The possibility of an incredibly distant Planet Nine has aroused great debate among astronomers. If it exists at all, its faintness will make it a challenge to find. Nevertheless, astronomers at Yale University have presented a method that may yield results and have started searching in the most likely places.

The evidence for Planet Nine is based on patterns in the orbits of Kuiper Belt objects. The theory is that something with a mass of 5 to 10 times that of the Earth is acting as a gravitational sheepdog. Deeper analysis and the discovery of more comets will no doubt strengthen or weaken Planet Nine's case, but to truly settle the question we need to actually see something moving in the darkness.

Calculations pointing to the planet's existence suggest its orbit is 14 to 27 times as far from the Sun's as Neptune's and it might be lurking in a broad region of the sky. “This is a region of space that is almost entirely unexplored,” said Professor Gregory Laughlin in a statement. 

Laughlin and PhD student Malena Rice will look for faint objects appearing to move very slowly through this part of the sky using a method called “shifting and stacking”. They will collect images from the Transiting Exoplanet Survey Satellite (TESS) and align them along possible orbital paths, shifting from one patch of sky to the next. TESS captures the parts of the sky it studies frequently, allowing Laughlin and Rice to combine hundreds of images, which they refer to as stacking, bringing exceptionally faint objects to the point of detectability.

Other astronomers have used the same idea to discover moons, but this required searching small patches of the sky around Uranus and Neptune, whose positions are well known. Looking for a planet whose location has only been vaguely theorized is a very different prospect. To validate their method, the pair successfully used TESS images to search for signs of Sedna and two other known trans-Neptunian objects.

Laughlin and Rice also tried looking at two sky patches where Planet Nine might reside. At the annual meeting of the American Astronomical Society’s Division for Planetary Sciences, the pair report they have found 17 potential objects. Some of these may turn out to be illusions, and anything real will most likely be an ordinary Kuiper Belt object, but further analysis may reveal one is something more.

Even if Planet Nine isn't there, clues to its location might be. “If even one of these candidate objects is real, it would help us to understand the dynamics of the outer solar system and the likely properties of Planet Nine,” Rice said. “It’s compelling new information.”

If Planet Nine is not in the area searched, Laughlin and Rice plan to expand to larger areas of the sky. A paper reporting the work has been accepted for the Planetary Science Journal (preprint on arXiv). 

At 20 times Neptune's distance from the Sun and probably considerably smaller in surface area, Planet Nine, if it exists, is likely to be around a million times fainter than the current outermost planet. We have spotted many objects much smaller than Planet Nine well beyond Neptune, even temporarily raising one to planetary status. However, these have all been much, much closer when we found them, despite how distant the outer part of their orbits may be.

Rice calls herself “agnostic” on whether Planet Nine exists at all, but added: “It would be beautiful if it’s out there.” Particularly, one imagines, if you're the one to find it.