A New Planet in Our Solar System?

Astronomy being a long-time interest of mine (as should be obvious by the number of related posts on this blog), I was naturally intrigued by the flurry of news reports on an announcement by astronomers Konstantin Batygin and Michael Brown that their calculations show a large planet (which they rather provocatively are calling "Planet Nine") exists in the outer Solar System, far beyond Pluto and Eris. The proposed planet is believed to orbit between about 200 AU (300 billion km) at perihelion and as much as 1200 AU (1.2 trillion km) at aphelion, with an orbital period of between 10,000 and 20,000 Earth years, a period comparable or even more than that of Sedna, which at around 11,400 Earth years has the longest orbital period of any known large object (for comparison, Pluto takes 248 Earth years to orbit the Sun and Eris takes 558 Earth years). Batygin and Brown estimate that the planet should be about ten times as massive as Earth, putting it between Earth and Neptune (17 Earth masses) in size, making it a super-Earth or more properly a mini-Neptune, a category of planet that has been found in many star systems but is (so far) unknown in our own Solar System.

Some of the initial headlines about this hypothetical planet were a bit exaggerated, to say the least. One even stated that a new planet had been “discovered” even though the article to which headline was attached made it fairly clear that it had only been predicted and is not known for certain to exist. As another article pointed out, despite the famous and impressive prediction and discovery of Neptune, no other prediction of a planet in our Solar System has turned out to be accurate, as even Pluto’s discovery by Clyde Tombaugh while searching for a planet predicted by Percival Lowell turned out to be just a fortunate coincidence. Neptune was discovered due to residuals in the orbit of Uranus, that is to say, Uranus was not moving as it should have been based on the gravitational effects of the Sun and the known planets, meaning that there was something else affecting it. As impressive as the mathematical prediction leading to Neptune’s discovery was, the residuals in that case were fairly substantial, allowing a precise prediction of the unknown planet’s size and position. Attempts to find other planets by similar methods have been based on much slimmer evidence that in many cases was later to shown to be wrong anyway (for example, the problems with Mercury’s orbit which led to predictions of a planet closer to the Sun disappeared when Einsteinian physics was applied, and the apparent remaining problems with Uranus’s orbit that led to predictions of an additional planet beyond Neptune disappeared after estimates of Neptune’s mass were corrected following Voyager 2’s encounter with the planet). In other cases, even at the time the supposed evidence upon which the claims were based was dubious, such as in the case of Nemesis, the claim that the Sun has a distant red dwarf or brown dwarf companion that is causing periodic extinction events on Earth by sending showers of comets toward the Earth from the Oort cloud in the far outer reaches of the Solar System. Many experts doubt the claim that extinctions occur on a regular, periodic basis, and some astronomers have pointed out that an object like the predicted one would have measurable effects on objects closer to the Sun, effects that are clear not happening. Finally, infrared surveys of the Sun’s neighborhood seem to have pretty much ruled out anything of Jupiter-size or greater to a distance far beyond that predicted.

So historically speaking the record for planet predictions, despite the one spectacular success in the case of Neptune, is pretty poor. However, in this case Batygin and Brown do seem to have a slightly stronger case than that made for most previous hypothetical planets. Their prediction is based on the odd grouping of the orbits of half a dozen extremely distant objects, including Sedna. They calculate that it is extremely unlikely that this correlation in orbits would occur by chance. One might argue that we simply haven’t found enough of the objects that may exist in that region of the Solar System to be sure that the correlation holds true for all or even most of them, but then the odds against our first discoveries out of a large population just coincidentally having similar orbits also has to be very small. Another factor perhaps just as strongly in favor is the fact that Batygin and Brown did not start out by looking for evidence of a planet; instead they were originally hoping to disprove a suggestion made by Scott Sheppard and Chad Trujillo that a large, distant planet might be responsible for the orbits of Sedna and similar objects. Instead, their simulations showed the opposite of what they expected. Why do their original intentions matter? Because even scientists can fall prey to confirmation bias. If a scientist starts out hoping to find a planet, they may tend to exaggerate the reliability of the evidence they are using. Since Batygin and Brown, at least by their own account, started out hoping to disprove the idea that there might be a planet, the evidence that changed their minds would have to be at least somewhat more convincing than for someone who was hoping to prove there was a planet.

If this planet exists, it is probably near to aphelion, the part of its orbit farthest from the Sun, which would explain why it hasn’t been found yet. If it is of the speculated size, it could possibly be seen in a good amateur telescope near perihelion (when it is closest to the Sun). A planet that bright would have been found long ago, perhaps by Clyde Tombaugh, who covered much of the sky after finding Pluto. But if this planet is at aphelion, its last perihelion was at least 5,000 years ago, long before telescopes were invented. At its probable current distance, it would be much harder to see and it may be lost among the stars of the Milky Way. Nevertheless, a determined search will probably find it – if it is there.

Of course, even if the planet exists and is actually spotted, it won’t resolve the controversy over how many planets there are in the Solar System. Brown takes what I must say (despite my great respect for his work in planetary astronomy) is somewhat juvenile pride in being the “Pluto killer”, since it was his discovery of Eris that led to the “demotion” of Pluto from full planetary status. But as I have discussed in the past, a good case can be made for a definition of “planet” that includes Pluto, Charon, Eris and a few other of the larger trans-Neptunian objects, or alternatively for saying that while Pluto et al may be called “dwarf planets”, that is still a kind of planet. As there are still many, including at least some astronomers, who still consider Pluto (and presumably the similarly sized Eris) a planet, the term “Planet Nine” given by Batygin and Brown to their hypothetical planet is, as I noted at the beginning, somewhat provocative. Nevertheless, if the planet really turns up, it will be a fascinating addition to our Solar System, even though it may be many decades before we are able to explore it more closely, considering how incredibly far away it is, even in comparison with Pluto.