8 Biggest, Weirdest Mysteries In Space

2. The Kuiper Cliff

The solar system doesn't just end when the planets do, there's a whole region of space around us called the Kuiper Belt. This is a rocky ring just beyond Neptune, much like the asteroid belt between Mars and Jupiter, and it's where everybody's favourite dwarf planet, Pluto, lives.

This region of space is understandably relatively unexplored, and is mysterious enough anyway, but it's the Kuiper Cliff that really puzzles people. The number of KBOs (Kuiper Belt Objects) appears to drop off rapidly at a distance of 50 AU (astronomical units) from the Sun. Based on the estimations for the mass required to form planets such as Uranus and Neptune, models suggested that the amount of KBOs should actually increase beyond 50 AU, so the sudden cliff is totally unexpected.

There are many possible explanations for the sudden drop, one of which being that there is some kind of massive, unseen planetary mass hoovering up and kicking out all of the smaller objects in the region. The New Horizons probe, the one that brought us those images of pluto, is currently pootling around the Kuiper Belt, so perhaps the data it collects could clear up the mystery once and for all, only time will tell.