A giant planet may slide through the outer solar system. It may give other objects around it a gentle nudge, causing them to follow similar orbits around the Sun. Despite several years of searching, though, no one has seen it. So many astronomers consider it a phantom.

Several astronomers have proposed the existence of such a world, known as Planet Nine or Planet X.

The most recent proposal came about five years ago. Two astronomers looked at the orbits of many big chunks of ice and rock beyond Neptune, the Sun’s outermost major planet. And they found that several of the objects follow similar paths. The orbits are tilted at about the same angle, and the objects come closest to the Sun in the same region.

Such a grouping didn’t seem random. So the astronomers suggested that a heavier object was acting as a “shepherd,” pushing the smaller objects into similar orbits. They calculated that it was a planet perhaps five to ten times as massive as Earth. It would be hundreds of times farther from the Sun than Earth is — more remote than anything yet seen in the solar system.

Searches have covered a good bit of the area where Planet Nine might be found. So far, they’ve come up empty. But the planet would be quite faint, so it could take some time to find it.

On the other hand, some astronomers are saying that there are other ways to explain the odd orbits besides a planet — suggesting the search for Planet Nine may come up empty.

Script by Damond Benningfield