In 1995, astronomers detected a planet orbiting a Sun-like star fifty light-years away—not by photographing it, but by watching its star wobble. That discovery, 51 Pegasi b, inaugurated a revolution in planetary science built entirely on indirect evidence. We have since catalogued over 5,500 confirmed exoplanets, yet we possess direct images of fewer than two dozen.

The challenge is staggering. Imagine standing in New York and trying to spot a firefly hovering beside a lighthouse in Los Angeles. Stars outshine their planets by factors of millions to billions, rendering most worlds invisible against their stellar hosts' glare. Yet astronomers have developed remarkably clever techniques that transform this apparent impossibility into routine discovery.

These methods exploit fundamental physics—gravity's subtle influence on motion, light's predictable behavior as it encounters matter, even the warping of spacetime itself. Each technique reveals different planetary populations and properties, together assembling a census of worlds we cannot directly see. Understanding these methods illuminates not just how we find exoplanets, but what makes each discovery scientifically valuable.

Stars do not sit motionless while planets orbit them. Instead, star and planet orbit their common center of mass—and for systems with massive planets or tight orbits, this gravitational dance creates measurable stellar motion. The radial velocity method detects this wobble by measuring how starlight shifts as the star moves toward and away from Earth.

When a star approaches us, its light waves compress slightly, shifting toward bluer wavelengths. As it recedes, the light stretches toward red. This Doppler shift is extraordinarily subtle—Jupiter causes our Sun to wobble at roughly 12 meters per second, while Earth produces a mere 9 centimeters per second. Modern spectrographs can now detect stellar velocities with precision approaching 30 centimeters per second.

The technique works best for massive planets orbiting close to their stars, where gravitational influence is strongest. The first exoplanet discoveries emerged from radial velocity surveys precisely because hot Jupiters—gas giants in tight orbits—produce the largest, fastest wobbles. Yet improved instrumentation has extended sensitivity toward smaller, more distant worlds.

Critically, radial velocity measurements reveal a planet's minimum mass and orbital period, but not its true mass unless we know the orbital inclination. A planet orbiting face-on produces no detectable wobble along our line of sight, while edge-on systems show maximum effect. This limitation becomes important when combining detection methods to fully characterize planetary systems.

Takeaway

Planets betray their presence through gravitational influence before they reveal themselves visually. When you cannot see something directly, look for the effects it creates on things you can observe.

When a planet passes between its star and Earth, it blocks a tiny fraction of starlight—a transit that produces a characteristic dip in brightness. Jupiter crossing our Sun would dim it by about one percent; Earth would cause a mere 0.01 percent reduction. Yet space telescopes like Kepler and TESS have measured such infinitesimal dimming with extraordinary precision, revealing thousands of transiting worlds.

The transit method yields information radial velocity cannot provide. The depth of dimming reveals the planet's physical size relative to its star. The duration indicates orbital distance and geometry. Combined with radial velocity mass measurements, astronomers can calculate planetary density—distinguishing rocky worlds from gas giants and water-rich sub-Neptunes.

More remarkably, transits enable atmospheric characterization through transmission spectroscopy. As starlight filters through a planet's atmosphere during transit, specific wavelengths are absorbed by particular molecules. Comparing in-transit and out-of-transit stellar spectra reveals atmospheric composition—water vapor, carbon dioxide, methane, even potential biosignatures. The James Webb Space Telescope has begun such measurements with unprecedented sensitivity.

Transit detection requires precise geometric alignment; only systems viewed nearly edge-on produce observable dimming. This means transit surveys miss most planets, but those detected become prime targets for detailed follow-up. The method favors large planets on short-period orbits, though extended observations have revealed Earth-sized worlds in habitable zones around small, cool stars.

Takeaway

Transit observations transform a simple brightness measurement into a planetary profile including size, density, and atmospheric composition. Precision measurement of ordinary phenomena often reveals extraordinary information.

Einstein's general relativity predicts that massive objects bend the fabric of spacetime, deflecting light that passes nearby. When a foreground star drifts across our line of sight to a more distant star, its gravity acts as a lens—temporarily magnifying and brightening the background source. If planets orbit the lensing star, they create additional brief magnification spikes superimposed on the main event.

Microlensing requires no light from the planet or even its host star. The technique detects mass directly through gravitational influence on passing light. This makes microlensing uniquely sensitive to populations invisible to other methods: planets orbiting faint red dwarfs, worlds at large orbital separations, and even free-floating planets wandering through interstellar space without stellar hosts.

The catch is that microlensing events are singular, unrepeatable occurrences. The precise alignment required happens only once as the lensing system drifts across our view of the background star. There is no opportunity for follow-up observations to refine measurements or characterize atmospheres. Each detection provides a snapshot of a planetary system we will never observe again.

Despite this limitation, microlensing surveys toward the galactic bulge—where stellar density maximizes alignment probability—have revealed that planets are extraordinarily common. Statistical analyses suggest the Milky Way contains more planets than stars, with billions of Earth-mass worlds potentially drifting unbound between stellar systems. This cosmic census reveals planetary formation's abundance even when individual worlds remain forever anonymous.

Takeaway

Microlensing detects worlds through pure gravitational physics, finding planets no other method can reach. Some of the most important discoveries come from techniques that sacrifice detailed study for unprecedented reach.

Each detection method illuminates a different population of worlds, together revealing that planets pervade our galaxy in staggering variety. Radial velocity traces gravitational choreography, transits capture shadow geometry, and microlensing witnesses spacetime curvature. No single technique provides complete understanding; their combination approaches something like a true planetary census.

The ingenuity required to find worlds we cannot photograph speaks to a deeper truth about astronomical discovery. We rarely observe cosmic phenomena directly—instead, we decode their influences, their shadows, their gravitational signatures. The universe reveals itself through the effects objects create, not merely through their appearance.

These invisible worlds challenge our intuitions about knowledge itself. We can determine an exoplanet's mass, size, density, orbital period, and atmospheric composition without ever capturing its image. Sometimes understanding what something does tells us more than seeing what it looks like.