Gary Moore, age 14, of Williamsport, Pennsylvania, for his question:

Can Pluto really be a moon of Neptune?

Astronomers have been trying to settle this question for several decades. The answer depends on precise measurements, including Pluto's exact mass. Andy attempts to report these findings, as they are verified. Recently, the mass of Pluto was measured anew. If this latest estimate is anywhere near accurate, little Pluto may be an escapee satellite that once belonged to the planet Neptune.
The study of Pluto is a little easier than trying to investigate a lighted candle on the moon    but not much easier. This remote member of the Solar System is 39 times farther from the sun than we are. It never comes closer to the earth than about 2.6 billion miles. So far, our investigations are somewhat sketchy and the task is more difficult because Pluto seems to be a small bundle of contradictions.
Its average distance from the sun is 4.583 billion.  However, because of its highly eccentric oval shaped orbit this distance varies from 2,766 million to 4,567 million miles. At times, this brings Pluto inside the enormous circular orbit of Neptune. In fact, right now it is about three million miles closer to the sun than Neptune. This strange orbital maneuver was computed several years ago. Since all the other planetary orbits are separated by vast distance, this oddity caused astronomers to wonder whether Pluto is really a moon that once orbited the giant planet Neptune.
The moons of the Solar System are smaller and much less massive than the planets. Before settling the status of Pluto, we needed to know these and other vital statistics that would qualify it as a moon or a planet type body. However, measuring the mass of such a small and remote object proved to be very tricky.
Careful measurements made in 1942 estimated Pluto to be a planet type body, almost as massive as the earth. Equally careful estimates made 13 years later reduced it to four fifths of the earth's mass, which still suggests a planetary type. In 1969, the massive problem was tackled again. These results indicated that the earth has about ten times more mass than Pluto.
In 1971, this estimate was verified by the U.S. Naval Observatory in Washington, D.C., using the most up to date techniques and equipment available. With only one tenth of the earth's mass, Pluto qualifies as a satellite type body. This, plus its odd orbit suggest that once upon a time it may have been a moon that belonged to Neptune.
However, at present we should class Pluto as a genuine planet. Whatever its status in the past, the little outsider, now travels a true planetary orbit around the sun. Unlike a satellite, it does not orbit around a planet as it goes. Nobody knows whether someday it may come close enough to Neptune to be recaptured. This is possible but unlikely. Chances are that runaway Pluto will continue to enjoy its freedom indefinitely. The following are the latest statisics on Pluto:

Pluto Statistics (minor planet)
Planetary Symbol: Name in Roman/Greek Mythology: Pluto / Hades
Diameter: 2,390 km (1,485 miles)
[0.187 x Earth's] Orbital Semi-major Axis: (average distance from Sun) 39.48 AU
(Earth = 1 AU)
Mass: 12.5 x 1021 kilograms (0.0021 x Earth's)
Orbital Eccentricity: 0.2488
Density: 1,750 kg/m3 (0.317 x Earth's)
Maximum Distance from Sun: 49.31 AU ( 7.376 billion km or 4.583 billion miles )
Surface Gravity: 0.58 m/s2 (0.06 x Earth's)
Minimum Distance from Sun: 29.66 AU ( 4.437 billion km or 2.757 billion miles )
Surface Temperature: 40 to 50 K ( -233° to -223° C or -387° to -369° F )
Minimum Distance from Earth: 28.6 AU ( 4.28 billion km or 2.66 billion miles )
Tilt of Axis: 123o Orbital Inclination: (tilt of orbit plane) 17.16°
Rotation Period about Axis:(length of Pluto's day) 6.387 days (retrograde)
Revolution Period about the Sun: (length of Pluto's year) 248 years
Satellites: 3 known