For years, scientists have believed Venus to be Earth's sister planet. It is only because exploration of the planet has allowed us to uncover its secrets. The 30 kilometre thick cloud which totally covers the planet, the levels of pressure on the surface which would feel similar to being 1 kilometre under the sea on Earth, the poisonous atmosphere and volcanic surface make Venus an unbelievably different planet to Earth. However, both planets, Earth and Venus are similar in other ways. Both planets are very close to each other in their positions in the Solar system. No other planet is closer to another one than Earth and Venus are to each other.
In the diagram above, you can see the orbits of the four inner planets around the Sun, and you can see how close Earth's orbit is to Venus. Compare this to the distance Earth is away from Mars, and the huge distance that Mars is away from Jupiter which only just fits in the picture!
Earth is only slightly bigger than Venus. Below is a diagram which shows how much bigger Earth is than Venus.
So, Venus and Earth are two planets about the same size as each other, orbiting the Sun close to each other. Both planets are both quite young geologically. Whereas the surfaces of Mercury and the Moon have remained the same for billions of years, with craters forming on top of craters, Earth and Venus have young surfaces. This is shown by the fact that both planets have very few impact craters from meteorites on their surfaces. The landscapes of each planet have both been reformed in the last 200 to 800 million years by geological processes. Rain, water, sea, wind, earthquakes and volcanoes have formed Earth's surface, and still do (Earth is a "geologically living" planet). Venus may have had similar conditions in the past. It is known that the planet's present surface was formed by volcanoes. The planet's surface is covered in volcanoes, and may have once had liquid lava flowing around it, just as there is water flowing around Earth. Missions to watch Venus' surface (such as the Magallen mission from 1990-1994) failed to see any volcanoes erupting, but this does not mean that all volcanoes on Venus are extinct. Perhaps Venus, like Earth, is still geologically alive.
So far we can see a few similarities between Earth and its "sister planet", Venus. They are similar sizes, similar distances from the Sun and about the same age as each other. But this is where the similarities end. The main difference between the two planets is that one planet is able to support life, whereas the conditions on the other make life impossible. Firstly, Earth's temperature is about 22°c on average, whereas Venus' is about 480°c. Earth's atmosphere contains oxygen, a breathable gas. Venus' atmosphere contains carbon dioxide, a poisonous gas. Earth's air contains droplets of water in clouds. Venus' air contains deadly sulphuric acid. But, even with these differences, we can see similarities between Earth and Venus. Earth's atmosphere does also contain some carbon dioxide. Rainwater on Earth is also dangerous to drink because it contains very small amounts of sulphuric acid. This is why rain is sometimes known as "Acid Rain." These are signs that maybe, one day, in millions of years, Earth will turn into another Venus. It may also suggest that, millions of years ago, Venus was like Earth is now.