Image: Double-star systems
Karen Teramura (With Adaptation  /  Artist's impression by Karen Teramura (UH Institute for Astronomy), background photograph by Wei-Hao Wang
The widest binaries and triple systems have very elongated orbits, so the stars spend most of their time far apart. But once in every orbital revolution they are at their closest approach.
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updated 1/6/2013 9:00:49 PM ET 2013-01-07T02:00:49

Alien planets born in widely separated two-star systems face a grave danger of being booted into interstellar space, a new study suggests.

Exoplanets circling a star with a far-flung stellar companion — worlds that are part of "wide binary" systems — are susceptible to violent and dramatic orbital disruptions, including outright ejection, the study found.

Such effects are generally limited to sprawling planetary systems with at least one distantly orbiting world, while more compact systems are relatively immune. This finding, which observational evidence supports, should help astronomers better understand the structure and evolution of alien solar systems across the galaxy, researchers said.

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"The fact that planets observed within wide binaries tend to have more eccentric (or 'excited') orbits than those around isolated stars tells us that wide binaries do often disrupt planetary systems," lead author Nathan Kaib, of Northwestern University and the University of Toronto, told SPACE.com via email. [ The Strangest Alien Planets (Gallery) ]

"Thus, we believe most planetary systems are extended, with outer planets orbiting at tens of AU from their host stars," Kaib added. (One AU, or astronomical unit, equals the distance from Earth to the sun — about 93 million miles.)

The study was published Jan. 6 in the journal Nature and will be presented by Kaib at the 221st meeting of the American Astronomical Society in Long Beach, Calif., on Jan. 7.

Shifting stellar orbits
Two-star systems occur commonly throughout our galaxy; indeed, astronomers think the Milky Way harbors about as many binary systems as single stars. Recently, astronomers have begun discovering planets in binary systems, some of them "Tatooine" worlds with two suns in their skies, like Luke Skywalker's home planent in the "Star Wars" films.

Many double-star systems throughout the galaxy are wide binaries, in which 1,000 AU or more separate the stellar companions on average.

The distance between stars in a wide binary often changes dramatically over time, however, since their orbits around a common center of mass can be far from circular.

"The stellar orbits of wide binaries are very sensitive to disturbances from other passing stars as well as the tidal field of the   Milky Way," Kaib said in a statement. "This causes their stellar orbits to constantly change their eccentricity, their degree of circularity. If a wide binary lasts long enough, it eventually will find itself with a very high orbital eccentricity at some point in its life.”

Eccentric orbits bring the two stars quite close together from time to time, even if the wide binary has a large average separation distance. And these close encounters can wreak havoc on planetary systems, the researchers found after performing about 3,000 computer simulations. 

In one set of runs, for example, the team added a wide-binary companion to our own solar system. In nearly half of the simulations, at least one giant planet — Jupiter, Saturn, Uranus or Neptune — got booted out into space.

Signficant orbital disruption generally takes hundreds of millions or billions of years to manifest, Kaib and his colleagues calculated.

"Consequently, planets in these systems initially form and evolve as if they orbited an isolated star," Kaib said. "It is only much later that they begin to feel the effects of their companion star, which often times leads to disruption of the planetary system."

Shedding light on extrasolar systems
Such destabilization, which is more dramatic in wide binaries than in more tightly orbiting two-star systems, does not always take the form of planetary ejection. Often, exoplanets just get tugged from their orginal, near-circular orbits into more eccentric ones, the simulations showed.

The researchers also looked at orbital eccentricities of actual exoplanets. The team found that planets in wide binaries have more eccentric orbits than worlds that circle single stars, suggesting the computer models are on the money.

"The eccentric planetary orbits seen in these systems are essentially scars from past disruptions caused by the companion star," co-author Sean Raymond, of the University of Bordeaux and the National Center for Scientific Research in France, said in a statement.

The team's computer simulations further suggest that these disruptions generally happen only in planetary systems that extend at least 10 AU or so from the host star.

