The biggest astronomical discovery in decades may happen in 2017 in the form of a whole new world: Planet Nine from outer space! If it exists anywhere along the suspected eccentric orbit, sweeping silently through the edges of the solar system more than one-thousand times farther from the sun than Earth is at its most distant point, the new planet won’t be able to hide for much longer. And the debate over its origin will be fantastic:
Stars are born in groups, so when they are young, solar systems regularly pass by each other. As Planet Nine passed by our solar system, the Sun's gravitational pull on it was stronger than that of its own star. As a result, Planet Nine was torn away and set onto its current orbit.
Maybe, maybe not. If Planet Nine exists—and some astronomers are saying there’s easily room for more than one sizable new planet out there in the Oort—labeling it an alien capture versus a homegrown world could get kind of complicated. Planets accrue from the debris around infant stars, those stars in turn tend to form in groups close together. There is a stage in every solar system, so we think anyway, where there are hundreds to thousands of sizable worlds, all interacting chaotically. Some get slung out, and in a stellar nursery with stars as close to one another as a tenth of a light or less, these baby planets could get passed around from star to star like a conspiracy theory among wingnuts.
For all we know, the Earth, or any of the largish sub-planets that would eventually make up the Earth, could have started as a seed growing around a completely different star and been “adopted” by our familiar sun as a young tyke, before growing into the familiar world we know and love today. Distant planets like the hypothetical Planet Nine might be able to tell us more about that untidy process, a process that might produce not just distant, eccentric worlds with many moons way out in a star’s cometary halo, but could send rogue worlds by the billions wandering untethered through interstellar space. We might even get a spacecraft to Planet Nine, if it went really fast, in as little as 20 years, and look for telltale isotopes from other stars or galaxies. What a mission of discovery that would make!