Exoplanet Detection Methods

The radial velocity method detects alien planets indirectly by tracking the periodic Doppler shifts in a star's spectral absorption lines. As a massive planet orbits, its gravitational pull causes the parent star to execute a small counter-orbit around their shared centre of mass. When the star wobbles towards Earth, its light shifts slightly towards blue wavelengths; as it recedes, the light shifts towards red. This technique is particularly effective at identifying massive gas giants orbiting close to their host stars, because stronger gravitational interactions produce more pronounced and easily detectable velocity variations.

Gravitational microlensing exploits general relativity, wherein the gravitational field of a foreground star acts as a natural cosmic lens, bending and magnifying the light of a distant background star. If an exoplanet orbits the lensing star, its own secondary gravitational field produces an additional, brief spike in the background star's brightness curve. Unlike other detection techniques, microlensing is uniquely sensitive to low-mass worlds at wide separations and can even uncover unbound rogue planets. However, because these cosmic alignments are completely random and non-repeating, follow-up observations of specific events are practically impossible.

Directly imaging exoplanets remains technically daunting because a parent star typically outshines its planetary companions by several orders of magnitude. To overcome this overwhelming glare, astronomers employ coronagraphs—optical masks installed within telescopes that physically block the central starlight while allowing dimmer, off-axis planetary light to pass through. When combined with adaptive optics to correct atmospheric blurring, direct imaging enables researchers to capture photons emitted directly by the planet itself. This capability makes coronagraphy indispensable for conducting spectroscopic analyses of planetary atmospheres, searching for chemical signatures such as methane, water vapour, and carbon dioxide.

Astrometry detects extrasolar planets by measuring the precise physical movements of a star across the celestial sphere over time. While the radial velocity technique tracks movement along the observer's line of sight, astrometry measures two-dimensional transverse displacement against fixed background reference points. Because the physical displacement caused by an orbiting body increases with distance from the host star, astrometry is most sensitive to planets with vast orbital separations. However, because stellar shifts are extraordinarily minute—often measuring fractions of a milliarcsecond—astrometric surveys require ultra-stable instruments and years of continuous monitoring to map entire orbits.