For most of human history, the question of whether other stars host planets remained unanswered. That changed in 1995, when Michel Mayor and Didier Queloz detected 51 Pegasi b, a Jupiter-sized planet orbiting a Sun-like star 50 light-years away, earning them the 2019 Nobel Prize in Physics. Since then, astronomers have confirmed over 5,500 exoplanets, revealing a stunning diversity of worlds that has overturned long-held assumptions about planetary systems and intensified the search for habitable environments beyond Earth.
Detection Methods
Exoplanets are extraordinarily difficult to observe directly because they are vastly outshone by their host stars. Astronomers rely on several indirect detection methods. The transit method, used by NASA’s Kepler and TESS missions, detects the tiny dimming of starlight when a planet passes in front of its star. By measuring the depth and duration of these transits, scientists determine the planet’s size, orbital period, and, when combined with other data, its density and likely composition.
The radial velocity method detects the gravitational wobble a planet induces in its host star as both orbit their common centre of mass. Precise spectrographs measure minute Doppler shifts in starlight, revealing the planet’s mass and orbital characteristics. Direct imaging, while technically demanding, has captured actual photographs of young, hot giant planets orbiting far from their stars, and is advancing rapidly.
Gravitational microlensing detects planets through the gravitational bending of light from distant background stars, and is particularly sensitive to planets at large orbital distances, a regime where transit and radial velocity methods are less effective.
The Diversity of Worlds
Exoplanet discoveries have revealed planetary types with no analogue in our solar system. Hot Jupiters are gas giants orbiting closer to their stars than Mercury does to the Sun, with surface temperatures exceeding 1,000 degrees Celsius. Super-Earths are rocky worlds with masses between Earth and Neptune, and appear to be the most common type of planet in the galaxy. Mini-Neptunes, water worlds, and even “diamond planets” with carbon-rich compositions expand the catalogue of known planetary types.
The habitable zone, the orbital region where liquid water could exist on a planet’s surface, remains a primary focus of exoplanet research. The TRAPPIST-1 system, with seven Earth-sized rocky planets (three in the habitable zone) orbiting an ultracool dwarf star just 40 light-years away, is one of the most studied planetary systems for habitability assessment.
Atmospheric Characterisation and the Search for Life
The James Webb Space Telescope (JWST) has inaugurated a new era in exoplanet science by analysing the atmospheres of transiting planets through transmission spectroscopy. By measuring how starlight is filtered through a planet’s atmosphere during transit, JWST can identify atmospheric gases including water vapour, carbon dioxide, methane, and potentially biosignatures, chemical signatures of biological activity.
Future missions including the Habitable Worlds Observatory (NASA) and PLATO (ESA) will extend this capability to smaller, more Earth-like planets. The ultimate goal, detecting unambiguous signs of life in the atmosphere of an exoplanet, may be achievable within the next two decades, representing one of the most profound potential discoveries in the history of science.
