Will all the stars Kepler observes have transiting planets?
Transits are only seen when a star’s planetary system is nearly perfectly aligned with our line of sight. There is no preferred alignment of the plane of planetary systems. The orbits of the planets can be at any angle to our line of sight. Therefore, the Kepler Mission is designed to look at more than 100,000 stars to find the small percentage that will actually show transits. For a Jupiter-size planet orbiting close to its star, the chance of a transit is about 1 in 10 (10%). For an Earth-size planet in an Earth-size orbit, the chance of it being aligned to produce a transit is less than 1%. For Kepler to detect a transiting planet, its orbit plane must be lined up with our line of sight. Most of the time, the extrasolar planets’ orbital planes do not line up. For Earth-like planets around Sun-like stars, the chances of randomly oriented orbital planes being in the correct orientation for Kepler to see a transit is about 0.5%. That is why the design of Kepler called for a very wide f
