BEYOND PLUTO

From Planet X to objects frozen in time – what truly lurks outside the Solar System’s chaotic frontier

- James Romero

Like an archaeological dig into the history of our Solar System.” That’s how New Horizons principal investigator Howard Stern described the spacecraft’s mission to Pluto and the outer Solar System. In recent decades, our ability to peer into the murky edges of the Solar System and map the populations of icy bodies that reside there has not only changed our understanding of the true scale and nature of the Solar System, but has also shone a light on the past, on how the current arrangement of rocky and icy worlds came to be and how interactions with the wider galaxy might shape its future.

Residents of the outer Solar System can be divided into various populations by their current orbits, history of orbital interactions or their compositional makeup. Kuiper Belt objects (KBOs) are the first population encountered as you move beyond the orbit of Neptune at around 30 astronomical units (AU) – one AU is the Earth-Sun distance. This sparsely populated ring extends out to 2,000 AU and includes icy bodies left over from the formation of the Solar System. Larger residents include Pluto, as well as Eris, Makemake and Haumea, which along with many much smaller inhabitants form a large subgroup known as ‘hot’ Kuiper belt objects.

Hot KBOs owe their current positions to an ancient eviction of between 10 and 30 Earth masses worth of small bodies from the Solar System’s inner regions, likely caused by ancient jostling of young gas and ice giants. This violent event is still evident in hot KBOs’ eccentric orbits, often at significant angles from the general plane of the Solar System.