There was a time when our solar system had nine planets. Then, in 2006, astronomers decided Pluto didn’t count anymore. There might still be nine planets, though. Astronomers Mike Brown and Konstantin Batygin have proposed an undiscovered ninth planet in the outer solar system. Efforts to find the alleged planet are still in the early phases, but Brown and Batygin just finished a major survey with the Zwicky Transient Facility (ZTF) telescope. They didn’t find Planet 9, but now we know where not to look, says Bad Astronomy.
Most of the planets in our solar system are visible with the naked eye at least occasionally, but Planet 9 is theorized to orbit far beyond Neptune. If it exists, it’s so far away that we can’t get a good look at it even with huge telescopes. In the void of interplanetary space, its only company would be small Trans-Neptunian Objects (TNOs). The clustering of TNO orbits is what initially drew Brown and Batygin to the possibility of Planet 9. Everything else, though, is a guess.
Brown and Batygin used a number of educated guesses to develop software to simulate Planet 9 for various values of size, reflectivity, and orbital shape. The ZTF isn’t a particularly big telescope at just 1.2 meters, but it can view a huge 47×47-degree patch of the sky in each image. It looks for objects that move or change brightness, making it perfect to look for some versions of Planet 9.
Because we’re still uncertain where Planet 9 is (if it is at all), Brown and Batygin had to start someplace. There’s a possibility Planet 9 is smaller and closer to us, and simulations showed that it would appear in ZTF images about half the time if that were true. The pair used the software simulation to create a database of positions and brightnesses for Planet 9 and scanned the ZTF database for a match, going back to when it began operations in 2017, but they didn’t find one.
This doesn’t mean that Planet 9 is a fantasy — it just means it most likely won’t be found in a closer orbit. That means we’re looking for something larger than Earth orbiting billions of kilometers away. There is hope that the upcoming Vera Rubin Observatory with its 8.4-meter mirror will be able to pin down the location. Either way, at least we know where Planet 9 isn’t, which is a step in the right direction.