It's generally accepted that very many species have yet to be discovered, but until now nobody has devised a reliable way to find out just how many.
Previously unseen letters suggest that Wallis Simpson was an insecure woman who felt trapped by her relationship with Edward.
After a half century of successes in space, low-Earth orbit has become a very messy place. More than 17,000 pieces of space junk bigger than a hand's breadth are floating around up there; the largest of them weighs nearly ten tonnes.
This new theory builds on the idea that around four billion years ago the Earth was struck by a Mars-like planet, but instead of the smash producing enough debris for one moon, this paper argues there was enough for two.