Both Mercury and Earth are inner planets. They both orbit the sun, but their orbital periods are different. Mercury and Earth are both rocky planets made up of land and rock. The Earth's orbital period is 365 days, and it takes twenty-four hours to complete a full day and night cycle. The mercury planet has only 88 days from one end of its orbit to the other. There is no life as we know it today on either Mercury or Earth, although each may have had at some time in their history conditions suitable for supporting life. The early Earth was very hot, with average temperatures around 140 degrees Fahrenheit (60 degrees Celsius). Any living organisms that arose then would have been burned away by the heat within a few months. The last time there was water on Earth was about 3.5 billion years ago. Even though plants come in many different forms, they all contain water inside their cells. Without this water, the cells would be dried out like a fruit drop overnight. Water is also needed to create organic chemicals that build up the soil and help crops grow. So without water, there would be no life as we know it today. In November 2008, the New Horizons spacecraft flew by Pluto. What it found surprised scientists. There is evidence that Pluto once may have had an atmosphere, but it has now evaporated leaving behind a cold, dry world.