- Norman Smith, Hinesburg, VT
First, a brief review. The Kepler Space Probe is currently observing about 100,000 stars within a small region* of our galaxy with the aim of detecting exo-planets, defined as planets around other stars. Kepler employs the "transit method" to discover these worlds. When a planet moves directly in front of its parent star from our perspective - called a 'transit' - the star's brightness diminishes slightly. By measuring the brightness reduction, the time period between successive transits and the duration of each transit, astronomers can ascertain not only the existence of exo-planets, but also their size, orbital period and orbital radius.
(For more information about these methods, refer to another question on the "Ask the Staff Astronomer" page: "How can we know what we know about exo-planets?")
Your question pertains to one of the weaknesses of this method: not every planet will appear to transit its parent star from our perspective. Astronomers have calculated that the probability of finding a given planet by the transit technique is about 1-2%. This probability is something of an approximation for all stars. The chance of observing a transiting planet increases with the size of the planet and the star, but decreases with the size of its orbit.
Regard, for instance, the Earth-Sun system. Let's pretend we made an exact replica of this Sun-planet system and tossed it out about 100 light years away from us in such a way so the planet's orbit could assume any alignment. The probability that this replica Earth would be aligned edge on to us is only about 0.05. Let's further pretend that we made thousands of replicas and scattered them through our part of the galaxy. We should expect that, on average, only about one out of 200 will be aligned so that we could observe the planet transiting its star.**
Remember that though the galaxy resembles a disc, it is thousands of light years "thick," so the star systems within it are not clustered around the same plane as are most planets in our solar system. Our own solar system is inclined by about 62 degrees relative to the "Galactic Equator."
The Kepler mission has discovered more than 1,000 confirmed planets and will likely find thousands more. However, as mission scientists, themselves, would concede, they are detecting only a small percentage of all the planets lurking within Kepler's view.
I hope this answer proves helpful!
*To give you a scale model, if the Solar System were the size of a quarter, the Milky Way Galaxy would be as large as North America. The area the Kepler Probe is observing would be the size of Connecticut.
**Important note about probabilities: we could not guarantee that we would observe a transit in one out of 200 systems. That is the tricky issue with probabilities. It is possible, through not probable, that hundreds of planets have the proper alignments. It is also possible, through not probable, that no planet in 10,000 has the proper alignment.