Wide Binary Stars Can Disrupt Planetary Systems
An international team of astrophysicists has shown that planetary systems with very distant binary stars are particularly susceptible to violent disruptions, more so than if the systems had two stellar companions with tighter orbits around each other.
The team, led by Northwestern University’s Nathan Kaib, conducted 3,000 computer simulations to study the effects of binary stellar companions (some with tight orbits around each other and others with wide or distant orbits) on the formation and evolution of planetary systems.
The researchers found that wide binary stars in planetary systems can lead to dramatic events over time. In one hypothetical system, the researchers added a wide binary companion to the Earth’s solar system. This triggered at least one of four giant planets (Jupiter, Saturn, Uranus and Neptune) to be ejected in almost half of the simulations.
In the computer models, these ejections typically were delayed by billions of years, so the planetary systems would spend the first parts of their lives feeling no effects from the binary stars. Only after binary orbits became very eccentric did they catastrophically disrupt the planetary systems.
The astrophysicists also found substantial evidence that this process occurs regularly in known extrasolar planetary systems.
The study was published Jan. 6 by the journal Nature. Kaib also presented the findings Jan. 7 at the 221st meeting of the American Astronomical Society in Long Beach, Calif.
Unlike the Sun, many stars are members of binary star systems — where two stars orbit one another — and these stars’ planetary systems can be altered by the gravity of their companion binary stars, which themselves can be affected by other forces.
The orbits of very distant or wide stellar companions often become very eccentric — 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 planets orbit the less distant star in the binary.) 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," said Kaib, a postdoctoral fellow in the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and the department of physics and astronomy at Northwestern.
"This causes their stellar orbits to constantly change their eccentricity — their degree of circularity," he said. "If a wide binary lasts long enough, it eventually will find itself with a very high orbital eccentricity at some point in its life."
Kaib was interested in studying wide binaries because, unlike tight binary stars, these systems have been virtually unstudied. The stars in wide binaries are separated by more than 1,000 astronomical units (AUs) with one AU representing the distance between the Earth and sun.
When a wide binary orbit becomes very eccentric, the two stars will pass very close together once per orbit on one side of the orbital ellipse, while being very far apart on the other side of the ellipse. This can have dire consequences for planets in these systems since the gravity of a close-passing star can radically change planetary orbits around the other star, causing planets to scatter off of one another and sometimes get ejected to interstellar space.
The process of a planetary system being disrupted by a wide binary takes hundreds of millions of years, if not billions of years, to occur.
"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."
Kaib, who also is a National Fellow in the Canadian Institute for Theoretical Astrophysics at the University of Toronto, conducted the computer simulations of the process with Queen’s University physics professor Martin Duncan and Sean N. Raymond, a researcher at the University of Bordeaux and the Center national de la recherche scientifique in France.
"We also found that there is substantial evidence that this process occurs regularly in known extrasolar planetary systems," Duncan said. "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, such as our Sun."
The researchers believe this is a telltale signature of past planetary scattering events and that those with eccentric orbits are often interpreted to be the survivors of system-wide instabilities.
"The eccentric planetary orbits seen in these systems are essentially scars from past disruptions caused by the companion star," Raymond said.
The researchers note that this observational signature only could be reproduced well when they assumed that the typical planetary system extends from its host star as much as 10 times the distance between the Earth and the Sun. Otherwise, the planetary system is too compact to be affected by even a stellar companion on a very eccentric orbit.
"Recently, planets orbiting at wide distances around their host stars have been directly imaged," Duncan said. "Our work predicts that such planets are common but have so far gone largely undetected."
The title of the Nature paper is "Planetary System Disruption by Galactic Perturbations to Wide Binary Stars."