The long and winding history of Planet X

Standing up in public and claiming that you have found evidence for a new planet in the solar system almost certainly means that you are about to join a century-long list of people who have been wrong. I mean, look, le Verrier and Adams did it in 1846 when they predicted the existence of Neptune, sure, but since then there have been dozens (hundreds?) of similar claims (including le Verrier himself, who thought he had found a Planet Vulcan inside the orbit of Mercury) which have all lead to absolutely nothing.

For most of the 160 years, astronomers tried to use the positions of the planets themselves to infer the existence of another planet (amusingly: it was the search for this alleged planet that led to the inadvertent discovery of Pluto, which is why the New York Times headline on the day of the Pluto announcement suggests that the planet might be bigger than Jupiter, which it is not).  This idea was finally put to rest in 1993, when a careful analysis of modern data showed that all of the planets are exactly where they are supposed to be (links to papers here may lead to irritating pay walls, but you can at least read the abstracts, I think).

When it was clear that the planets were where they were supposed to be, astronomers moved on to the comets, which traverse the regions where Planet X might be, and could be effected by them. A suggestions was made that Planet X causes comet showers  and modulates the positions of incoming comets, but these suggestions have rarely been given much credibility when more detailed analyses are performed.

The idea of trying to detect Planet X by its gravitational effects was re-picked up soon after the discovery of the Kuiper belt. Early on, astronomers proposed a Mars-like planet at ~60 AU to explain an apparently abrupt outer edge to the Kuiper belt, but then quickly ruled it out. But the idea kept floating around, and many papers suggested some sort of variant.

Things began to get interesting with the discovery of Sedna, which was the first object discovered in the solar system whose orbit could only be explained by interaction with something besides the known planets (I have a whole series on this over on my blog. Also, now I know how to end the series). In the discovery paper, I suggested, among other possibilities, a new planet. But a ~Earth sized one around 70 AU. Really, though, for the past 12 years I have been convinced that Sedna's strange orbit is a left over relic from the formation of the sun in a cluster of stars 4.5 billion years ago. Others suggested a ~Mars mass planet a little further out as the culprit, or even a planet that was once in the outer solar system, but is now gone. Planets are popular things to propose when you see some gravitational effect that you cannot otherwise explain.

After Sedna, the most interesting developments in the Planet-X-ology came from two papers over the past few years. First, Chad Trujillo and Scott Sheppard noted some unusual orbital alignments among distant Kuiper belt objects (arguments of perihelion appeared clustered around zero, to be specific). They proposed a ~2 earth mass planet at a few hundred AU organizing these objects through a mechanism called a Kozai resonance, though they pointed out in their own paper that they couldn't make the idea work out. And detailed simulations showed that their proposed planet did not have the hoped for effect and thus could not be real. Once again. Though this time, critically, the data looked good, just not the explanation.


A more recent study also had important clues about Planet Nine. In September, Rodney Gomes and colleagues proposed that an entirely separate set of objects were being influenced by some sort of distant planet. While they were not able to use the observations to say much about the planet, they concluded that it was likely that these objects were being pushed around by something large and far away.

So by this last September, then, if you were paying extremely close attention, you might have been clued in that something was going on in the outer solar system and that a large planet might well be a good explanation, but no one could figure out exactly what was going on. Was there some way to connect Sedna, the strange alignments of Trujillo and Sheppard, and the distant objects of Gomes et al?


This moment reminds me of the time right after the discovery of Uranus. In 1783, Anders Johan Lexell published the first calculation of the orbit of Uranus (proving that it was on an elliptical, rather than parabolic orbit, and thus not a comet). Lexell found that, while the orbit of Uranus was elliptical, it also appears to be being perturbed by an external body, presumably a planet. More detailed calculations of the orbit of Uranus in 1821 by Alexis Bouvard confirmed the orbital irregularities and reinforced the idea that there must be an eighth planet out there. These calculation are the ones that led Adams and le Verrier to separately try to see if they could figure out not just if there was a planet out there, but where there was a planet.

This point is where Konstantin and I started. We could tell that the data from Trujillo and Sheppard was right: something was happening in the outer solar system, but no one could quite figure out what (the paper by Gomes et al. hadn't been published yet, but would eventually be a critical part of the story). We quickly re-confirmed that the Kozai resonance hypothesis of Trujillo and Sheppard didn't work, so what was it? At this point you can read Konstantin's account of how we got to the answer. Are we right this time? After all of these years? We think so. The data are good, the understanding of how the gravitational forces push everything around is good. We think that, this time, we've actually identified something real out there. We're confident enough to stand up in public and say that we think we have found evidence for a new planet in the solar system.

And we think it is time to retire the long and winding history of Planet X, and start anew with Planet Nine.