Mercury is the first planet in the solar system, only about 60 million km away from the sun.It takes Mercury about 88 days to go round the sun once and one Mercury day is about 58 Earth days long.Mercury is a small planet - not much larger than our moon.Temperatures go up to 500 °C when it faces the sun and -150°C on its dark side. Some scientists believe that Mercury is really the moon of another planet that flew off course and began its own orbit as a planet.
That means that you would be crushed if you tried to walk on the surface. The clouds that surround the planet are thick and yellow.
This makes it almost impossible to see the surface of Venus.
The clouds are not made up of water, like on Earth.
They have poisonous gases, mostly sulphur, in them.
Below the clouds there is a thick atmosphere of carbon dioxide- so it is impossible for us to breathe on this planet. This is because the sunlight that gets into the atmosphere is trapped there and can't get out. We call this the greenhouse effect - the same thing happens on our Earth and warms up our atmosphere.