Binary Star Orbit Shifts Could Force Planetary Ejections
January 7, 2013

Binary Chaos: Wide Binary Systems Can Result In Planetary Ejections

redOrbit Staff & Wire Reports - Your Universe Online

The orbits of distant binary stars can become altered over time, potentially causing violent disruptions in their planetary systems and possibly causing worlds to become scattered or ejected, an international team of astrophysicists has discovered in a new study.

According to the researchers, all planetary systems in which one star is orbited by a second, companion star, can be altered by the gravity of the companion star, but those where the companion star has a tighter orbit are less susceptible to the chaotic effects caused by changes in that star's orbit over time.

"The orbits of very distant or wide stellar companions often become very eccentric -- ie. less circular -- over time, driving the once-distant star into a plunging orbit that passes very close to the planets once per orbital period," the University of Toronto, one of the institutions involved in the research, said in a statement on Sunday. "The gravity of this close-passing companion can then wreak havoc on planetary systems, triggering planetary scatterings and even ejections."

"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," Nathan Kaib, a postdoctoral fellow in the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), a National Fellow in the Canadian Institute for Theoretical Astrophysics (CITA), and lead author of the study, added. "This causes their stellar orbits to constantly change their eccentricity — their degree of circularity. If a wide binary lasts long enough, it will eventually find itself with a very high orbital eccentricity at some point in its life."

Writing in the journal Nature, Kaib and colleagues from Queen's University and the University of Bordeaux in France, reported that when a wide binary orbit becomes highly eccentric, the two stars will pass close together once per orbit on one side of the orbital ellipse, while also being far apart from each other on the opposite side.

They warn that this could have "dire consequences" for the planets in these binary systems, as the gravity of a star that passes nearby can "radically" change their orbits around the other star. That could mean that planets could be scattered off of one another, or even be ejected out of the system and into deep space.

To test their theory, they added a hypothetical wide binary companion star to go with the Sun in our solar system, and ran a series of computer simulations to see what happened. As a result of the companion star's presence, at least one of the four giant planets -- Jupiter, Saturn, Uranus, or Neptune -- wound up being ejected into interstellar space in nearly 50-percent of the simulations, according to the university.

"This process takes hundreds of millions of years if not billions of years to occur in these binaries. 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."

"We also found that there is substantial evidence that this process occurs regularly in known extrasolar planetary systems," added Queen's University physics professor Martin Duncan. "Planets are believed to form on circular orbits, and they are only thought to attain highly eccentric orbits through powerful and/or violent perturbations. When we looked at the orbital eccentricities of planets that are known to reside in wide binaries, we found that they are statistically more eccentric than planets around isolated stars like our Sun."

The research is being presented at the 221st meeting of the American Astronomical Society in Long Beach, California.