
The transit method detects exoplanets by observing periodic dips in a star's brightness as a planet passes in front.Imagine looking at a light bulb, and a moth flies in front of it. The bulb dims slightly for a moment. That's exactly how scientists find planets around distant stars: they watch for tiny 'eclipses' when a planet passes across its star's disk.
The first successful detection of an exoplanet transit occurred in 1999 for the star HD 209458. The theoretical possibility of the method had been discussed since the mid-20th century, but technical difficulty required space-based surveys. The breakthrough was achieved by William Borucki, who since the 1980s had been developing the concept of the Kepler mission. Launched in 2009, Kepler observed about 150,000 stars simultaneously and discovered thousands of candidates, proving that planets are common in the Galaxy. In 2018, TESS followed, continuing transit surveys of nearby stars.
When a planet crosses a star's disk, the total radiation flux decreases by an amount proportional to the planet's disk area: ΔF/F = (R_p/R_s)². The transit light curve has a characteristic shape with gradual ingress/egress and a flat bottom (if the star is uniform). The period between transits gives the orbital period, and the transit duration gives the impact parameter and inclination. The secondary transit (when the planet passes behind the star) allows measuring the planet's own emission. Transit spectroscopy reveals absorption lines in the atmosphere — for example, water vapor, methane, carbon dioxide.
If aliens looked at the Sun using the transit method, they would notice Earth blocking just 0.008% of its light! Such precision is only possible with space telescopes.
| δ | variable | transit depth — transit depth |
| R_p | variable | planet radius — radius metre |
| R_* | variable | stellar radius — radius metre |





























