Planet 9 is running out of places to hide

We have a pretty good idea of ​​what’s lurking in our solar system. We know that there is no Mars-sized planet orbiting between Jupiter and Saturn, nor is there a brown dwarf nemesis heading our way. Anything large and fairly close to the sun would be easily spotted. But we can’t rule out a smaller, more distant world, like the hypothetical Planet 9 (or Planet 10, if you want to throw about Pluto). The probability that such a planet exists is quite high, and a recent study finds it even less likely.

Many astronomers have wondered about the existence of planets that might be hiding on the fringes of our solar system, especially when the power of our telescopes has been quite limited. But when major sky surveys began scanning the sky, they found nothing but asteroid-sized worlds. But the orbits of the worlds we found seemed clustered in a statistically odd way, as if they were being gravitationally disturbed by a larger object. If that were the case, this planet 9 would have a mass of about five Earths and an orbit of a few hundred to thousands of astronomical units. In other words, just small enough and far enough away that it wouldn’t be easy to see in sky surveys.

Of course, that motivated people to look for the world, but it’s not easy. Planet 9 would be too far away to be seen by reflected light, so you’d have to look for it using its faint infrared light. And with a mass of only five Earths, it wouldn’t give off much heat. Add to that the fact that a planet that far away would be orbiting very slowly, so with a single observation group you wouldn’t even notice it was moving. This is where this new study comes in.

To search for distant planets, the team used two infrared sky surveys, one from the InfraRed Astronomical Satellite (IRAS) and one from the AKARI Space Telescope. The two surveys were taken more than twenty years apart, giving each hypothetical planet ample time to move to a slightly different part of the sky. They assumed that all of the distant planets would be fairly close to the equatorial plane, and then combed through the data to note possible planets.

Surprisingly, they found more than 500 candidates. Based on the energy distribution of their spectra, most of these candidates had orbital distances less than 1,000 AU and masses less than Neptune, which is exactly the range expected for Planet 9. But you shouldn’t get too excited. When the team looked at the infrared signatures by hand, they found that none of them were that convincing. Most of them were either within or near a faint integrated river nebula, also known as the galactic cirrus. They are diffuse clouds of interstellar gas that are not easily seen at visible wavelengths but emit infrared light.

So it turns out these candidates are not planets, but rather the echoes of a faint nebula. Which pretty much rules out Planet 9. Lost hopes of another planet in the clouds.