We have a pretty good idea of what lies within our solar system. We know that there is no Mars-sized planet orbiting between Jupiter and Saturn, and no brown dwarf enemy moving toward us. Anything large and fairly close to the Sun will 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 above Pluto). The odds against the existence of such a planet are quite high, and a recent study finds it even less likely.
Many astronomers have wondered about the existence of planets that might be lurking within the boundaries of our solar system, especially when the power of our telescopes was quite limited. But when large sky surveys began scanning the heavens, they found nothing more than asteroid-sized worlds. But the orbits of the worlds we found seemed to be clustered in a statistically odd way, as if they were being gravitationally perturbed by a larger object. If that were the case, this “Planet 9” would have a mass of about five Earths and an orbital distance of several hundred to thousands of AU. In other words, small enough and far enough away not to be easily seen in sky surveys.
Naturally, this motivates people to search the world, but it is not easy. Planet 9 would be too far away to be seen by reflected light, so you’ll have to look for it by its faint infrared glow. And with a mass of only five Earths, it wouldn’t give off much heat. Adding to this is the fact that a planet this far away would be orbiting very slowly, so within a set of observations you wouldn’t notice it moving at all. That’s 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 done more than twenty years apart, giving each hypothetical planet enough time to move to a slightly different part of the sky. They assumed that all the distant planets would be relatively close to the equatorial plane, then sifted through the data, taking potential planets into account.
A weak integrated stream nebula near Polaris. Credit: Kush Chandaria, CC BY-SA 4.0
Surprisingly, they found more than 500 candidates. Based on the energy distribution of their spectra, most of these candidates had orbital distances below 1000 AU and masses less than Neptune, which is exactly the range expected for Planet 9. But don’t get too excited. When the team looked at the infrared signatures by hand, they found that none of them were all that compelling. Most of these tended to be either within or near a faint integrated stream nebula, also known as a galactic plume. They are diffuse clouds of interstellar gas that are not easily seen at visible wavelengths, but rather emit infrared light.
So it turns out that these candidates are not planets, but rather echoes from a faint nebula. Which pretty much rules out Planet 9. Hopes for another planet lost in the clouds.
Reference: Sedgwick, Chris and Stephen Serjeant. “Searching for giant planets in the outer Solar System with far-infrared all-sky surveys.” arXiv preprint arXiv:2207.09985 (2022).
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