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 stare at the twin suns in Star Wars: A New Hope. It turns out that some planets in the real-life versions of these systems may have gotten a much more literal kick out of it, too.
New research suggests that “rogue planets” wandering through the Milky Way — or planets that are isolated from their parent stars and live as cosmic orphans — may be 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 specifically from “warped Tatooine” systems. These are systems in which the stars and the planets orbiting them are misaligned, causing them to be at tilted angles from each other.
As telescopes have improved, the detection of these rogue planets has advanced to the point where astronomers think there are many more free-floating planetary bodies than the stars in cozy arrangements, like the solar system, in the Milky Way. Recent projects estimate the number of rogue planets ejected from their home systems in our Milky Way at one quadrillion (10 followed by 14 zeros). These new, twisted Tatooine findings may help explain this Why rogue planets are so common.
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“A normal planetary system, such as our solar system, consists of multiple planets orbiting a single star. On the other hand, binary stars are also common, accounting for more than 50% of galaxies,” said Cheng Chen, team leader and researcher. astrophysicist at the University of Leeds, told Space.com. “If there are planets orbiting a binary star, we call it a ‘circumbinary planetary system.'”
Although we know that planet formation is a byproduct of star formation, we still don’t know the production rate of planets. That’s because we can’t be sure how many of them will be expelled from their systems and wander around the Milky Way as hard-to-spot, icy bodies. These would be bodies that are not illuminated or warmed by a parent star.
“More accurate demographic determinations of rogue planets could help us complete the final piece of the planet formation puzzle,” Chen said.
He went on to explain that the orbital planes of some of these planets may not align with the orbital planes of their host binaries. Astronomers call these ’tilted orbiting planets’ and examining them could reveal the dominant mechanism for generating free-floating planets.
Kicked out of their cosmic homes
Scientists believe that orphan planets form around young stars, just as planets that remain ‘bound’ to their stellar parent do. Stars are actually born from a collapsing cloud of gas and dust, but this process does not consume all the material in that cloud, leaving stars surrounded by ‘protoplanetary disks’. Too dense regions of these disks collapse and give rise to planets. About 4.6 billion years ago, this gave rise to the solar system and its planets, including Earth.
The period following planet formation is thought to be particularly chaotic for these young systems, leading to ‘messy’ gravitational interactions that could lead to eject planets.
Researchers even theorize that our solar system once had a fifth giant planet, joining Jupiter, Saturn, Uranus and Neptune, during a chaotic period called “late instability.” The idea is that this “extra” planet was displaced from its orbit, after which gravitational interactions with the other giant planets would have caused the unfortunate planet to be banished from the solar system altogether.
However, Chen and his team didn’t focus on a relatively simple single-star system like the Solar System to investigate the origins of rogue planets. Instead, they focused on more complex binary star systems, like those seen in that iconic sunset over Tatooine.
“Three-or-more-body problems are much more complicated than two-body problems. The inner galaxy can disturb the planet due to dynamical effects,” Chen said. “On the other hand, planet-planet interactions can also disrupt planetary orbits.”
The team simulated planetary systems in which two planets are separated from their stars by some distance and plot orbits oblique to the orbit of the central binary stars.
Chen and colleagues experimented with a variety of systems with different ranges of orbital inclinations and a range of planetary distances. They also tinkered with the masses of the planets involved, while still keeping some planets more massive than others.
“We found that a huge planet like Jupiter could disrupt other smaller planets around the binary star and eject them from the system. This can happen when two planets are close to each other or when they are near orbital resonance regions,” said Chen.
The University of Leeds researcher further explained that the team’s previous research showed that two Jupiter-sized planets around a single star can become unstable if their distance is very small, less than twice the distance between Earth and the sun. However, in this study they found that one tilted massive planet around a binary can cause small planetary ejections even if their distances are greater.
This surprised Chen and colleagues because it revealed that a greater diversity of planets could be ejected from twisted Tatooine systems than previously predicted.
“Initially, we thought that only two massive planets around the binary could be ejected due to the strong dynamical effects between the two planets and the planet-binary interactions,” Chen said. “We did not 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. “As a result, these systems may contribute to the population of rogue planets in the universe.”
This means that the systems the team examined could be responsible for the predicted wealth of Earth-sized rogue planets.
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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 past planetary systems and cause a gravitational disturbance that results in a planet being banished. This could be a fairly efficient way to produce rogue planets, whether they come from a single star or from a binary star system.
It is unlikely that Chen will abandon his research into rogue planets. This means that the Taiwanese astronomer’s efforts could help bring these cosmic orphans, separated from their stars, ‘out of the cold’ – at least figuratively.
“I love planets! When I was eight years old, I decided to become an astronomer and study the nine planets in our solar system before Mike Brown changed that by reclassifying Pluto,” Chen joked. “Today, however, more than 10,000 exoplanets have been found that exhibit unexpected features that we can study. Rogue planets are not the only ones; rather than orphans, we should consider them members of our planetary family.”
The team’s research has been published in The Astrophysical Journal Letters.