"Can you explain the transit method that astronomers use to find exo-planets?"

Today, we pick a question out of Pandora's Jar.*  The query relates to how astronomers find exo-planets, those worlds in orbit around other stars.


We'll begin with defining "transit." A transit occurs when a planet moves directly in front of a star. We sometimes see transits of Mercury and Venus** from Earth. Astronomers can use transits of planets around other stars to detect them. As these stars are so far away, they can't actually observe the transit. Instead, astronomers can measure the slight brightness decrease which a star experiences when  a planet passes in front of it from our perspective.
Unlike the radial velocity ("wobble") method which can only detect planets sufficiently massive to induce detectable gravitational tugs, the transit method can be employed to discover smaller, Earth-sized planets. As it requires very sensitive equipment, the transit method has only been in use for slightly more than ten years. In 2002, astronomers found OGLE-TR-56b***, the first exo-planet discovered through use of the transit method. Since this first discovery, hundreds of exo-planets have been found through this technique, principally by the Kepler Probe, a spacecraft that has been in service since 2009. This craft is monitoring more than 145,000 stars within the Cygnus region for evidence of periodic dimming related to transiting planets.

As some planets require years to complete a single orbit, the transit method requires prolonged observation times to establish that the brightness diminishment is periodic. Brilliantly, some planets can be indirectly discovered through the transit method as astronomers can measure the slight perturbations that other planets exert on those whose transits they are observing.   

Through the transit method, terrestrial planets have been found within the habitable zones of their parent stars. These zones are neither excessively hot nor cold and could therefore allow for the formation and evolution of life forms. (Less than fifty such worlds have so far been identified.) While astronomers have not gathered conclusive evidence that life actually exists elsewhere in the Universe, the discovery that such planets are not uncommon lends hope, Pandora, to those of us who hope that life proliferates in the Universe.

I hope this response is helpful.  

*Pandora, the first human woman created by the Greek Gods, was the personification of curiosity and so we wanted to name the question vessel after her. She was said to have opened a jar, not a box, which released all of humanity's dread and despair. By the time she managed to close the jar, only hope remained enclosed within it.    

**Transits of Mercury and Venus are not common events. The last transit of Venus occurred in June 2012. The next Venus transit won't occur until December 2117. The last transit of Mercury was in November 2006. The next occurs in May 2016. Transits are visible from other planets, as well. For instance, someone on Mars would have seen a transit of Earth in 1984. The next transit of Earth from Mars happens in November 2084.   

***That strange name relates to the Optical Gravitational Lensing experiment. Though its focus is on the discovery of dark matter through gravitational lensing, it has also been utilized to discover exo-planets.