Our tidy arrangement of planets looks unusual among thousands of known systems
Small rocky worlds close to the Sun, giant planets farther out: it feels like the natural order. Yet among 4,809 planetary systems confirmed by April 2026, few resemble ours. More often, astronomers find families of similar-sized planets packed closely together, or gas giants skimming their stars with years lasting only a few days.
Imagining other suns with planets is old. Aristarchus of Samos suggested in the third century BC that the Sun, not Earth, sits at the centre, though most ancient astronomers rejected him. Copernicus published the first mathematically predictive Sun-centred model in 1543, and Galileo, Kepler and Newton built the physics that made it convincing. In the sixteenth century Giordano Bruno argued that the stars were suns with planets of their own, and Newton later speculated that any such systems would follow a similar design.
Proof took until 1992, when several planets of roughly Earth's mass turned up orbiting a pulsar, the collapsed remnant of an exploded star. In 1995 came 51 Pegasi b, a giant circling an ordinary nearby star every four days. By 23 April 2026 the tally reached 6,416 confirmed exoplanets, and 1,061 systems were known to hold more than one.
One classification sorts systems by how mass is spread among their planets. In the commonest type, which includes TRAPPIST-1, the planets have similar masses, like peas in a pod, with smaller ones packed tighter and larger ones spaced more widely. Mixed systems swing up and down in mass; Gliese 876 is one. Our Solar System is ordered, with mass rising outward. The reverse, big planets inside and small ones outside, has never been observed.
Systems also change over time. During formation, some planets get flung out entirely and wander as rogues, though in crowded star clusters they can be captured again into wide orbits. When a star swells into a red giant, it swallows its inner planets while the survivors drift outward as it loses mass. Around low-mass stars, sparse disks rarely build gas giants, so their compact families tend toward rock and ice. Belts of debris are common too; our own system has at least four, including the asteroid belt and the distant Oort cloud.
Source: Planetary system
