I should preface this article with the acknowledgement that, though highly probable*, the existence of the body now dubbed “Planet Nine” is hypothetical. Consequently, any theory about its origin will be highly speculative. That having been said -or written- some astronomers have actually proposed that Planet Nine could be a rogue planet.** Paul A Mason and James Vesper, two research astronomers at New Mexico State University, wrote a paper (published by the American Astronomical Society in Jan 2017) entitled “Simulation of Rogue Planet Encounters with the Solar System: Is Planet Nine a Rogue Planet?”

They conducted 156 computer simulations involving the interaction of rogue planets with the solar system. Mason and Vesper assumed that the rogue planet in their simulation was similar to that of the proposed planet nine: at least ten times the size of Earth. The analysis of these simulations showed that

• In 60% of the encounters, the rogue planet simply entered the solar system’s outer fringes and then departed. At times it took a small dwarf planet with it.
• In 40% of the encounters, the Sun managed to capture the rogue planet, which would remain at a great distance from the Sun.

Intriguingly, the simulation suggests that the captured body would establish a highly elongated orbit at a distance of 1000 AU (or, 1000 times farther from the Sun than Earth.) This distance is about equal to the presumed mean distance of Planet Nine. Even though the Sun’s gravitational influence is tenuous at such a distance, the Sun’s gravity will still be dominant over the nearest star system, Alpha Centauri, which is 261,935 AU away.

An artistic depiction of the hypothetical planet Nine. As it has never been observed -and might not exist in the first place- details about its appearance are, well, scant. Notice the small illuminated ellipse at the upper right. That ellipse represents Neptune’s orbit. Image credit: Wikimedia Commons

Of course, this is all highly speculative. Unsurprisingly, astronomers have not yet found Planet Nine. At such a distance, it must be quite faint, if it exists. Moreover, with a mean distance of 1000 AU, its orbital period would likely exceed 31,600 years. Such a planet’s existence would be confirmed by noting alterations in its position over time. [Pluto was discovered this way after a year’s searching, and it only requires 248 years to complete one orbit. Granted, that was in 1929–1930 using what would now be regarded as an antiquated blink comparitor.] Hopefully, the James Webb Space Telescope might help astronomers locate Planet Nine. If it is found, astronomers will then study it as closely as possible (probes are not a feasible option at this point) and could gain some real insight into its origin.

It might well be a rogue planet that now belongs to us. Physics allows it.

*Though not yet observed, many astronomers believe that Planet Nine exists because of the peculiar “clustering” of Trans-Neptunian objects

**A rogue planet is defined as one that is not gravitationally bound to any star. It seems highly probable that the Milky Way harbors billions of these rogue worlds. That high number is not surprising because simulations suggest that most, if not all, solar systems will propel at least one -but often more- planets away during the tumultuous formation process.