Which planet is called Earth's twin because of its size?

Venus, which is 95 per cent of Earth's diameter and 82 per cent of its mass, made of much the same material, and formed next door. On any inventory of ingredients the two are near-identical, and the outcomes could hardly be further apart.

The surface is around 465 degrees Celsius everywhere, day and night, pole and equator - hotter than Mercury despite being further out. The atmosphere is carbon dioxide at ninety times our pressure, equivalent to being a kilometre underwater, with clouds of sulphuric acid. Soviet landers reached the surface in the 1970s and 80s and sent back photographs; the longest survived just over two hours before being cooked and crushed.

The reason it matters is that Venus may not have started this way. Models suggest it could have held liquid water oceans for a billion years or more before a runaway greenhouse took hold - water vapour trapping heat, driving more evaporation, until the oceans were gone and the hydrogen escaped to space. It is the nearest thing available to a controlled experiment on what a thick carbon dioxide atmosphere does to a rocky planet, which is why it is studied with rather more attention than a hot rock deserves.