we have a pretty good idea of what lurks inside our Solar System. We know there is no Mars-sized planet orbiting between Jupiter and Saturn and no brown dwarf nemesis heading our way. Anything large and fairly close to the Sun would be easily detected. But we can’t rule out a smaller, more distant world, like the hypothetical Planet 9 (or Planet 10 if you want to launch it on Pluto). The odds that such a planet exists are pretty high, and a recent study finds it even less likely.
Many astronomers had wondered about the existence of planets that might lurk at the edge of our Solar System, particularly when the power of our telescopes was quite limited. But when the big sky surveys began scanning the skies, they found nothing beyond asteroid-sized worlds. But the orbits of the worlds we found seemed to be bunched together in a strange statistical 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 a few hundred to a thousand astronomical units. In other words, small and distant enough that it is not easily seen in sky surveys.
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’d 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. Add to that the fact that such a distant planet would orbit very slowly, so that within a single set of observations, you wouldn’t notice it moving at all. 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 20 years apart, 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, then went through the data, noting potential 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 1000 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 tended to be within or near a faint integrated flow nebula, also known as a galactic cirrus. They are diffuse clouds of interstellar gas that are not easily seen 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. Hopes for another planet lost in the clouds.
This article was originally published on universe today by Brian Koberlin. Read the original article here.