Why are the planets where they are instead of somewhere else?
The relatively small Earth, with its sizable moon, is the only planet we know of with life on it. Coincidence?
Where did the nine planets that orbit our sun come from?
Nowadays everybody knows there are nine planets in our solar system, but that information was hard won.
On last count, there were nine planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Soon, however, we might need to add a tenth planet to the solar lineup. Learn more on this Moment of Science.