The scientists who made headlines this week by announcing evidence for a new planet in our solar system are basing the claim entirely on a mathematical model. Nobody’s seen the thing, but the math says it’s there. This isn’t the first time scientists have found a new planet before really finding it, but this technique also has produced outright blunders. This time, though, astronomers say there’s reason to take the new potential planet seriously.
Planet Nine, as the astronomers have dubbed it, would be the first genuine planet discovered in our solar system since Neptune was found in 1846. (Pluto, found in 1930, was officially removed from the list of planets in 2006 by the International Astronomical Union.) And it’d be a big one. The model predicts that Planet Nine is five to 10 times the mass of Earth. Astronomers say it is possible such a substantial object could have eluded detection, because it allegedly lurks in the farthest outskirts of the solar system — moving in a lopsided path whose closest approach to the sun is still 40 times as far as Neptune. Each orbit would take upward of 10,000 years.
Caltech astronomers Michael Brown and Konstantin Batygin say they’ve observed six icy bodies in the distant solar system moving in skinny, oblong orbits that are roughly aligned, like pencils pointing in approximately the same direction. Such a configuration is unlikely, they say, without something else forcing them to exist like that. That something else, they think, is Planet Nine.
Their paper, published in The Astronomical Journal on Wednesday, says the odds are only .007 percent that this pattern would occur “by chance,” but that doesn’t mean there’s a 99.993 percent chance the planet is there. There are still many ways that the scientists could have made an error or overlooked some subtle source of bias, or there could be another explanation for their data. “I’d be willing to bet about 50/50 odds, maybe better, that there’s another planet out there,” said Scott Tremaine, an astronomer at the Institute for Advanced Study in Princeton. He said he places the claim “somewhere between prediction and speculation.”
The case for the new planet builds on a 2014 paper in the journal Nature by Scott Sheppard and Chadwick Trujillo. They observed the properties of about a dozen objects, including a brand-new object the pair found in the outer fringes of the solar system. (Like Pluto, these icy bodies are not big enough to qualify as planets.)
Sheppard, of the Carnegie Institution for Science, said it was not a huge sample but it was enough to recognize an improbable pattern in the objects’ orbits. Their paper explored the possibility that a massive unseen planet had “herded” the smaller objects into an otherwise unlikely alignment.
In the new paper, Brown and Batygin took things a step further, proposing a very elongated orbit for the unseen planet and a more detailed mechanism by which it might have herded the smaller bodies.
Brown is highly respected in astronomy circles. He’s best known for his role in the reclassification of Pluto as a dwarf planet — a change that was brought on partly by his skill of discovering other small, icy bodies comparable in size to Pluto. Either these had to be classified as planets or Pluto had to go. He popularized the episode in a book called “How I Killed Pluto and Why It Had It Coming,” and he tweets under the handle @plutokiller.
Batygin said in an interview that their Planet Nine prediction follows the same general concept that led to the discovery of Neptune. In the mid-1800s, astronomers and mathematicians realized that the orbit of Uranus, then the farthest known planet, wasn’t behaving the way Newton’s law of gravity predicted. They calculated that it might be under the influence of a more distant, unseen planet. That turned out to be Neptune, but in that case, “the signal to noise ratio was much higher” than it is for Planet Nine, Tremaine said.
Astronomer Hal Levison, of the Southwest Research Institute, said the case for Planet Nine is plausible but, invoking the words of Carl Sagan, the researchers haven’t yet presented the kind of extraordinary evidence needed to back such an extraordinary claim. Although mathematical models have helped scientists completely rewrite the formation of the solar system in the past 20 years, history holds some important cautionary tales.
In the early 20th century, astronomer Percival Lowell calculated a discrepancy in the orbit of Neptune that he attributed to an unseen Planet X. Lowell hired Clyde Tombaugh to look for Planet X, and lo and behold, Tombaugh found Pluto. In 1930, Pluto was thought to be a big planet — considerably more massive than Earth. It took until the 1970s for people to figure out that Pluto was a lightweight, only a fraction of the mass of the moon and too small to be nudging Neptune’s orbit. By then it also came out that Lowell’s calculations were based on an incorrect mass for Neptune. With that adjustment, there was no need for a Planet X to explain Neptune’s motion. “This was one of the amazing coincidences in the history of astronomy,” Levison said. This little shiny thing was exactly where Lowell’s erroneous calculations led them.
Levison said that if Planet Nine does exist, there’s no good explanation for how it got to its remote location. It couldn’t have been born there, he said. There isn’t enough material out there to form a planet. It might have been flung outward after formation, but with a hard enough push to get it out that far, it’s hard to understand why it didn’t just keep going right out of the solar system.
Then there’s the question of how a planet more than five times the size of the Earth hasn’t been detected yet. Astronomer Renu Malhotra of the University of Arizona said it’s possible Planet Nine is being blocked by glare from stars in the densest part of our galaxy. The planet itself may even have a dark surface, making it unusually hard to spot, she said.
Multiple scientists I spoke with agreed it’s premature to believe in Planet Nine but that the evidence is compelling enough to justify a search for it. “I think there’s still a lot of unexplored space in the outer solar system where planets could be lurking,” Malhotra said. Planet X turned up Pluto. Who knows what Planet Nine will bring.