There's an awful lot of stars, it would be very unlikely that we look at all of them from the top down where we could not detect a planet passing in front of th

There's an awful lot of stars, it would be very unlikely that we look at all of them from the top down where we could not detect a planet passing in front of the star. Kepler glares at single piece of our galaxy and monitors about 150,000 stars simultaneously. Kepler is nice for finding large planets that orbit their stars in days. If you wanted to find planets like Pluto using the transit method, you'd have to look at its star for about 750 years (generally one wants to observe at least 3 transits) - albeit Pluto being even smaller than our own Moon, I'd guess it could not be detected.