If Planet Nine exists, why has no one seen it?Strange things are happening at the outer edges of our solar system. An object up to ten times the mass of Earth is pulling others towards it. Is it a planet, or something else? As we head towards the end of another extraordinary year, BBC Future is taking a look back at some of our favourite stories for our "Best of 2021" collection. Discover more of our picks here. The 19th-Century travel writer
How Big Is Space – Interactive version
Uranus: Why we should visit the most unloved planet
