Scientists are still uncertain whether Planet Nine even exists, but assuming it does, some now suggest that it originated in another solar system – and our sun stole it.
If true, it would be the first exoplanet discovered in our own solar system, and it would thereby throw the very definition of what constitutes an exoplanet into confusion.
"While the existence of Planet 9 remains unproven," write the authors of a paper proposing this scenario, "we consider capture from one of the Sun’s young brethren a plausible route to explain such an object’s orbit."
The theory goes like this: Stars emerge in clusters, and in this early phase of their existence, they often pass near one another. Under such circumstances, if a planet finds itself shoved into an orbit that takes it far from its parent star, it becomes susceptible to the pull of other young stars in the cluster, if their paths cross at the right time.
According to computer simulations undertaken by the researchers at Lund University, Sweden, and Laboratoire d'Astrophysique de Bordeaux, France, this is likely what happened to Planet Nine about 4.5 billion years ago. And then, as the sun left its birth cluster, Planet Nine was stuck in our solar system.
"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," said lead author Alexander Mustill, astronomer at Lund University, in a press release.
If Planet Nine does turn out to be an exoplanet, it could prove invaluable to our understanding of the universe, "the only exoplanet that we, realistically, would be able to reach using a space probe," as Dr. Mustill puts it.
Of course, before scientists can determine whether Planet Nine is, in fact, an exoplanet, there is another important step to take: Confirm it exists at all.
The original study that proposed the existence of Planet Nine based its reasoning on "the orbits of distant Kuiper Belt objects," suggesting that the distinct patterns of their motion could be explained by the presence of a planet many times more massive than Earth.
The research was based solely on computer modeling, with the astronomers explaining our ignorance of the planet’s existence by the fact that a planet of its size, lying as far from Earth as it does, would be all but undetectable by today’s telescopes.
"There is still no image of Planet 9, not even a point of light," Mustill said in the press release. "We don't know if it is made up of rock, ice, or gas. All we know is that its mass is probably around ten times the mass of earth."