For centuries, planets were known only in our own solar system. Then came 1992, and the first confirmed exoplanet—a planet orbiting another star—was discovered. Since then, our catalog of alien worlds has exploded, with more than 5,000 confirmed planets and thousands more candidates. And one method stands above the rest for sheer volume and reliability: the transit method.
The transit method detects a planet by watching for periodic dips in the brightness of a star. When a planet crosses—or “transits”—in front of its host star from our point of view, it blocks a small fraction of the star’s light. Sensitive space telescopes like Kepler, TESS, and soon the European Space Agency’s PLATO monitor these dips, looking for patterns that suggest orbiting bodies.
From these light curves, astronomers can calculate the planet’s size, orbital period, and—if paired with radial velocity data—even its density. The method works best when a planet’s orbit is edge-on from Earth’s perspective, which limits the number of planets we can detect this way. But with enough stars being monitored, the math still favors massive discovery potential.
Transit detections have revealed everything from super-Earths to hot Jupiters, mini-Neptunes, and possible ocean worlds. The famed TRAPPIST-1 system, which hosts seven Earth-sized planets, was discovered via transits. So was Kepler-452b, the first confirmed Earth-sized exoplanet in the habitable zone of a Sun-like star.
There are limitations. False positives can occur due to binary stars, stellar variability, or instrumental error. That’s why transit discoveries are often confirmed with spectroscopic data. But the method’s power remains unmatched for statistical surveys: Kepler’s four-year mission alone suggested that planets outnumber stars in our galaxy.
The transit method has also opened the door to atmospheric science. During transits, starlight filters through a planet’s atmosphere—if it has one—allowing us to detect molecules like water vapor, methane, or even biosignatures. This technique, known as transmission spectroscopy, is being honed by the James Webb Space Telescope to analyze the atmospheres of temperate, rocky exoplanets.
As next-generation missions like PLATO and the Roman Space Telescope launch in the coming years, the transit method will continue to lead the charge in finding new worlds—and perhaps, one day, new life.
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