How Astronomers Detect Exoplanets Orbiting Distant Stars

Finding a Candle Beside a Lighthouse from a Thousand Miles Away

An Earth-sized planet orbiting a Sun-like star at 1 AU blocks about 0.008% of its star's light during a transit. Detecting that signal requires measuring brightness changes of less than 1 part in 10,000, maintained over hours, from a star hundreds of light-years away. Yet astronomers have confirmed over 5,700 exoplanets as of 2025, and thousands more await confirmation. The methods used to find them range from precision photometry to the bending of spacetime, each sensitive to different classes of worlds.

The first confirmed exoplanet orbiting a Sun-like star was 51 Pegasi b, detected in 1995 by Michel Mayor and Didier Queloz using the radial velocity method. They received the 2019 Nobel Prize in Physics for the discovery. Since then, the field has transformed from finding individual curiosities to characterising planetary populations across the galaxy.

Transit Photometry

Transit photometry is the most productive exoplanet detection method by count. When a planet's orbital plane is aligned such that it passes in front of its host star as seen from Earth, it blocks a fraction of the starlight. This produces a characteristic dip in the light curve.

• The depth of the dip is proportional to the ratio of the planet's area to the star's area: δ = (R_planet/R_star)².
• A Jupiter-sized planet transiting a Sun-like star produces a 1% brightness dip; an Earth-sized planet produces about 0.01%.
• The orbital period is determined by the time between successive transits; Kepler's third law then gives the orbital distance.
• Transit events must repeat with consistent periodicity to distinguish them from stellar variability and instrument noise.

NASA's Kepler Space Telescope operated from 2009 to 2018, staring at roughly 150,000 stars in a fixed field of view. It discovered over 2,600 confirmed exoplanets. Its successor, TESS (Transiting Exoplanet Survey Satellite), surveys the entire sky in two-year cycles, finding planets around bright, nearby stars more amenable to follow-up observation.

Radial Velocity

A planet does not orbit its star — both planet and star orbit their common centre of mass. The star wobbles slightly. If the orbital plane is roughly edge-on to Earth, this wobble produces a periodic Doppler shift in the star's spectral lines: blueshift as the star moves toward us, redshift as it moves away. This is the radial velocity or Doppler spectroscopy method.

The HARPS spectrograph at La Silla Observatory achieves velocity precision below 1 m/s, while ESPRESSO at the Very Large Telescope approaches 10 cm/s. Earth-mass planets at habitable-zone distances produce signals of just 9 cm/s — at the frontier of current capability.

Direct Imaging

Direct imaging captures reflected or thermal light from the planet itself, separate from the star. Extreme contrast ratios — a planet may be 10⁹ times fainter than its star in visible light — make this technically challenging. Coronagraphs block the star's light; adaptive optics systems correct for atmospheric turbulence in real time.

• Direct imaging is most effective for large planets at large orbital separations — young, hot gas giants emit significant infrared radiation and are separated far enough from the star to resolve.
• HR 8799 b, c, d, and e, imaged in 2008 and 2010, were the first multi-planet system directly imaged. All four are gas giants beyond 14 AU.
• The Nancy Grace Roman Space Telescope, planned for the late 2020s, will use a coronagraph instrument capable of imaging planets up to 10⁹ times fainter than their stars.
• Direct imaging also enables spectroscopy of the planet's atmosphere, detecting signatures of water, carbon dioxide, and potentially biosignatures.

Gravitational Microlensing

General relativity predicts that a massive object bends light passing near it. When a foreground star aligned with a background star acts as a gravitational lens, the background star brightens in a characteristic pattern. A planet orbiting the foreground star adds a short additional brightening spike to the lensing event.

Microlensing is uniquely sensitive to planets in the 1–10 AU range and can detect Earth-mass planets around distant stars. Events are non-repeating, so follow-up is impossible, but statistical surveys — particularly by the Korea Microlensing Telescope Network (KMTNet) and the Optical Gravitational Lensing Experiment (OGLE) — have yielded dozens of detections.

Atmospheric Characterisation

Detection is only the beginning. Transmission spectroscopy during a transit allows astronomers to probe a planet's atmosphere. As starlight filters through the atmospheric limb, molecules absorb specific wavelengths. The James Webb Space Telescope, operational since 2022, detects carbon dioxide, methane, water, and sulphur dioxide in exoplanet atmospheres with unprecedented sensitivity.

In 2023, JWST detected carbon dioxide and sulphur dioxide in the atmosphere of WASP-39b, a hot Saturn 700 light-years away. The detection of dimethyl sulphide — a potential biosignature produced by marine organisms on Earth — in the atmosphere of K2-18b was reported in 2023, though its interpretation remains contested. The tools now exist, in principle, to detect signs of life on a distant world.