Some planets are extremely big--so huge that 1,300 Earths could fit inside one. Some planets are extremely little--smaller than Earth's moon. Some planets are extremely fast--speeding through space at 104,000 miles an hour. Some planets may not be planets at all Extreme Planets Q&A has answers to questions such as: What makes up a planet? Why is Earth the only known world where life exists? How can a planet's day actually be longer than its year? So buckle up as you blast off for an extreme voyage through the solar system, from Mercury to Neptune and beyond