Planet Nine Is Either a Gas Giant or an Icy Snowball

Wednesday, 04 May 2016 - 10:12AM
Solar System
Wednesday, 04 May 2016 - 10:12AM
Planet Nine Is Either a Gas Giant or an Icy Snowball
Planet Nine is becoming more mysterious by the minute. If it does, in fact, exist, which is looking increasingly likely, then there are two primary explanations for its existence--and they're complete polar opposites. According to several studies from the Harvard Smithsonian Center for Astrophysics, Planet Nine is either a Jupiter-like gas planet or a giant, dense snowball, like a humongous version of Pluto.

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 postulate 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.

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"The evidence points to Planet Nine existing, but we can't explain for certain how it was produced," CfA astronomer Gongjie Li, lead author on a paper accepted for publication in the Astrophysical Journal Letters, said in a statement.
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The main point of contention involves where exactly Planet Nine formed in our solar system. Two other Harvard astronomers, Scott Kenyon and Benjamin Bromley, ran computer simulations of the formation of a potential ninth planet, and found that the most likely theory involves the early formation of a gas giant in the inner solar system. The first few million years of our Sun's life were very chaotic, and astronomers believe that gas giants were continually forming, colliding with each other, and affecting each other's orbits.

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"The odds that other gas giants were formed in the early solar system are very good," Kenyon told Gizmodo. "You could have made ten and Jupiter could have eaten a few."
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If this is the case, then Planet Nine could have formed closer to the Sun, and then been evicted to the outskirts of the solar system by a series of gravitational "kicks" from other gas giants, particularly Jupiter and Saturn.

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"Think of it like pushing a kid on a swing. If you give them a shove at the right time, over and over, they'll go higher and higher," said Kenyon. "Then the challenge becomes not shoving the planet so much that you eject it from the solar system."
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The other scenario, which is significantly less likely but still possible, is that the planet formed exactly where it is now. In that case, the planet would have been created from the same process that formed the Earth: very slow, cold, and gradual matter accretion over time. In this case, the matter that formed the planet would have been displaced from the "gaseous disk" that surrounded our Sun for the first 20 million years of its life.

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"Our idea is that as the gaseous disk is going away, it develops a hole, which gets bigger and bigger until the disk is gone," Kenyon said. "As this hole is getting bigger, material outside the hole sweeps up solid particles like a snowplow, and deposits them at a large distance."
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This is a question with extremely disparate answers, but luckily, it might not be a question for too much longer. Although Planet Nine is still technically a hypothetical planet, once we confirm its existence, it should be fairly easy to discover which of these scenarios is true.

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"The nice thing about these scenarios is that they're observationally testable," Kenyon said. "A scattered gas giant will look like a cold Neptune, while a planet that formed in place will resemble a giant Pluto with no gas."
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Image Credit: Caltech/R. Hurt (IPAC)

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Space
Solar System

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