In 2006, the Sun’s family was truncated. The International Astronomical Union “demoted” the ninth planet of our solar system to “dwarf-planet” status. Pluto was handed its walking papers. It was disowned.
The move set off cries of disappointment among nerdy school kids and space enthusiasts alike. It also ruffled the feathers of planetary scientists, who disagreed with the I.A.U.’s ruling.
The controversy isn’t over. Astronomy magazine has called for a debate over Pluto’s nature.
Members of the I.A.U. have said that a celestial object, in order to be considered a planet, has to: (1) orbit the Sun; (2) have a gravity, colossal enough to pull itself into a sphere; and (3) have “cleared the neighborhood” of smaller bodies within its orbit.
Clearly, Pluto does orbit the Sun and is spherical. But it fails to meet the third criterion because it crosses into Neptune’s orbit.
As a planet forms, it tends to become the dominant force in its neighborhood. As it interfaces with other, smaller objects—asteroids, comets, and other debris—it sweeps them up or slingshots them away, paving a clear path along its orbit. That’s what it means to have “cleared the neighborhood” of smaller bodies within its orbit.
Pluto doesn’t, they claim. It and its five moons—Charon, Styx, Nix, Kerberos, and Hydra—are a few among a litter of icy cosmic crumbs, jostling in an area in the outer rim of the solar system called the Kuiper Belt, which extends from about 30 astronomical units out to 55 astronomical units (where one astronomical unit is the distance from Earth to the Sun, roughly 93 million miles.)
A house is a house whether it stands in a city or in the countryside. Shouldn’t a planet be a planet regardless of where it exists? At the Pluto-like distance of 40 astronomical units—40 times farther away from the Sun than we are now—Earth would not clear its orbit of asteroids, and so, would Earth then not be classified as a planet?
Pluto may not be a full-fledged planet, but it’s still worthy of a NASA mission. The New Horizons probe is on target to arrive at its doorstep on July 14, 2015. The flyby is likely to clarify whether or not it should be brought back into the planetary fold.