PHYSICS
Venus is Earth-like in many ways. It is not much closer to the sun than is the Earth, 0.72 Astronomical Units (AU) versus the Earth's 1.0 AU. Moreover, the high albedo of the Venus clouds, 0.8, reflects much more of the sunlight than does the average albedo of the Earth, 0.3. The net result is that less solar energy enters the lower atmosphere of Venus than the terrestrial atmosphere.
The paradoxical high surface temperature of Venus occurs because unlike the Earth, the surface of Venus cannot radiate into space. The radiating layer, which emits the infrared radiation to balance the solar flux, occurs high in the atmosphere. Since temperature rises with decreasing altitude (and increasing pressure) in a planetary atmosphere, the surface becomes much, much warmer than on Earth. This is the so-called greenhouse effect. The efficiency of this effect on Venus, in which just a fraction of the solar energy received by the Earth can cause such high temperature, is of great interest to terrestrial scientists who find the Earth to be slowly warming and "greenhouse" gases to be increasing too.
Venus is similar to the Earth in size and mass and therefore in density. Its similar density suggests similar composition.