Moulton Lava

Moultonic Musings

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Location: New England, United States

Thursday, August 24, 2006


The International Astronomical Union has downgraded Pluto's status to something less than a planet.

Originally, the term planet referred to an object (other than the Earth's own moon) that 'wandered' against the background stars. Pluto was first predicted by Clyde Tombaugh, who noticed perturbations in the orbit of Neptune, and hypothesized that they were caused by a nearby object. Tombaugh worked out the math of the Newtonian gravitational mechanics and told astronomers where to look. In 1930, they spotted Pluto right where Tombaugh had predicted.

If you were to look down on the Solar System from a point above the plane of the ecliptic, you would see that as the Earth/Moon system traveled around the Sun, the Moon weaves an undulating trail as it swings toward and away from the Sun on its monthly orbit around the Earth. And the Earth, in turn, also meanders in a slight slalom, as it oscillates about the mutual center of mass of the Earth/Moon pair.

The Pluto/Neptune system is a little more complicated than that, but each perturbs the other into a distorted ellipse, with the swerving being most pronounced when the two objects make their closest approach to each other.

Pluto's orbit around the sun is inclined to the plane of the ecliptic, and it swings inside the orbit of Neptune at its closest approach to the Sun. It's not a captive moon of Neptune, but neither is it a totally independent planet. Rather, Pluto reflects one of the phenomena first predicted by Henri Poincaré — that three gravitating bodies weave a dance that is not quite periodic, but rather a mathematically complicated do-si-do.

The Solar System is full of small bodies — including asteroids and comets — that interact subtly with the larger bodies, perturbing them from perfectly elliptical orbits, with the perturbations being most pronounced during the rare events of a close encounter, like two ships passing in the night sky.

Astronomers are wrestling with the technical definition of 'planet' in the wake of the discovery of many other small objects which, like Pluto, orbit the Sun while dancing with other nearby companions. There is no shortage of planetesimals, of which Pluto is the best known. Coming up with a sharp definition separating planets from asteroids, moons, and cometary objects is proving to be a controversial task.