We have a good idea of what's going on. There isn't a Mars-sized planet in the vicinity of Jupiter and Saturn. It is easy to see anything large and close to the sun. We can't rule out a smaller, more distant world like Planet 9 or Planet 10. A recent study found that the chances of such a planet existing are very low.

Astronomers have wondered about the existence of planets that may be hidden at the edge of our solar system when the power of our telescopes was not as powerful. The sky surveys found nothing beyond the asteroids. The worlds we found seemed to be clustered in a way that made them look like they were perturbed by a larger object. The planet would have a mass of about five Earths and a distance of a few hundred to a thousand units from the sun. It wouldn't be easy to see in a sky survey.

It isn't easy to find the world. Planet 9 is too far away to be seen by reflected light. It wouldn't give off much heat because it's only five Earths. Adding to this is the fact that within a single set of observations, you wouldn't notice the planet was moving. This is where the study begins.

The team used two telescopes to look for planets. There is plenty of time for a hypothetical planet to move to a different part of the sky. They assumed distant planets would be close to the plane and took note of possible planets.

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

They found a lot of candidates. The range for Planet 9 is expected to be less than 1,000AU and less than Neptune. You shouldn't get too giddy. The team looked at the signatures by hand and didn't find anything compelling. The majority of them were either within or close to a faint integrated flux nebula. They are clouds of gas that are invisible to the naked eye.

The candidates aren't planets but the echoes of a faint nebula. Which seems to rule out Planet 9. There were hopes that another planet would be found.

There is a reference to Chris and Stephen. ArXiv preprints arXiv: 2207.09985, about searching for giant planets in the outer Solar System.