With a radius of 3,959 miles, Earth is the fifth largest planet in our solar system, and it's the only one known for sure to have liquid water on its surface. Earth is also unique in terms of ...

Earth formed over 4.5 billion years ago from a swirling cloud of gas and dust squished together by gravity. That same cloud gave rise to our entire solar system, including our star, the sun.

With a radius of 2,106 miles, Mars is the seventh largest planet in our solar system and about half the diameter of Earth. Its surface gravity is 37.5 percent of Earth’s. Mars rotates on its ...