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 a brown dwarf enemy heading our way. Anything large and close enough 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 pounce on Pluto). The odds of such a planet existing are quite high, and a recent study finds it even less likely.

Many astronomers have wondered about the existence of planets that could hide at the edge of our solar system, especially when the power of our telescopes was quite limited. But as large sky surveys began to scan the skies, they found nothing beyond asteroid-sized worlds. But the orbits of the worlds we found seemed to be clustered together in a statistically odd way, as if they were gravitationally perturbed by a larger object. If so, this “Planet 9” would have a mass of about five Earths and an orbital distance of a few hundred to a thousand astronomical units. In other words, just small enough and far enough away that it’s not easily visible in sky charts.

Naturally, this motivated 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 would have to search for it by its faint infrared glow. And with a mass of only five Earths, it wouldn’t give off much heat. Added to this is the fact that such a distant planet would orbit very slowly, so that in a single 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 surveys of the sky, one from the Infrared Astronomical Satellite (IRAS) and the other from the AKARI Space Telescope. The two readings were taken more than twenty years apart, giving any hypothetical planet plenty of time to move to a slightly different part of the sky. They assumed that all the distant planets would be fairly close to the equatorial plane, then combed through the data, taking note of potential planets.

A faint integrated flux nebula near Polaris. Credit: Kush Chandaria, CC BY-SA 4.0

Surprisingly, they found over 500 candidates. Based on the energy distribution of their spectra, most of these candidates had orbital distances below 1,000 AU and masses below Neptune, which is exactly within the expected range for Planet 9. But you shouldn’t be too excited. When the team examined the infrared signatures by hand, they found none of them so convincing. Most of them tended to be inside or near a faint embedded flux nebula, also known as a galactic cirrus. They are diffuse clouds of interstellar gas that are not readily visible at visible wavelengths, but instead 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. The hopes of another planet lost in the clouds.

Reference: Sedgwick, Chris and Stephen Sergeant. “Searching for giant planets in the outer solar system with far-infrared full-sky surveys.” arXiv preprint arXiv:2207.09985 (2022).

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