I don’t get how it can detect 5,000 planets and we can’t confirm one the size of earth is in our solar system.
It uses gravitational microlensing to find the distant planets. If it was in the right place and knew where to look, the planet you’re talking about could be seen.
Because it detects planets in distant star systems as the planet passes in front of the star. If you are referring to so-called planet 8 then that is another kettle of fish altogether. If it is exists at all they can’t use those same detection mechanisms.