Q. What is the smallest planet in the solar system?

F.R.

Quincy

A. Pluto, with a diameter at the equator of just 1,429 miles, is the smallest of the nine planets. Discovered in 1930, it's made of the same ice and dust that other objects that far from the sun are made of, and some say that because of its small size, composition, and elliptical-instead-of-circular orbit, Pluto shouldn't even be called a planet.

But other astronomers say it's not fair to pick on the little guy. They point out that one suggested astronomical definition of a planet is anything wider than 621 miles (1,000 kilometers) that orbits the sun. Pluto qualifies.

But it is small. Earth's moon is nearly 50 percent bigger, measuring 2,159 miles across. Charon, Pluto's moon, is almost as big as Pluto itself. Together they are the closest thing to a double planet in our solar system.

The next smallest planet is Mercury, which is only 3,032 miles in diameter and the closest planet to the Sun. Earth's diameter is 7,927 miles. Jupiter is the solar system king, more than eleven times bigger than Earth at a median diameter of 88,875 miles. Jupiter, made up mostly of gasses, is spinning so fast it's a little bit oval, so they don't just measure it equatorially.

Compare all that to the sun. At 870,105 miles across, it's nearly ten times larger than Jupiter.