Circumbinary castaways: Short-period binary systems can eject orbiting worlds

This artist’s concept illustrates Kepler-16b, the first
planet known to orbit two stars – what’s called a
circumbinary planet. The planet, which can be seen in the
foreground, was discovered by NASA’s Kepler mission. New
research from the University of Washington indicates that
certain shot-period binary star systems eject circumbinary
planets as a consequence of the host stars’ evolution.
Credit: NASA/JPL-Caltech/T. Pyle

Planets orbiting “short-period” binary stars, or stars locked
in close orbital embrace, can be ejected off into space as a
consequence of their host stars’ evolution, according to new
research from the University of Washington.

The findings help explain why astronomers have detected few
circumbinary planets—which orbit that in turn orbit each other—despite
observing thousands of short-term binary stars, or ones with
orbital periods of 10 days or less.

It also means that such binary star systems are a poor place to
aim coming ground- and space-based telescopes to look for
habitable planets and life beyond Earth.

There are several different types of binary stars, such as
visual and spectroscopic binaries, named for the ways
astronomers are able to observe them. In a paper accepted for
publication in Astrophysical Journal, lead author David
Fleming, a UW astronomy doctoral student, studies 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.
Fleming will present the paper at the Division on Dynamical
Astronomy conference April 15-19.

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 from the
stellar rotations to the orbits. They slow down the stellar
rotations, expanding the orbital period.”

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 Rory Barnes, 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

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, Fleming said, is that
planets tend to 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 , Fleming and co-authors found
that this stellar-tidal evolution of binary stars removes at
least one planet in 87 percent of multiplanet circumbinary
systems, and often more. And even this is likely a conservative
estimate; Barnes said the number may be as high as 99 percent.

The researchers have dubbed the process the Stellar Tidal
Evolution Ejection of Planets, or STEEP. Future detections—”or
non-detections”—of circumbinary around short-period binary
stars, the authors write, will “will provide the best indirect
observational test of the STEEP process.

The shortest-period binary star system around which a
circumbinary planet has been discovered was Kepler 47, with a
period of about 7.45 days. The co-authors suggest that future
studies looking to find and study possibly around short-term should focus on those with longer
orbital periods than about 7.5 days.

Fleming and Barnes’ co-authors are UW astronomy professor Tom
Quinn, post-doctoral researcher Rodrigo Luger and undergraduate
student David E. Graham. This work used storage and networking
infrastructure provided by the Hyak supercomputer system at the
UW, funded by the UW’s Student Technology Fee.

As for habitability and the search for life, Fleming said
orbiting short-term eclipsing binaries
might otherwise be attractive targets for closer study, with
their edge-on angle showing eclipses, and more, to the distant

“But this mechanism tends to kill them,” he added. “So, it’s
not a good place to look.”

Explore further:

Astronomers discover an M-dwarf eclipsing binary system

More information: On The Lack of Circumbinary Planets
Orbiting Isolated Binary Stars, arXiv:1804.03676 [astro-ph.SR]

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