We would certainly see something that is six times the mass of the earth sharing our orbit. The wiki estimates the missing planet to be 6.3 times as massive as

We would certainly see something that is six times the mass of the earth sharing our orbit. The wiki estimates the missing planet to be 6.3 times as massive as the Earth, could be up to 2.3 times more or −1.5 times less than this figure. "Based on earlier considerations, this hypothetical super-Earth-sized planet would have had a predicted mass of five to ten times that of the Earth, and an elongated orbit 400 to 800 times farther from the Sun than the Earth is." https://en.wikipedia.org/wiki/Planet_Nine https://en.wikipedia.org/wiki/Planet_Nine