Many astronomers have questioned the existence of planets that might be hiding at the edge of our solar system, especially when the power of our telescopes has been somewhat limited. But when the Great Sky Surveys began scanning the sky, they found nothing but asteroid-sized worlds. But the orbits of the worlds we found seemed to be clustered in a statistically strange way, as if they were gravitationally perturbed by a larger object. If so, this “Planet 9” would have a mass of about five terrestrial planets, and an orbital distance of a few hundred to a thousand astronomical units. In other words, it is small enough and far enough that it is not easily seen in sky surveys.
Naturally, this motivated people to search for the world, but this is not easy. Planet 9 will be too far away to be seen with reflected light, so you’ll have to look for it by its faint infrared glow. And with its mass only five that of Earth, it wouldn’t emit much heat. Added to that is the fact that such a distant planet would rotate so slowly that within one set of observations you wouldn’t even notice it was moving at all. This is where this new study comes in.
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To search for distant planets, the team used two infrared sky surveys, one from the Infrared Astronomy Satellite (IRAS) and one from the Akari Space Telescope. The two surveys were taken over twenty years ago, giving any hypothetical planet plenty of time to move to a slightly different part of the sky. They assumed that any distant planets would be fairly close to the equatorial plane, and then combed the data while noting possible planets.
Surprisingly, they found more than 500 candidates. Based on the energy distribution of their spectra, most of these candidates had orbital distances of less than 1,000 AU, and masses less than Neptune, which is exactly the expected range for Planet 9. But you don’t have to be too excited. When the team looked at the infrared signatures manually, they found none of them convincing. Most of them tended to be either within or near a faint integrated flow nebula, also known as the cirrus galaxy. They are diffuse clouds of interstellar gas that are not easily seen at visible wavelengths, but emit infrared light.
So it turns out that these candidates are not planets, but echoes of a faint nebula. Which largely rules out Planet 9. Hopes for another planet are lost in the clouds.
Reference: Sedgwick, Chris and Stephen Sergent. “Searching for Giant Planets in the Outer Solar System Using Far Infrared Surveys of All Sky.” arXiv preprint arXiv: 2207.09985 (2022).