Planetary systems with very distant binary stars are particularly susceptible to violent disruptions, more so than if they had stellar companions with tighter orbits around them, according to a new paper.

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 stars. 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 gravity of this close-passing companion can then wreak havoc on planetary systems, triggering planetary scatterings and even ejections. 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 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 Nathan Kaib, lead author of the study. "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." 

The researchers conducted computer simulations of the process and added a a hypothetical wide binary companion to the Earth's solar system which eventually triggered at least one of four giant planets (Jupiter, Saturn, Uranus and Neptune) to be ejected in almost half of the simulations.

Two simulations of planetary system disruption by galactic disturbances to wide binary stars. On the left is a zoomed-out view showing the orbit of a hypothetical 0.1 solar mass binary star around our own solar system with an initial orbital separation of 10,000 AU (1 AU is the distance between the Earth and the sun). On the right is a zoomed-in view of the orbits of Jupiter, Saturn, Uranus, and Neptune. As the binary orbit becomes eccentric, this eventually excites the planetary orbits and Uranus and Neptune are both ejected. Credit: Nathan Kaib

"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," said Kaib, who will present the findings this week at the 221st meeting of the American Astronomical Society in Long Beach, California. "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."

The researchers believe this process occurs regularly in known extrasolar planetary systems and is a telltale signature of past planetary scattering event. Those with eccentric orbits are often interpreted to be the survivors of system-wide instabilities.

The researchers note that this observational signature could only 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.

Published in Nature.