The nearest planets to Earth are Mars and Venus.
Mars is farther from the Sun than Earth. On Mars, there are 687 days in a year because it takes 687 days for Mars to circle, or orbit, around the Sun. Olympus Mons, the largest volcano on Mars, is also the largest volcano in our solar system and more than three times as high as Mount Everest, the world's largest peak. Olympus Mons is very old and has not erupted in millions of years. There is plenty of red dust on Mars, which is dry as a desert. The dust creates huge storms on Mars.
Among Earth, Mars, and Venus, the planet closest to the sun is Venus. A year on Venus is 225 days. Clouds in Venus trap heat, which makes Venus hotter than all of the other planets. These clouds are dangerous to life because they are made of drops of acid, instead of drops of water like clouds on Earth. This makes them poisonous. Venus also is covered with hundreds of volcanoes.
Mars and Venus are unique in their own ways, which makes them interesting to study.
[Source: 100 Things You Should Know About Space]