How did the Kepler satellite discover thousands of possible planets around distant stars?

The Kepler satellite discovered thousands of potential planets by monitoring the brightness of more than 100,000 stars for the subtle dimming caused by a transit. A transit occurs when a planet passes directly between its host star and the observer, momentarily blocking a tiny fraction of the star's light.12

To achieve the high precision required to detect these minute changes, the spacecraft utilized several specialized methods:

• Continuous Observation: By operating from a heliocentric orbit, Kepler avoided the atmospheric distortion and day-night cycles that hinder ground-based telescopes, allowing it to stare at the same patch of sky in the constellation Cygnus for years.1
• Defocused Imaging: The telescope was intentionally kept slightly out of focus so that each star covered multiple pixels on its charge-coupled devices (CCDs). This prevented pixel saturation and improved the accuracy of brightness measurements.1
• Light Curve Analysis: Scientists analyzed the resulting light curve—a graph of a star's brightness over time—to identify the periodic dips that signify the presence of an orbiting body.13

This technique is most effective when a planet's orbital plane is aligned with the line of sight to the telescope.12