Astrophysicists find wide binary stars wreak havoc in planetary systems

Jan 06, 2013

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 they had stellar companions with tighter orbits around them.

Unlike the Sun, many are members of – where two stars orbit one another – and these stars' planetary systems can be altered by the gravity of their . The orbits of very distant or wide often become very eccentric – ie. less circular – over time, driving the once- into a plunging orbit that passes very close to the planets once per . 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 Nathan Kaib, lead author of a study published today in Nature describing the findings. "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."

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This movie shows 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

When a wide 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 .

Kaib, a postdoctoral fellow in the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and the Department of Physics and Astronomy at Northwestern University and a National Fellow in the Canadian Institute for Theoretical Astrophysics at the University of Toronto, conducted 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 Centre national de la recherche scientifique in France. They 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.

"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."

"We also found that there is substantial evidence that this process occurs regularly in known extrasolar ," said 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 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," said Raymond.

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.

"Recently, planets orbiting at wide distances around their host stars have been directly imaged. Our work predicts that such are common but have so far gone largely undetected," says Duncan.

Explore further: Computers beat brainpower when it comes to counting stars

More information: Paper DOI: 10.1038/nature11780

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1.5 / 5 (8) Jan 06, 2013
3.4 / 5 (5) Jan 06, 2013
I once done a simulation for a brown dwarf orbiting the Sun from a distance of 1 light year (around 63000 AU) so a little bit beyond the Oort cloud.

It did not have any significant effect on the solar system except the time it passed near the Oort cloud. The period was around 26000000 years, I kid you not ;)

Unfortunately the precision of the simulation was not enough to for any meaningful result. Would be cool to see such simulation dome by professionals though :D
5 / 5 (1) Jan 07, 2013
So basically the long-distance planet mediates energy to kinetically heat the system (except the very cool close systems that refuse to "vaporize" more planets).
5 / 5 (1) Jan 07, 2013
It did not have any significant effect on the solar system except the time it passed near the Oort cloud.

Such a system wouldn't have an Oort Cloud (if at all it would have an Oort cloud that encloses the combined system. But likely not even that)

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