Planet Nine Might Be the First Exoplanet in Our Solar System

Wednesday, 01 June 2016 - 10:01AM
Solar System
Wednesday, 01 June 2016 - 10:01AM
Planet Nine Might Be the First Exoplanet in Our Solar System
Planet Nine has many mysteries surrounding it--most notably whether it actually exists or not. But now, Swedish researchers have discovered that the object might be even more exotic than we thought, as it's highly likely that it was stolen from another star system.

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"It is almost ironic that while astronomers often find exoplanets hundreds of light years away in other solar systems, there's probably one hiding in our own backyard," team member Alexander Mustill from Lund University said in a statement.
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Planet Nine is a hypothesized ninth planet in our solar system announced back in January. There has been no official confirmation that it is real, but if it is, then researchers estimate that it orbits the Sun at approximately 20 times the distance of Neptune's orbit, which is 2.8 billion miles away. It would be too large to be a dwarf, so if proven to exist, it would definitely be the ninth planet of our solar system.

But if it is real, scientists had no idea how it formed (and how we missed it for so long), especially since it's so far away from the Sun. Physicists theorized that it either formed as a gas giant close to the Sun, and was subsequently "kicked" by gravitational pulls from other planets to its current position, or that it formed exactly where it is now, as an accumulation of icy debris. But neither theory was perfect, which led the researchers in the current study to look for alternative explanations.

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"What we were wondering is, how could you actually form a planet out there?" Mustill says in the above video. "Because the rest of the Solar System appears to be so compact, and then you have this enormous gap where there's really not very much, and then a very large planet, nearly 10 times the mass of Earth. It seems a little implausible to us that you would just 'create' this thing out at such a distance."
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When the researchers used computer simulations to investigate how the planet could have formed and ended up in its current position, and they found a counterintuitive but plausible explanation: Planet Nine is actually an exoplanet. It originally formed in a neighboring system and orbited another star before it was "stolen" by our solar system during a close encounter 4.5 billion years ago.

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"While the existence of Planet 9 remains unproven," the authors wrote in their paper, published in Monthly Notices of the Royal Astronomical Society: Letters, "we consider capture from one of the Sun's young brethren a plausible route to explain such an object's orbit."
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In the simulations, the researchers found that there was a 50% chance that our solar system would capture an object from a system with a wide-orbit planet, which sounds extremely promising. However, that doesn't mean that there's a 50% chance that Planet Nine is actually an exoplanet, since the simulations already assume several conditions, most notably that neighboring systems have wide-orbit planets like Planet Nine. So taking everything into account, the chances are between .1 and 2 percent.

Mustill insists that these probabilities are still encouraging, since it is still much more likely than other options we've come up with. For example, there's a 300 percent higher chance that it's an exoplanet than that it formed in our solar system by random chance.

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"Although these probabilities seem low, you have to compare them to each other, and not absolutely," Mustill told New Scientist. "Because ultimately, any very specific outcome is very unlikely."
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And if it is an exoplanet, that would be a game-changer for all sorts of reasons, not least because it would be much, much easier to actually visit than any other exoplanet in the Universe.

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"This is the only exoplanet that we, realistically, would be able to reach using a space probe," said Mustill.
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