The orbits of Kuiper Belt objects and of Planet Nine. The entire solar system is tiny in comparison. (Image: Caltech/R. Hurt (IPAC))
Ever since Percival Lowell began searching in 1906 for a "Planet X" that explained strange patterns in Uranus' orbit, astronomers have been captivated by the idea that there's a mysterious planet out there that hasn't been found yet. On the other side, many astronomers who believe any search for Planet X is misguided: If there were more significant planets orbiting in the sun, we likely would have found them by now.
Now, a new paper from Caltech astronomers Michael Brown (the notorious killer of Pluto's planetary status) and Konstantin Batygin argues for a rather large planet far out beyond Pluto. They're calling it Planet Nine. If it exists, it would:
- Be as much as four times the size of the Earth (but somewhat smaller than Neptune)
- Take 15,000 years to orbit the sun
- Be, at its closest orbit, 200 astronomical units of distance away from the center of our solar system (IE, about seven times the space between the sun and Neptune)
No one has seen this planet, although a telescope in Hawai'i called Subaru (!) is now looking for it. Although it would be a rather large planet, as planets go, because it's so far away, it could be very hard to catch a glimpse of, even with a very powerful telescope.
Brown and Batygin were among the Planet X skeptics, but their modeling shows that certain weirdnesses in the orbits of objects in the Kuiper Belt, beyond the orbit of Neptune, could be explained by the existence of a body like Planet Nine. Their analysis shows that there's little probability (just 0.007%) of those weirdnesses being due to chance, but as one scientist cautioned in Science Magazine, the statistical significance of that probability is less than one might hope.
In other words, it's still possible that this might be a false alarm, although as one astronomer told the Washington Post, this new research increases the odds that Planet Nine is real from about 40 percent to 60 percent. We won't know for sure, though, until astronomers see direct evidence of its existence.