Many astronomers have wondered about the existence of planets that might be lurking on the edge of our solar system, especially when the power of our telescopes was quite limited. But as the big sky surveys began to sweep the skies, they found nothing but asteroid-sized worlds. But the orbits of the worlds we found seemed to cluster together in a statistically odd way, as if they were being gravitationally perturbed by a larger object. If this were the case, this “Planet 9” would have a mass of about five Earths and an orbital distance of a few hundred to a thousand AU. In other words, small enough and distant enough not to be easily seen in sky surveys. Of course, this motivated people to search for 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 won’t give off much heat. Added to this is the fact that such a distant planet would orbit very slowly, so that within a single set of observations you would not notice it moving at all. This is where this new study comes in. Remove all ads on Universe Today Join our Patreon for just $3! Get the ad-free experience of a lifetime 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 more than twenty years apart, giving each hypothetical planet plenty of time to move to a slightly different part of the sky. They assumed that any distant planets would be close enough to the equatorial plane and then combed the data to account for possible planets. A faint embedded 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 less than 1,000 AU and masses less than Neptune, which is exactly the expected range for Planet 9. But don’t get too excited. When the team examined the infrared signatures by hand, they found that none of them were all that exciting. Most of these tended to be located either in or near a faint embedded stream nebula, also known as a galactic cube. 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 rather the echoes of a faint nebula. Which pretty much rules out Planet 9. Another planet’s hopes lost in the clouds. Citation: Sedgwick, Chris and Stephen Serjeant. “Searching for giant planets in the outer Solar System with far-infrared sky surveys”. arXiv preprint arXiv:2207.09985 (2022).
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