If Planet Nine exists, it's been through one hell of an ordeal. That's the takeaway from a series of new studies that ask how in the name of Uranus a planet could have gotten itself into such a whacked-out orbit. This in turn might help explain the unlikely orbits of half a dozen Kuiper Belt objects.
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Planet Nine is a hypothetical world roughly the mass of Neptune that orbits our Sun in a giant ellipse, at a distance of 64 to over 160 billion kilometres. Although astronomers have proposed hidden ninth planets for years, this latest version — the brainchild of Caltech's Mike Brown and Konstantin Batygin — has gained quite a bit of traction since it was announced in January. The potential planet is so compelling that many astronomers have penned follow-up papers describing how we might find it and what it could look like.
Almost as interesting as Planet Nine's present location is how this behemoth wound up cooling its heels ten times further from the Sun than Pluto. Several new modelling efforts led by researchers at the Harvard-Smithsonian Center for Astrophysics are now attempting to answer that question.
There are essentially three competing hypotheses. The most likely, proposed by astronomers Scott Kenyon and Benjamin Bromley, posits that Planet Nine is a gas giant that formed in the inner solar solar system before straying too close to Jupiter and getting itself punted out.
"The odds that other gas giants were formed in the early solar system are very good," Kenyon said, adding that the few first million years of our Sun's life were a chaotic time, with young gas giants sucking up matter and crashing into each other all willy-nilly. "You could have made ten and Jupiter could have eaten a few."
There's just one problem: once gravity punted Planet Nine into the boondocks, what caused it to brake? Why didn't it keep on going into interstellar space? One possibility, Kenyon said, is that the so-called "gaseous disk" that surrounded our Sun for the first 20 million years of its life produced enough friction to slow Planet Nine down. "If you have the right mass of planet and the right mass of gas, you can damp the orbit and circularise it," he said.
Next, the two men proposed an even stranger possibility: maybe Planet Nine formed in place. In this scenario, Planet Nine is a giant snowball rather than a gas giant, that packed itself up in a very slow and very cold version of the accretionary process that formed the Earth.
"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." Over the course of hundreds of millions of years, all of those plowed up ice shavings snowballed into one another, resulting in a jawbreaker about twice the size of the Earth.
Kenyon admits the idea is speculative. Fortunately, there's a very good way to test these competing hypotheses — by observing Planet Nine. "The nice thing about these two scenarios is that they predict different things for the planet," he said. "If it's a scattered gas giant, it will looks like a cold version of Neptune. If it's an icy object that formed at 500 AU, it'd be a large version of Pluto."
The final scenario sounds like a plot line from a B-grade sci-fi movie, and it seems to be comparably unlikely. Planet Nine could be an extraterrestrial invader. "Planet 9 may be an exoplanet in our own solar system," said Gongjie Li, another astronomer at Harvard's Center for Astrophysics whose recent modelling paper explores this very possibility, among others.
Astronomers believe our star formed as part of a densely packed cluster that dissipated after about 100 million years. In the early days of the solar system, close encounters with neighbouring stars would have been much more common. It's possible — albeit unlikely — that the outer edges of our solar system once exchanged material, including planets, with another.
"This is a possibility, but I think everything has to work out just right," Kenyon said. "It's a very finely tuned scenario."
Li's models also reveal a small but significant chance that a passing star could have perturbed Planet Nine's orbit without breaking it. This could explain why the planet is stuck in such an unusually stretched out ellipse today.
One could argue that all of this speculation will be moot if we never find a massive object lurking 160 billion kilometres away. On the other hand, it's kind of awesome that the mere thought of Planet Nine is causing scientists to consider all the crazy stuff that could have gone down in the early solar system. Even if our telescopes turn up nothing, a system where rogue exoplanets and larger-than-Earth-sized snowballs are possible is an amazing place to live.
