Why must an extrasolar planet’s orbit be aligned just right for a transit to be observed?

For an extrasolar planet to be observed via the transit method, its orbit must be oriented in space such that the planet periodically passes directly between its star and the observer's telescope.12

This specific alignment is necessary because a transit is defined as the passage of a smaller body across the disk of a larger body. If the orbital plane is not aligned with the line of sight from the telescope, the planet will pass above or below the star from the observer's perspective, failing to occult any portion of the star's disk. Consequently, the characteristic drop in the star's brightness—which astronomers use to detect the planet and determine its size and orbital period—will not occur. Because this geometric requirement is strict, the transit technique is inherently biased toward detecting planets that orbit very close to their host stars, as these are more likely to have orbital orientations that result in frequent transits.12