Gliese 876 worlds are locked in a 2-to-1 resonance, orbiting the star every 30.3 days and 60.9 days (presumably with a slight back-and-forth drift, or "libration," around this perfect ratio). You can watch a movie of how it works. The resonance means that the planets gravitationally interact with each other repeatedly each time their positions line up; that's how they stay in resonance. And this planet-planet interaction affects the star’s radial velocity (its gravitational wobble toward and away from Earth) in a measurable way, as has been tracked by Geoff Marcy (University of California, Berkeley) and his colleagues at the Keck Observatory in Hawaii.

By combining this information with Hubble Space Telescope observations of the star’s motion across the sky, Bean could perform a Monte Carlo statistical analysis. In essence, he allowed a computer to test a huge number of possible orbital parameters, and let the computer sift through them to see which ones best matched the observations. The results clearly showed that the only probable outcomes are ones with the planets within about 5° of being in the same orbital plane.

“This is what we expected, but it’s nice to actually show it,” says Bean. “The near co-planarity is consistent with planet formation in a disk, like the planets in our solar system.”

The study also reveals that the planets’ orbits are inclined a good 50° to the plane of the sky. This allowed Bean to convert the measured minimum masses that are given by the star's radial-velocity wobbles to the planets' actual masses. The middle planet has about 0.8 times the mass of Jupiter, and the outer planet weighs 2.6 Jupiters.

Gliese 876’s third, innermost planet is a "super-Earth" with only about 8 Earth masses (0.025 Jupiter mass). It's much closer to the star than the other two, orbiting it in just 1.94 days. Bean’s study does not reveal its orbital inclination. But it, too, is likely to be coplanar with its outer two companions. Further study of the system will eventually reveal its inclination, says Bean.

"This is another very interesting result from a planetary system that has produced groundbreaking insights time and again," comments exoplanet specialist Greg Laughlin (University of California Santa Cruz). "Gliese 876 affords us by far our most detailed and accurate map of any extrasolar planetary system in the solar neighborhood, and while in most respects it's utterly alien in comparison to the solar system, it is reassuring to see a kinship in the most fundamental property of system coplanarity."


Image of Rod


January 9, 2009 at 6:42 pm

While the observations indicate the exoplanets orbit in a similar plane like planets in our solar system in the ecliptic, we need to remember that this is a M4V red dwarf star and the exoplanets are giants in eccentric orbits compared to those in our system. The larger exoplanet near 2.6 Jupiters is about 0.21 AU for its semimajor axis and e = 0.27, so challenges remain for exoplanet origin theory and there is no dust disk observed around Gliese 876. The idea that this large, exoplanet formed farther out in a disk and migrated inwards has problems.

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Image of Gerald Nordley

Gerald Nordley

January 15, 2009 at 10:18 am

The two outer planets' periods are very close to a 2:1 resonance, and the current eccentricities and node alignments would reflect the history of their mutual interactions more than past migration. Detecting a distant debris ring around a red dwarf might be too difficult to claim absence with any certainty, so I think we set that aside pending more observations.
The Gliese 876 system is an interesting counterexample to a couple of often stated generalizations about exoplanets. The system is much more massive than Sol's, despite the star being much less massive. Depending on what one chooses for the star's bolometric luminosity, both outer planets (and any large moons they may have) may lie in its habitability zone, illustrating that if one goes by relative period instead of distance, a red dwarf can fit as many planets in its habitability zone as other stars.

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