There are few solar system mysteries quite as compelling as "Planet Nine."
The hypothetical world, thought to be orbiting at an extreme distance from the sun, has some observational evidence that supports its existence. But as it is so far away and therefore so dim, it's not very easy to actually directly observe it.
Calculated to scoot as far away as 1,000 AU from the sun (when 1 AU, or astronomical unit, is the average distance at which the Earth orbits the sun), there's a huge question mark hanging over how it even formed - it had to have evolved far closer to the sun and then some gravitational process migrated it away. This extreme orbit has led to other ideas that Planet Nine might have been pilfered by our sun when it passed close to another star system in the past.
But there's an alternative hypotheses that suggests Planet Nine is actually an alien intruder; a captured "rogue" or orphaned planet that was adopted by the solar system.
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At the 229th meeting of the American Astronomical Society in Grapevine, Texas, last week, researchers from New Mexico State University (NMSU) presented results from their simulations of rogue planets that get captured by the gravity of the sun. It is thought that interstellar space is heavily populated with these orphaned worlds - basically exoplanets that have been kicked from their parent star systems. Over time, these worlds may meander into the path of other star systems, careening into the orbits of resident words, causing all kinds of dynamical chaos. In some cases, resident planets are sling-shotted away by the incoming rogue, but in others, the rogue may be itself captured and adopted by its host star.
Could this be the case for our solar system? Did a rogue planet from another star get captured, becoming Planet Nine?
After carrying out 156 simulations of solar system encounters with interstellar rogue planets, NMSU undergraduate student James Vesper and professor Paul Mason found that, in a surprising number of situations, the rogue planet was captured, settling into a stable orbit around the sun.
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According to their calculations, 60 percent of rogue planet encounters result in the rogue being flung back out of the solar system, sometimes taking another planet with it - like a cue ball upsetting the otherwise orderly rack of balls during a perfect pool table break. But it may not be as dramatic as that; the mere gravitational presence of a new planet in the solar system could cause all kinds of dynamical instabilities.
The remaining 40 percent of "rogue planet capture" scenarios resulted in the gentle capture and integration of the world, so long as the interloper was less than the mass of Neptune (around 17 times the mass of Earth) and the orbit was wide. From observations by Caltech's Konstantin Batygin and Mike Brown of the peculiar motion of distant objects in the solar system last year, Planet Nine should be around ten times the mass of the Earth, but definitely smaller than Neptune.
Before we can confirm Planet Nine's origins, however, we first need to find the thing. So astronomers are hard at work surveying possible locations in the sky that could be home to another large, undiscovered planet of the solar system and the Caltech astronomers are confident that 2017 could be the year they finally reveal Planet Nine's true identity.
WATCH VIDEO: How the Mysterious Planet 9 is Tilting Our Solar System