By Ron Cowen
Any fourth grader knows the names of the solar system’s planets. There’s Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Oh yeah, and Pluto, right?
Maybe not anymore.
An oddball compared with the other eight orbs, Pluto is smaller than Earth’s moon. It orbits the sun at a 17-degree tilt relative to the plane in which the other planets move. Pluto doesn’t belong to either of the two groups in which astronomers classify the other planets. It isn’t rocky like the so-called terrestrials–Mercury, Venus, Earth, and Mars. Nor is it one of the gas giants–like Jupiter, Saturn, Uranus, and Neptune. Pluto’s icy composition resembles that of a comet.