From John Reynolds
Stuart Clark’s alternative moon-formation theory based on a massive natural explosion inside Earth was intriguing (6 July, p 30). It made me wonder if a self-propelling planet could exist somewhere. It would need a less explosive but still significant heat output from a natural, internal nuclear reactor, and to be near enough to a reservoir of the right propellant – a nice deep ocean, say.
That planet would be a lonely wanderer that had escaped from its parent star, powered by a nuclear steam engine. Worryingly, budding Darth Vaders may not have to bother building a Death Star – they could just grab one that nature made earlier.
Canonbie, Dumfriesshire, UK
