Andy Diehl brings up a question worth considering over a tasty beverage. How come Pluto’s a dog and Goofy’s a dog but Pluto gets the collar end of the leash? Hardly seems fair.
Which brings us to that other controversial Pluto, the one that NASA’s New Horizon spacecraft visited last July. (News flash — on 28 October, NASA announced that they’d received the very last of the data NH accumulated during that 2½-hour visit.) Official Astronomy has reclassified Pluto from “planet” to “dwarf planet,” but NH honcho Alan Stern and much of the rest of the world say, “No way!”
The traditionalist position is, “But we’ve always called Pluto the ninth planet.” Well, “always” only goes back to when the preternaturally persistent Clyde Tombaugh discovered the object in 1930. At the time he found it Pluto was indeed the ninth “planet” out from the Sun. However, it spends about 10% of each orbit* closer to the Sun than the eighth planet, Neptune. So should we call it the “seven-and-a-fraction-th” planet?
No, because (1) that contravenes Official Astronomy’s rules, and (2) it’d be silly.
So what are the rules for what’s a planet?
- The object must be in orbit around its star.
- The object must be massive enough to be rounded by its own gravity.
- It must have cleared the neighborhood around its orbit.
“Rounded” is a bit tricky. It doesn’t mean “spherical” because if you spin a sphere, centrifugal forces move mass towards its equator. Earth’s equator is 13.3 miles further away from its center than its poles are. Miller’s Planet in the Interstellar movie is also a spheroid, even further deformed by elongation towards the black hole it orbits, yet it still rates as “rounded by its gravity” and qualifies as a planet.
“Clearing the neighborhood” means “my gravity dominates the motion of everything in my orbit.” Earth and Jupiter, both acknowledged planets, each have retinues of asteroids in the Trojan positions, at the same distance from the Sun as the host planet but in regions 60º ahead of or behind it. Even so, both planets often suffer messy encounters (remember Chicxulub and Chelyabinsk?) with asteroids and such that hadn’t gotten the memo.
Neptune meets all three criteria. Its gravity dominates Pluto’s motion even though Pluto’s in a separate orbit. For every three of Neptune’s trips around the Sun, Pluto makes exactly two. The gravitational converse doesn’t hold, though. Pluto’s mass is 0.1% of Neptune’s so the big guy doesn’t care.
This video, from an Orbits Table display at the Denver Museum of Nature and Science, shows a different Plutonian weirdness. We’re circling the Solar System at about 50 times Earth’s distance from the Sun (50 AU). Reading inward, the white lines represent the orbits of Neptune, Uranus, Saturn and Jupiter. The Asteroid Belt is the small greenish ring close to the Sun. The four terrestrial planets are even further in. The Kuiper Belt is the greenish ring that encloses the lot.
The yellow-orange line is Pluto’s orbit. Most of the Solar System lies within a thin pancake, the Plane of The Ecliptic. Pluto’s orbit is inclined 17º out of the Plane. That’s odd.
Theory says that the System evolved from an eddy in a primordial cloud of dust and gas. Gravity shrank that blob of stuff to form a disk at the eddy’s equator as it drew 99.9% of the system’s mass to form the Sun at the disk’s center.
Newton’s First Law is all about Conservation of Momentum. When applied to circular motion, it says that if you’re whirling in a certain plane, you’ll continue whirling in that plane unless something knocks you out of that plane. Hence, the Plane of The Ecliptic.
Pluto’s path is a puzzling challenge to the theory. It was only a minor puzzle until the 1990’s when astronomers discovered a plethora of Pluto-type objects outside of Neptune’s orbit. Most run way out of the Plane. Worst is Eris, at inclination 44º . Clearly, Pluto’s not special. It belongs to a large tribe that Astrophysicists must explain if they’re to claim to understand the Solar System.
~~ Rich Olcott
* – During its current 248-year orbit, Pluto was inside Neptune’s orbit between 1979 and 1999.