The Solar System’s outer limits
Thanks to recent discoveries, we now have a clearer picture of what lies beyond Pluto
For 60 years after its discovery in 1930, Pluto – together with its largest moon Charon, discovered in 1978 – marked the outermost limit of the Solar System. With an average distance from the Sun of 39 astronomical units (AU) – with one AU the distance from Earth to the Sun – that’s pretty far out. But the 1990s saw the discovery of numerous other ‘trans-neptunian objects’ beyond the orbit of Neptune, the most distant of the major planets, 30 AU from the Sun, with further discoveries coming in ever since. Pluto actually resides in a relatively populous neighbourhood called the Kuiper Belt – a doughnut-shaped region extending from around 30 to 50 AU which contains hundreds of thousands of bodies larger than 62 miles in size. Pluto is still the largest Kuiper Belt object (KBO) we know of. With a diameter of 1,473 miles, it has enough gravity to pull it into a spherical shape, classifying it as a ‘dwarf planet’ – a status it shares with other large KBOS such as Makemake and Haumea. Smaller KBOS are more irregular-looking, resembling the asteroids found closer to the Sun. Unlike asteroids, however, which tend to be rocky in composition, KBOS are predominantly made of water ice and frozen methane and ammonia. The region beyond the Kuiper Belt, called the ‘scattered disc’, contains icy bodies that have been scattered by Neptune’s gravity into highly eccentric orbits. These may stray hundreds of AU from the Sun, rising far above the central plane, before their orbits take them back almost to the orbit of Neptune. The largest scattered-disc object discovered so far, Eris, is similar in size to Pluto. In the course of its 557-year orbit, it roams all the way from inside Pluto’s orbit to almost 100 AU from the Sun.