Someday soon, we could find a real ninth planet in our solar system, one bigger than Earth. Like Neptune, the math adds up, and it's just a matter of time before we find it unless something spectacularly weird is going on in our solar system. But in the meantime, there's a lot of weirdness and scapegoating surrounding the probably-planet.
So let's back up a little. Astronomers Konstantin Batygin and Mike Brown looked at the orbit of a bunch of objects at the outskirts of our solar system in the possible Inner Oort Cloud and found some irregularities. Sedna, 2012VP113, and around five other small dwarf planets all have orbits that take them farther out than any other known objects in the solar system. Yet they all seem to converge at the same point in their nearest approach to the sun.
In the same way that Neptune shepherds the orbit of the Kuiper Belt Objects like Pluto and Makemake, Brown and Batygin say that a distant planet in between Earth and Neptune in size pulls Sedna and its siblings to this point. With the discovery of 2012VP113, evidence grew for a distant unseen planet, but this was the first formal hypothesis.
Brown, for his part, has been on the hunt for new, distant planets in our solar system for decades. His groundbreaking work led to the discovery of Eris, an object around the size of Pluto that helped usher in the term dwarf planet and usher out Pluto's planetary status. The quest was detailed in Brown's book How I Killed Pluto (And Why It Had It Coming.) The book never found its ninth planet. But now, Brown thinks he might just know where it is.
Since the hypothesis was laid out earlier this year, it's been simply called Planet Nine. It both captivated the public imagination and has led to a lot of misinformation. Just yesterday, NASA had to release a public statement to refute theories that Planet Nine was tugging at its intrepid, Saturn-orbiting Cassini probe.
"An undiscovered planet outside the orbit of Neptune, 10 times the mass of Earth, would affect the orbit of Saturn, not Cassini," William Folkner, a planetary scientist at NASA and Caltech's Jet Propulsion Laboratory, said in the statement. "This could produce a signature in the measurements of Cassini while in orbit about Saturn if the planet was close enough to the sun. But we do not see any unexplained signature above the level of the measurement noise in Cassini data taken from 2004 to 2016."
Folkner further explained that, if Brown and Batygin's orbital parameters for Planet Nine were correct, there wouldn't be any kind of detectable tug until around 2020. That contradicts a report that seems to have originated in Scientific American that says small perturbations are occurring on the craft right now. Already, that report has been updated to say that there may be small tugs on Saturn's orbit, rather than Cassini's, but even then, NASA says they haven't seen a tug on Saturn of yet.
That makes for, more or less, a hypothesis that could, eventually, be true. But another supposed hypothesis? Maybe not so much.
If the British tabloid press is to believed, Planet Nine is the culprit behind periodic bombardments of asteroids on Earth, like the one that killed the dinosaurs. The Daily Mail and Metro both ran with it. Yahoo News carried it stateside. But thankfully, the British tabloid press is almost never to be believed when it comes to science stories. So what gives?
Around the same time as talk of Planet Nine heated up, astronomer Daniel Whitmire published a study in the Monthly Notices of the Royal Astronomical Society that resurrects the old idea of an unseen planet or small mass star that causes periodic extinctions on Earth. Whitmire's paper toys with the original "Nemesis" idea, suggesting other possible culprits including a very inclined Earth-like planet between Neptune and Planet Nine or yet another trans-Neptunian planet. Planet Nine is mentioned in the paper (pre-print available here) not as the culprit beyond these once-every-27-million-year bombardments, but as evidence that we may have other planets unknown to us.
Planets of certain sizes have been all-but ruled out in many sky surveys. In a 2014 infrared survey from NASA's WISE telescope, Saturn-mass planets were ruled out at 10,000 times the distance of the Sun-to-Earth, and Jupiter-mass planets were ruled out beyond 26,000 times that distance. Planet Nine, as it stands, is instead somewhere in between a very large rocky planet and a very small gas giant. These Super-Earths or Mini-Neptunes are common in other solar systems, but if discovered, Planet Nine will be the first known planet of that class in our solar system. But it may not be the last.
But nowhere does Whitmire blame Planet Nine, continually calling the hypothetical comet-or-asteroid-thrower "Planet X" throughout. A confusing press release from the University of Arkansas, where Whitmire is an adjunct mathematics professor after his official retirement, does little to dispel this.
Both Brown and Batygin are, judging from their Twitter posts, not happy about these errors.
There are, of course, other weird stories floating out there about Planet Nine. A non-peer reviewed study suggests that, rather than forming in the same region as the gas giants and migrating out, Planet Nine was captured from another solar system, explaining its strange orbit. It's based on a series of simulations from how a passing planet might be captured without wiping out the Kuiper Belt. It's an interesting idea, and indeed, another Arxiv preprint (that didn't go to press in a peer reviewed journal) suggested Sedna could have been captured from another passing solar system.
Both are interesting ideas. Both have some numerical suggestions that it's probable. Neither has, as of yet, been peer reviewed and published beyond Arxiv.
It's easy to get minds racing with the hint that we could discover a new planet. Indeed, Brown and Batygin believe it's an all-but-inevitability, with a likely .001% chance it's a statistical error after the most recent dwarf planet find in the region, according to Brown's Twitter. But that often opens the door for various hypotheses of various stripes of believability. Indeed, there is no Planet Nine yet, for sure, but that isn't preventing a lot of misconceptions about it by a shoddily done game of "connect the dots."
After we find Planet Nine, and study it, we can start to worry about what it means. Some intriguing ideas may come up in the interim, but take them all with a grain of salt, because a new super-Earth is not a panacea to the mysteries of our solar system.