When eclipsing binaries orbit each other closely, within about 10 days or less, Fleming and co-authors wondered, do tides — the gravitational forces each exerts on the other — have "dynamical consequences" to the star system? "That's actually what we found" using computer simulations, Fleming said. "Tidal forces transport angular momentum [youtube.com] [?m?s video] from the stellar rotations to the orbits. They slow down the stellar rotations, expanding the orbital period."
[...] The expanding stellar orbit "engulfs planets that were originally safe, and then they are no longer safe — and they get thrown out of the system," said Rory Barnes [washington.edu], UW assistant professor of astronomy and a co-author on the paper. And the ejection of one planet in this way can perturb the orbits of other orbiting worlds in a sort of cascading effect, ultimately sending them out of the system as well.
Making things even more difficult for circumbinary planets is what astronomers call a "region of instability" created by the competing gravitational pulls of the two stars. "There's a region that you just can't cross — if you go in there, you get ejected from the system," Fleming said. "We've confirmed this in simulations, and many others have studied the region as well." This is called the "dynamical stability limit." It moves outward as the stellar orbit increases, enveloping planets and making their orbits unstable, and ultimately tossing them from the system.
On The Lack of Circumbinary Planets Orbiting Isolated Binary Stars [arxiv.org] (arXiv:1804.03676)