Venus has surface temperatures of up to 900 degrees Fahrenheit because it is much closer to the sun and has a thick, high-pressure atmosphere. When sunlight penetrates the atmosphere of Venus, it does not have the energy to escape efficiently and continues to heat the surface.
The crushing atmosphere of Venus consists of up to 96 percent carbon dioxide, which is also one of the major greenhouse gases on Earth. Venus also has 90 times the pressure of Earth's atmosphere. Carbon dioxide is a major greenhouse gas because its atomic structure allows it to absorb energy more easily and radiate it slowly, rather than allowing it to pass through unaffected by the toxic conditions on Venus.
Venus' orbit is also much closer to the sun than the Earth's orbit. Solar radiation strikes Earth more diffusely than it does on Venus, which heats Venus more than Earth. Mercury is the closest planet to the sun and gets even more heat than Venus during its day. However, the relative lack of atmosphere on Mercury means that this heat radiates away on the night side and heats up again during the day. These two factors combine to make Venus the hottest planet in the solar system.