Venus is known both as the "planet of love" and the Earth's "evil twin". And although research suggests its environment is more hellish than romantic, there's actually a lot we don't know about our celestial neighbour. Now Japanese scientists have made a surprising discovery: an enormous, bow-shaped feature in the planet's cloud region which seems fixed to the slowly rotating planet. Clouds around it, on the other hand, whizz by at about 100 metres per second. So what is it?