To date, over 5,500 exoplanets — planets that orbit stars other than the sun — have been discovered, and a large portion of those alien worlds have been observed via something called "the transit method."
It refers to when an exoplanet passes, or transits, in between its home star and Earth's line of sight to that star. For us back here on Earth, when this happens, the star's brightness momentarily dips, indicating to us that something is obstructing our view — and that "something" is almost always a planet.
