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EXCLUSIVE: Binary star systems may eject planets into space

Few theoretical prospects are more terrifying than that of Earth suddenly being flung out of orbit and into interstellar space, increasingly far from the light and heat of our sun.

Thankfully this is a remote possibility for our world– but it is a very real concern for exoplanets born into widely separated two-star systems.

Recent research suggests that powerful gravitational disruptions in wide binary systems can actually eject planets out of orbit and into space.

Wide binaries, systems in which 1,000 AU or more (1 Astronomical Unit equals about 93 million miles) separate two companion stars, encourage highly eccentric orbits among their exoplanets.

“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 researcher Nathan Kaib of Northwestern University in a statement. “This causes their stellar orbits to constantly change their eccentricity, their degree of circularity. If a wide binary lasts long enough, it eventually will find itself with a very high orbital eccentricity at some point in its life.”

Since major orbital disruptions usually take billions of years to manifest, planets are “initially free to form and evolve as if they orbited an isolated star,” continued Kaib. “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.”

Lacking the shared gravitational center of a single star, binary systems instead orbit a common center of mass. Though they spend most of their lives far apart, eccentric orbits will occasionally bring the two stars together as each follows its respective ellipse.

Formerly stable planetary systems orbiting one star in a wide binary can be thrown into chaos when the companion star pays a close visit. The result is increased orbital eccentricity– and sometimes outright ejection.

The team ran about 3,000 computer simulations to reach their conclusions. One model added a wide binary companion to our own solar system, with nearly half of the simulations resulting in at least one giant planet (Jupiter, Saturn, Uranus or Neptune) unceremoniously flung into space.

Astronomical observations of actual exoplanets show that wide binaries tend to generate more eccentric orbits than those of worlds circling single stars, supporting the team’s hypothesis.

In an exclusive interview with The Space Reporter, researchers Nathan Kaib of Northwestern and Sean Raymond of the Laboratoire d’Astrophysique de Bordeaux in France elaborated on their recent findings.

After a world is ejected from its home system, “the planet floats around in interstellar space,” said Mr. Raymond. “Instead of orbiting a star, the planet now orbits the center of the Galaxy, just like the Sun.”

“It just doesn’t have a host star to warm it anymore,” added Mr. Kaib. “It’s a cold, lonely fate.”

Raymond noted a connection between his research and a 2011 study by Takahiro Sumi of Osaka University published in Nature, which suggests that an abundant population of Jupiter-like planets float between the stars.

“[Binary] instabilities probably can’t explain the origin of the very large abundance interpreted by Sumi and his team”, said Raymond, “but there is some debate.”

The instabilities that wide binary stars can trigger in planetary systems don’t only affect giant planets. “In most of my simulations from a related project the smaller rocky planets end up being thrown into their parent stars, although in some cases they are also ejected,” Raymond continued. “Same goes for smaller bodies — asteroids that could end up as meteorites.”

Solitary stars are also guilty of casting stones. “Our own solar system has likely ejected many trillions of meteorites into interstellar space,” said Kaib. “As they orbit the Sun, these small rocks inevitably have close encounters with planets, whose gravity flings them out of the solar system.”

So do the same principles apply to potential instabilities on a larger scale? “It’s a little harder to fling stars or planets out of our Galaxy because the Milky Way’s gravity is a lot stronger than the Sun’s,” explained Raymond. “The speed you need to reach to escape the Sun’s gravity is much less than the Milky Way’s escape speed– although it depends on your position.”

“In low-density, or dwarf, galaxies, planets that are ejected from their stars might actually be ejected from their galaxies as well – that could happen if the galaxy’s escape speed were smaller, which is possible.”

Finally, the authors shared some of the potential implications of their research.

“One thing I find really exciting about the wide binary star mechanism is that there is this natural delay,” said Raymond. “It takes a long time — usually 100 million to several billion years — for the wide binary star’s orbit to change enough to trigger an instability.”

“It’s easy to imagine systems with Earth-like planets that may even have enough time to develop life, but after a long delay undergo catastrophic instabilities,” Raymond continued. “A tragic scenario, but a really interesting one too.”

Kaib explained how the team’s observations could shed light on unknown planetary systems. “We see observational evidence that these binary-triggered disruptions have actually taken place among known planetary systems,” he said.

“For this to occur, it means that planetary systems must typically be extended from their host star– similar to our own solar system. The reason is that if all the planets were closely huddled around their host star, it would be very unlikely for a wide binary to affect them, regardless of its evolution.

“Currently it is very difficult to find planets orbiting at large distances from their host stars,” added Kaib. “Understanding wide binary-triggered disruptions provides new insights into these unobserved regions of planetary systems.”

The study was published Sunday (Jan. 6) in the journal Nature and will be presented by Nathan Kaib at the 221st meeting of the American Astronomical Society in Long Beach, California on Monday (Jan. 7).