The first claim of an extrasolar planet that survived peer review came from radio astronomers, not optical ones. In 1992, Aleksander Wolszczan and Dale Frail announced the detection of two — later three — terrestrial-mass planets orbiting PSR B1257+12, a millisecond pulsar in Virgo, by measuring tiny variations in the pulsar's pulse-arrival times. Three years later, the 51 Pegasi b radial-velocity announcement opened the modern catalogue. The two discoveries together laid down a methodological lesson — every detection method picks up only some of the planets it could see, and the catalogue we see is the union of those distinct selection biases.
Radial velocity (RV) measures the star's wobble through Doppler shifts in its spectrum: a Jupiter-mass planet at 1 AU pulls its star around at about 13 m/s, well within current instrumental reach (HARPS, ESPRESSO operate at sub-m/s precision). The bias is toward massive planets in close orbits. Transit photometry — watching for the brief dip in stellar brightness when a planet passes in front — is the workhorse that built most of the modern catalogue: the Kepler mission (2009-2018) used it exclusively, and TESS (2018+) extended it to brighter, nearer stars. The bias is geometric: only a small fraction of randomly-oriented orbits transit at all (~0.5% for a sun-like star). Direct imaging — spatially resolving the planet from its host — is extraordinarily difficult because of the contrast (planets are 10⁶ to 10¹⁰ times fainter than their host stars at visible wavelengths); coronagraphs and adaptive optics make it possible at all, and the bias is toward young, hot, massive planets at wide separations. Microlensing — the brief brightening of a background star as a foreground planet's gravity bends its light — is biased toward more distant systems; the Roman Space Telescope (NASA, 2027 launch) is designed to expand microlensing catalogues by orders of magnitude. The catalogue is therefore not a fair sample of the underlying planet population, and a substantial part of exoplanet science consists of de-biasing the catalogue. Combining methods helps: a planet found by transits gives radius; a planet found by RV gives a mass-projection; a planet found by both gives radius and mass and so a density. Astrometry — tracking the star's position rather than its velocity or brightness — is the most recent addition; Gaia (ESA, 2014+) has the precision to detect Jupiter-mass planets around nearby stars by their astrometric wobble alone, and full Gaia DR4 (expected 2026) is anticipated to roughly double the catalogue. The catalogue Earth most wants — Earth-sized rocky planets in habitable zones of sun-like stars — is precisely the configuration where every existing method is at the edge of its sensitivity. The Habitable Worlds Observatory (NASA, late 2030s) and the Extremely Large Telescope (Cerro Armazones, first light ~2028) are the next-generation instruments designed to push past those edges.
The combinatorics of the next decade are striking. PLATO (ESA, 2026) runs a Kepler-style transit survey targeted at Earth-sized planets around bright stars, optimized so RV follow-up can confirm them. Roman (NASA, 2027) brings wide-field IR transit and microlensing surveys. Gaia DR4 (ESA, 2026) releases the largest astrometric exoplanet catalogue ever produced. ELT (2028+) directly images planets that imaging campaigns of the 2010s could not have resolved. By the early 2030s the catalogue will be larger, less biased, and heavier on the configurations the biosignature search needs.