"Mercury is a small world of extreme temperatures, with a global magnetic field like Earth’s, but much weaker. Neither Venus nor Mars has such a magnetic field, although the two planets are similar to Earth in many other ways. Venus, unlike the Earth, has a hellish temperature. Venus is farther from the Sun than Mercury but is even hotter. The high temperature is due to an extreme greenhouse effect, the process by which the atmospheric gases raise the temperature by absorbing outward flowing heat. Earth’s atmosphere may once have contained large amounts of carbon dioxide, the way Venus’s atmosphere does now. But on earth, the oceans absorbed much of carbon dioxide, so that gas could not trap as much heat in the atmosphere as it does on Venus. The three large terrestrial planets are like the bowls of cereals in the child’s story of Goldilocks. Venus is too hot, Mars is too cold, but Earth is just right to support liquid water and life as we know it."