The Sun is a lonesome star, its closest neighbour more than 40,000km away. But that was not always the case. When it formed some 4.6 billion years ago, the Sun was in a densely-packed star cluster, with the space between newborn stars teeming with dust, gas, comets, asteroids – perhaps even planets.
The gravity of the young Sun and the other sibling stars, as astronomers call them, tore at the smaller heavenly bodies, until over millions of years the objects all settled down into individual star systems. And that is how our present Solar System ended up as we know it today – according to the current theory, at least.
But this leaves two mysteries. The first is Planet 9, which calculations indicate may be orbiting somewhere on the outskirts of the Solar System, though it has never been observed. The second is the Oort
