Which planet is closest in size to the Earth?

Venus is most like Earth in terms of mass and size, and it is also the planet closest to Earth, but the two planets are far from identical twins. They spin in opposite directions, and whereas Earth has a temperate climate capable of supporting life, Venus is an inferno, with a thick, poisonous atmosphere and surface temperatures hot enough to melt lead. Most of what scientists know about Venus' topography has been obtained with radar imaging.

Venus is a terrestrial planet, like Earth, which means it is composed of rock, unlike the gas giants Jupiter, Saturn, Uranus and Neptune. Because of its proximity to the sun, it probably formed in the same way that Earth did, accreting matter from rocks and asteroids that circled the young sun. The retrograde motion of Venus is mysterious, however. Some scientists believe that it spins in the same direction as Earth, but its poles are oriented in the opposite direction. Two French scientists -- Alexandre Correira and Jacques Laskar -- believe that the sun's gravitation slowed Venus' rotation until the planet stopped and began turning in the opposite direction.