It used to be called Earth's twin. With much the same size, mass and composition as our home, Venus was a lush jungle planet in the popular imagination of the early 20th century. Muggier than Earth, perhaps, but otherwise not so very different.
That, of course, turned out to be entirely incorrect. Venus's surface is sweltering and its atmosphere suffocating: its being closer to the sun made a dramatic, not an incremental, difference to its fate.
That realisation has all but extinguished hopes of finding a twin for any earthly environment in our solar system. (Iced-over oceans on moons of the gas giants are almost our last hope.)
So the search for Earth's twin has moved much further afield: to the families of other stars. Work to identify the "habitable zones" in which such planets might exist has turned up some startling insights – not just about them, but also our own planet (see "Goodbye, Goldilocks: is life on Earth heading for an earlier demise?").
If the latest models are accurate, Earth and Venus really might have been twins, had the orbit of one been just a tiny bit different. But rather than two clement Earths, there might have been two infernal Venuses. That's a doubly humbling thought.
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