What Kind of Planet Are We Talking About?
If Planet Nine exists, it would likely be a gas or ice giant, not a rocky world like Earth or Mars. Based on gravitational models, scientists estimate it could be between five and ten times the mass of Earth. That would make it similar in size to Uranus or Neptune. It would be too small to be a star but large enough to have a major influence on objects around it.
Its orbit would also be incredibly long and stretched out. While Earth takes one year to orbit the Sun, and Neptune takes 165 years, Planet Nine might take between 10,000 and 20,000 years to complete a single trip around the Sun. That would place it in the outermost reaches of our solar system, far beyond what we normally explore.
Because of its extreme distance, Planet Nine would be extremely cold and would reflect very little sunlight. It would likely be faint and hard to spot, even with powerful telescopes. Detecting it would require scanning the skies carefully and patiently, especially since it would be moving slowly against the background of stars.