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 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 –
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 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."
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.
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 planetary systems," 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 planets are
common but have so far gone largely undetected," says Duncan.
Image credit: With thanks to wishfire.com
Source: The Daily Galaxy via University of Toronto
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