Taken together, the observational and modeling results imply that many extrasolar systems harbor distantly orbiting worlds, though such planets are tough for astronomers to detect at the moment, researchers said.

"Given that most planet-detection campaigns cannot detect planets beyond 5-10 AU from their host stars, our results provide new clues about the characteristics of planetary systems in a regime that is poorly constrained by current observations," Kaib told SPACE.com. "We believe that planets orbiting at distances of 10 AU or further from their host stars are common."

Follow SPACE.com senior writer Mike Wall on Twitter @michaeldwall or SPACE.com@Spacedotcom. We're also on Facebook and Google+.

© 2013 Space.com. All rights reserved. More from Space.com.

Photos: Month in Space: January 2014

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  1. Southern stargazing

    Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them. (Y. Beletsky / ESO) Back to slideshow navigation
  2. A balloon's view

    Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich. (Kyle Moroney / AP) Back to slideshow navigation
  3. Spacemen at work

    Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment. (NASA) Back to slideshow navigation
  4. Special delivery

    The International Space Station's Canadian-built robotic arm moves toward Orbital Sciences Corp.'s Cygnus autonomous cargo craft as it approaches the station for a Jan. 12 delivery. The mountains below are the southwestern Alps. (NASA) Back to slideshow navigation
  5. Accidental art

    A piece of art? A time-lapse photo? A flickering light show? At first glance, this image looks nothing like the images we're used to seeing from the Hubble Space Telescope. But it's a genuine Hubble frame that was released on Jan. 27. Hubble's team suspects that the telescope's Fine Guidance System locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in a remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288. (NASA / ESA) Back to slideshow navigation
  6. Supersonic test flight

    A camera looking back over Virgin Galactic's SpaceShipTwo's fuselage shows the rocket burn with a Mojave Desert vista in the background during a test flight of the rocket plane on Jan. 10. Cameras were mounted on the exterior of SpaceShipTwo as well as its carrier airplane, WhiteKnightTwo, to monitor the rocket engine's performance. The test was aimed at setting the stage for honest-to-goodness flights into outer space later this year, and eventual commercial space tours.

    More about SpaceShipTwo on PhotoBlog (Virgin Galactic) Back to slideshow navigation
  7. Red lagoon

    The VLT Survey Telescope at the European Southern Observatory's Paranal Observatory in Chile captured this richly detailed new image of the Lagoon Nebula, released on Jan. 22. This giant cloud of gas and dust is creating intensely bright young stars, and is home to young stellar clusters. This image is a tiny part of just one of 11 public surveys of the sky now in progress using ESO telescopes. (ESO/VPHAS team) Back to slideshow navigation
  8. Fire on the mountain

    This image provided by NASA shows a satellite view of smoke from the Colby Fire, taken by the Multi-angle Imaging SpectroRadiometer aboard NASA's Terra spacecraft as it passed over Southern California on Jan. 16. The fire burned more than 1,863 acres and forced the evacuation of 3,700 people. (NASA via AP) Back to slideshow navigation
  9. Where stars are born

    An image captured by NASA's Spitzer Space Telescope shows the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This false-color infrared view, released on Jan. 15, spans about 40 light-years across the region. The brightest portion of the nebula is centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But Spitzer also can detect stars still in the process of formation, seen here in red hues. (NASA / JPL-Caltech) Back to slideshow navigation
  10. Cygnus takes flight

    Orbital Sciences Corp.'s Antares rocket rises from NASA's Wallops Flight Facility on Wallops Island, Va, on Jan. 9. The rocket sent Orbital's Cygnus cargo capsule on its first official resupply mission to the International Space Station. (Chris Perry / NASA) Back to slideshow navigation
  11. A long, long time ago...

    This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang. (NASA / NASA via AFP - Getty Images) Back to slideshow navigation
  12. Frosty halo

    Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.

    Slideshow: The Year in Space (Brian Peterson / The Bismarck Tribune via AP) Back to slideshow navigation
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