Star Wars fans will definitely get a kick out of binary star systems nicknamed "Tatooine" systems — a reference to the planet Luke Skywalker stands on to gaze up at twin suns in Star Wars: A New Hope. As it turns out, some of the planets in the real-life versions of these systems may have been getting a much more literal kick out of them, too.
New research suggests "rogue planets" that wander the Milky Way — aka, planets that are isolated from parent stars and live as cosmic orphans — may be getting kicked out of double, or binary, star systems. But there's a twist (literally)!
The team found that rogue planets are more likely to be ejected from "twisted Tatooine" systems specifically. These are systems in which the stars and the planets that orbit those are misaligned, thus existing at tilted angles from one another.
As telescopes have improved, the detection of these rogue planets has burgeoned to the point that astronomers think free-floating planetary bodies vastly outnumber stars in cozy arrangements, like the solar system, in the Milky Way. Recent projects put the number of rogue planets ejected from their home systems in our galaxy as high as a quadrillion (10 followed by 14 zeroes). These new twisted-Tatooine findings could help explain why rogue planets are so common.
Related: NASA's TESS exoplanet hunter may have spotted its 1st rogue planet
"A normal planetary system, like our solar system, is comprised of multiple planets orbiting a single star. On the other hand, binary stars are also common, accounting for more than 50% of star systems," Cheng Chen, team leader and an astrophysicist at the University of Leeds, told Space.com. "If there are planets orbiting around a binary, we call it a 'circumbinary planetary system.'"
While we know that planet formation is a byproduct of star formation, we still don’t know the production rates of planets. That's because we can't be sure how many are being ejected from their systems and wandering the galaxy as tough-to-spot, icy bodies. These would be bodies that aren't illuminated nor heated by a parent star.
"More precise rogue planet demographic determinations can help us complete the last piece of the planet formation puzzle," Chen said.
He continued by explaining that some of these planets' orbital planes may not be aligned with the orbital planes of their host binary stars. Astronomers call these "tilted circumbinary planets," and investigating them could reveal the dominant mechanism for generating free-floating planets.
Kicked from their cosmic homes
Scientists think believe orphan planets form around infant stars just like planets that remain "bound" to their stellar parent do. Stars are basically born from a collapsing cloud of gas and dust, but this process doesn't consume all of the material in that cloud, thus leaving stars surrounded by "protoplanetary disks." Overly dense regions of these disks collapse and give rise to planets. Around 4.6 billion years ago, that's what gave rise to the solar system and its planets, including Earth.
The period following planet formation is believed to be especially chaotic for these fledgling systems, leading to "messy" gravitational interactions that can eject planets.
In fact, researchers theorize our solar system may once have had a fifth giant planet alongside Jupiter, Saturn, Uranus and Neptune during a chaotic period called "late instability." The idea is that this "extra" planet was shifted from its orbit, then gravitational interactions with the other giant planets would have caused the unfortunate planet to be exiled from the solar system altogether.
Chen and his team didn't focus on a relatively simple, single-star system like the solar system to investigate rogue planet origins, however. Instead, they focused on more complex binary star systems, like the one seen in that iconic sunset over Tatooine.
"Three or more body problems are much more complicated than two-body problems. The inner stellar binary can disturb the planet due to dynamical effects," Chen said. "On the other hand, planet-planet interactions can also disturb planetary orbits."
The team simulated planetary systems in which two planets are separated by some distance from their stars and trace out orbits that are inclined in relation to the orbit of the central binary stars.
Chen and colleagues experimented with a variety of systems featuring a differing range of orbital inclinations and a spread of planetary separations. They also tinkered with the masses of the planets involved while still keeping one planet more massive than the other.
"We found that a massive planet like Jupiter could disturb other smaller planets around the binary and eject them from the system. This can occur when two planets are close or when they are located around orbital resonance regions," Chen said.
The Leeds University researcher further explained that the team's previous study found that two Jupiter-sized planets around a single star can become unstable when their separation is very small, less than two times the distance between Earth and the sun. In this study, however, they found that one tilted massive planet around a binary can cause small planet ejections even when their separations are wider.
This surprised Chen and colleagues, as it revealed a more diverse range of planets could be ejected from twisted Tatooine systems than previously predicted.
"Initially, we thought only two massive planets around the binary could be ejected due to the strong dynamic effects between the two planets and the planet-binary interactions," Chen said. "We didn’t expect that small planets could be ejected so efficiently. As a result, circumbinary systems could produce rogue planets ranging from small to large."
"Small planets are more common than high-mass planets," he continued. "Consequently, these systems may contribute to the population of rogue planets in the universe."
This means the systems the team investigated could account for the predicted wealth of Earth-sized rogue planets.
Chen explained that the team is currently looking for other mechanisms that could also produce rogue planets. This includes the possibility that other stars could fly by planetary systems and cause a gravitational disturbance that leads to a planet being exiled. This could be a rather efficient way to produce rogue planets, whether from around a single star or from a binary system.
Chen is unlikely to give up his investigation into rogue planets. This means the Taiwanese astronomer's efforts could help to bring these cosmic orphans unbound from their stars "in from the cold" — at least figuratively.
"I like planets! When I was 8 years old, I decided to become an astronomer and studied the nine planets in our solar system before Mike Brown changed that by reclassifying Pluto," Chen joked. "However, nowadays, more than 10,000 exoplanets have been found, displaying unexpected characteristics for us to study. Rogue planets are not alone; we should not let them be orphans but consider them members of our planetary family."
The team's research is published in The Astrophysical Journal Letters.
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