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This may seem like a duplicate of A few questions regarding the transit of planets and Orbital plane evolution and planet discovery via transit method, but...

How is it that we've been able to discover so many exoplanets via the transit method when it's so rare to see transits of Venus and Mercury across our own Sun? The orbits of all the major planets of our solar system are very nearly co-planar, and yet most Sun-Mercury-Earth or Sun-Venus-Earth alignments don't result in a transit visible from Earth. For any given star/exoplanet, there is a limited region of space from which a transit could be observed. How is it that we happen to fall into such a region for so many exoplanets, thus allowing for their discovery?

If orbital orientation is purely random, we could expect only a tiny fraction of exoplanets to be discoverable via the transit method. If most planetary systems have orbital axes more-or-less parallel with each other (and presumably with the galaxy itself), then we could only find exoplanets via transit by looking out more-or-less along the plane of our own orbit, and again, not necessarily finding a large fraction of the systems that actually exist (all those stars above or below the ecliptic).

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  • $\begingroup$ added specific answer under first possible duplicate question. It is basically that Kepler searched so many stars 500,000 as compared with other methods radial velocity about 5,000. Such large numbers compensate for the small probability of having a transit if there is a planet. $\endgroup$ – TazAstroSpacial Nov 6 '19 at 4:50