The number of planets that orbit the sun depends on what you mean by “planet,” and that’s not so easy to define ...

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 ...

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 ...

All seven of the other planets in our solar system are about to become visible at once in a great planetary alignment – ...