How many earth years does this planet take to orbit the Sun???
If its closest approach to the sun is 300 astronomical units, that is 10 times the average distance from the sun to Neptune. It takes Neptune 164.79 earth years to orbit the sun once (it was discovered in 1846 so it finished one lap around the sun in 2011). But Planet Nine’s apparent average distance would be 400 AUs from the sun. Someone who is good at math could figure the time it would take Planet Nine to orbit the sun once using Kepler’s laws.
In the article “...Its huge orbit would mean it takes between 10,000 and 20,000 years to make a single pass around the sun...”