A Giant Planet Seems to Be Lurking Unseen in Our Solar System

"Astronomers had found all eight of the main planets by 1846. But that doesn't stop us from looking for more. In the past 100 years, we've found smaller distant bodies we call dwarf planets, which is what we now classify Pluto as.

The discovery of some of these dwarf planets has given us reason to believe something else might be lurking in the outskirts of the Solar System.
Could there be a ninth planet?

There's a good reason astronomers spend many hundreds of hours trying to locate a ninth planet, aka " Planet Nine" or "Planet X". And that's because the Solar System as we know it doesn't really make sense without it."

"Scientists all over the world have been on the hunt for visible evidence of Planet Nine for many years now.

Based on the computer models, we think Planet Nine is at least 20 times farther away from the Sun than Neptune. We try to detect it by looking for sunlight it can reflect – just like how the Moon shines from reflected sunlight at night.

However, because Planet Nine sits so far away from the Sun, we expect it to be very faint and difficult to spot for even the best telescopes on Earth. Also, we can't just look for it at any time of the year.

We only have small windows of nights where the conditions must be just right. Specifically, we have to wait for a night with no Moon, and on which the location we're observing from is facing the right part of the sky."
https://www.sciencealert.com/a-giant-planet-seems-to-be-lurking-unseen-in-our-solar-system