Beyond our solar system
For most of human history, the planets beyond our Solar System were invisible possibilities. Astronomers suspected that other stars might possess worlds of their own, yet even the nearest stars are extraordinarily distant. Proxima Centauri, our closest stellar neighbour after the Sun, lies about 4.24 light-years away. A light-year is the distance light travels in one year: roughly 9.46 trillion kilometres. Although light races through space at nearly 300,000 kilometres per second, a message sent at light speed to Proxima Centauri would take more than four years to arrive.
Planets produce little visible light of their own and are overwhelmed by the glare of the stars they orbit. Imagine trying to see a firefly beside a lighthouse from hundreds of kilometres away. Consequently, astronomers usually detect an exoplanet – a planet outside our Solar System – indirectly. The first confirmed exoplanets, announced in 1992, were found orbiting a pulsar: the rapidly spinning remains of an exploded star. In 1995, 51 Pegasi b became the first planet discovered around a Sun-like star. It surprised scientists because this gas giant orbits extremely close to its star, completing a year in about four Earth days.
One successful technique is the transit method. If a planet crosses, or transits, the face of its star from our viewpoint, it blocks a fraction of the starlight. A Jupiter-sized planet crossing a Sun-sized star may reduce its brightness by about one per cent. An Earth-sized planet causes a far smaller dip – roughly 0.008 per cent. Regular dips reveal the length of the planet’s year, while their depth helps scientists estimate its diameter. However, the method works only when an orbit is aligned almost edge-on from Earth. Many planets never cross their stars from our viewing angle and remain undetected by transit surveys.
NASA’s Kepler space telescope transformed this search. It monitored more than 150,000 stars in one region of the sky. During its original four-year mission, it searched for repeated patterns rather than relying on one dimming, which might have been caused by a starspot, an instrument error or another star. Kepler and its extended mission identified thousands of planets and candidates. Its results suggested something profound: planets are not rare exceptions but common products of star formation, and many stars may possess several worlds.
Astronomers also use the radial-velocity method. A planet does not circle a perfectly stationary star: both bodies orbit their shared centre of mass, making the star wobble. As the star moves towards Earth, its light shifts towards the blue end of the spectrum; as it moves away, the light shifts towards red. These changes reveal a planet’s minimum mass and orbital period. Transit observations indicate size, whereas radial velocity indicates mass. Together, the measurements allow researchers to calculate density and infer whether a planet is probably rocky, watery or dominated by gas.
Finding a planet is only the beginning. Scientists are interested in worlds within a star’s habitable zone, where temperatures might permit liquid surface water. Yet location alone does not make a planet habitable. Venus lies near the Sun’s habitable zone, but its thick carbon-dioxide atmosphere traps so much heat that its surface is about 465 degrees Celsius. Mars has a thin atmosphere and an average surface temperature near -63 degrees Celsius. Atmosphere, size, magnetic field, geology and the behaviour of the host star can all alter a world’s prospects.
The TRAPPIST-1 system illustrates both excitement and uncertainty. About 40 light-years away, its cool red-dwarf star is orbited by seven roughly Earth-sized planets. Three lie within its traditional habitable zone. Because the star is much dimmer than the Sun, those planets orbit far closer than Earth does to the Sun. Red dwarfs can also release powerful flares that may strip away atmospheres. Thus, promising positions do not prove that the planets contain oceans, air or life; they merely make the system an especially valuable laboratory.
The James Webb Space Telescope allows astronomers to investigate some atmospheres. Its 6.5-metre mirror collects infrared light, and it operates about 1.5 million kilometres from Earth. During a transit, some starlight filters through the planet’s atmosphere. Different gases absorb particular wavelengths, leaving patterns like chemical fingerprints. Water vapour, carbon dioxide and methane may therefore be detectable. Nevertheless, no single gas proves that life exists: geological processes can imitate biological signals, clouds can hide evidence, and starlight can contaminate measurements.
By 2025, more than 5,800 exoplanets had been confirmed, yet this is only a minute sample of the Milky Way’s probable population. Our galaxy contains hundreds of billions of stars, and current instruments favour large planets with short orbits because they create stronger, more frequent signals. An Earth twin, taking one year per orbit, would require years of observation before astronomers recorded several transits. The search is therefore a patient process of gathering repeated measurements, rejecting alternative explanations and combining evidence from several instruments. Each discovery answers one question while sharpening others: how do planets form, how unusual is our Solar System, and are any distant worlds alive?
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