So-called circumbinary planets — those planets that orbit around a binary star, like the fictional Tatooine from the Star Wars — can be ejected off into space as a consequence of their stars’ evolution, according to a new study to be published in the Astrophysical Journal (arXiv.org preprint). The findings help explain why astronomers have detected few circumbinary exoplanets despite observing thousands of short-term binary stars, or ones with orbital periods of 10 days or less.
This artist’s concept shows a hypothetical exoplanet around the binary star system of Kepler-35A and B. Image credit: NASA / JPL-Caltech.
There are several different types of binary stars, such as visual and spectroscopic binaries, named for the ways astronomers are able to observe them.
David Fleming, a doctoral student at the University of Washington, and colleagues study eclipsing binaries, or those where the orbital plane is so near the line of sight, both stars are seen to cross in front of each other.
When eclipsing binaries orbit each other closely, within about 10 days or less, the study 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. Tidal forces transport angular momentum from the stellar rotations to the orbits. They slow down the stellar rotations, expanding the orbital period,” Fleming said.
This transfer of angular momentum causes the orbits not only to enlarge but also to circularize, morphing from being eccentric, or football-shaped, to perfect circles.
And over very long time scales, the spins of the two stars also become synchronized, as the Moon is with the Earth, with each forever showing the same face to the other.
“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 co-author Dr. Rory Barnes, also from the University of Washington.
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. We’ve confirmed this in simulations, and many others have studied the region as well,” Fleming said.
“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.”
“Another intriguing characteristic of such binary systems, detected by others over the years, is that planets tend to orbit just outside this stability limit, to ‘pile up’ there. How planets get to the region is not fully known; they may form there, or they may migrate inward from further out in the system.”
Applying their model to known short-period binary star systems, the scientists found that this stellar-tidal evolution of binary stars removes at least one planet in 87% of multi-planet systems, and often more.
“And even this is likely a conservative estimate; the number may be as high as 99%,” Dr. Barnes said.
“We dubbed the process the Stellar Tidal Evolution Ejection of Planets (STEEP),” the researchers said.
“Future detections of circumbinary around short-period binary stars will provide the best indirect observational test of the STEEP process.”
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David P. Fleming et al. 2018. On the Lack of Circumbinary Planets Orbiting Isolated Binary Stars. ApJ, in press; arXiv: 1804.03676
