The Holy Grail of space science is the discovery of a planet just like ours: the right size, the right orbit around its sun, not too hot, not too cold – in the area dubbed the Goldilocks Zone. For millennia humans studying the stars had no idea if there were any other planets outside our solar system, let alone ones similar enough to ours to sustain life. Are we alone in the universe? We may be very close to finding Earth's twin planet.