How Are Exoplanets Discovered?

The transit method detects exoplanets by measuring the tiny dip in a star's brightness when a planet passes in front of it. This is the most productive detection technique, responsible for the majority of known exoplanets.

Space telescopes like Kepler (2009-2018) and TESS (2018-present) continuously monitor thousands of stars, looking for the characteristic periodic dimming caused by orbiting planets. The depth of the brightness dip reveals the planet's radius relative to the star, while the interval between transits gives the orbital period. Combined with radial velocity measurements, this allows astronomers to determine a planet's density and infer its composition. The transit method works best for planets with orbits aligned edge-on to our line of sight,statistically, only a small fraction of planets transit from our perspective.

The radial velocity method detects planets by measuring the subtle back-and-forth wobble of a star caused by an orbiting planet's gravitational pull. It was the first method to successfully find planets around Sun-like stars.

As a planet orbits, it exerts a gravitational tug on its star, causing the star to move in a small orbit of its own. This motion shifts the star's light spectrum, slightly blueward when moving toward us, and redward when moving away. Modern spectrographs can detect stellar velocities as small as 1 meter per second. The radial velocity method is especially sensitive to massive planets in close orbits and provides the planet's minimum mass. Combined with transit data, it gives the true mass and thus the planet's density.

Direct imaging captures actual photographs of exoplanets by blocking the overwhelming glare of their host star using specialized instruments called coronagraphs.

A star can be billions of times brighter than its planets, making direct imaging extraordinarily challenging. Coronagraphs physically block the star's light, while advanced adaptive optics correct for atmospheric distortion. This method works best for young, massive planets in wide orbits,they are still warm from formation and glow in infrared light. The James Webb Space Telescope and upcoming missions like the Habitable Worlds Observatory aim to directly image smaller, potentially Earth-like planets and analyze their atmospheres for biosignatures.

Microlensing uses Einstein's theory of general relativity to detect planets. When a star with a planet passes in front of a distant background star, its gravity bends the background light, creating a detectable signature.

The foreground star acts as a gravitational lens, magnifying the background star's light. If the lensing star has a planet, the planet creates an additional spike or anomaly in the magnification curve. This technique is uniquely sensitive to planets at moderate orbital distances (similar to Earth-Sun distance) and can detect planets around very distant or faint stars. The Nancy Grace Roman Space Telescope, launching in the late 2020s, will use microlensing to find thousands of new exoplanets, including free-floating planets not bound to any star.

Pulsar timing detects planets by measuring variations in the ultra-precise radio pulse arrival times from a rotating neutron star (pulsar).

Pulsars emit radio beams with clockwork precision. An orbiting planet causes the pulsar to wobble, creating measurable changes in pulse arrival times. This method produced the very first confirmed exoplanet discovery in 1992,two planets orbiting PSR B1257+12. Pulsar planets are extremely rare and exist in extreme environments bathed in high-energy radiation, making them very different from planets in our solar system.

Astrometry measures the precise positional wobble of a star on the sky, caused by the gravitational influence of orbiting planets.

While radial velocity measures a star's motion toward and away from us, astrometry measures its side-to-side motion across the sky. This requires extraordinarily precise position measurements,typically in microarcseconds. ESA's Gaia mission is mapping the positions of over a billion stars with unprecedented accuracy and is expected to reveal thousands of new exoplanets through their astrometric signatures, particularly massive planets in wide orbits.