Planet 9

Planet 9, or Planet X is a hypothetical planet orbiting beyond Neptune. This theory would explain the anomalies in the Kuiper Belt. Planet 9 could have a weight (mass) about 5 to 10 times the weight of Earth. Since this planet should orbit far away, it would take between 10000 to 20000 Earth years to orbit the Sun. The brightness (Magnitude) of Planet 9 could be around 21, which means that it is very dim.

Reference

https://science.nasa.gov/solar-system/planet-x/

https://en.wikipedia.org/wiki/Planet_Nine

Other websites

https://edition.cnn.com/2024/11/04/science/planet-nine-kuiper-belt-latest-evidence/index.html