Should Large Moons Be Called 'Satellite Planets'?

The blockbuster science fiction film "Avatar" introduced today's moviegoers to the idea that a moon could be more than just rocks and craters. The imaginary moon Pandora, orbiting a gas giant planet in the Alpha Centauri system, is a veritable paradise with lush forests and a rich diversity of life. If the film's writer/producer James Cameron had consulted with Pluto researcher Alan Stern he might have even introduced a new term to sci-fi audiences: Satellite Planet.

Huh? What kind of oxymoron is this?

Stern, the principal investigator on NASA's New Horizons mission to Pluto, says that we need a practical and simple definition for a planet, in the aftermath of the International Astronomical Union's battle just what to call Pluto. In 2006 the IAU voted to dethrone Pluto and insist the solar system has only eight "real" planets.

One IAU motive was not to get overwhelmed with naming too many planets. But that's as misguided as teaching school children that the only eight major rivers in North America are the Colorado, Mississippi, Columbia, Missouri, Rio Grande etc. and that the other rivers are not truly "rivers."

Stern insists the number of planets in our solar system is more like 1,000 - the vast majority being icy dwarfs in the 10 billion-mile-wide Kuiper belt, the solar system's largest piece of real estate.

Stern and his planetary rebel alliance of astronomers are regrouping to offer a new planet paradigm in light of the exploding diversity of exoplanet discoveries over the past 15 years. He says that we should define a celestial object by its physical attributes only.

Therefore, a good practical definition of a "planet" is purely geophysical: it is a celestial body that has enough gravity to pull itself into a sphere. It can be made of rock, ice or gas. A line simply has to be drawn where the object is not so massive as to be a star or brown dwarf. Stern describes this as a common-sense definition that is free of all the qualifiers that burden the IAU definition.

Among many reasons the "anti-Copernican" IAU plant definition is doomed is because it insists an object's location defines what it is. Pluto lies in the cluttered Kuiper belt of solar system debris that is the remnant circumstellar disk that encircled the sun 4.5 billion years ago.

The problem is that this dynamical definition of a planet becomes logically self inconsistent for a variety of reasons - the biggest being that you'd have to reclassify an object if you moved it to a different location. If you moved Earth out to the Kuiper belt it would be dethroned to a dwarf planet because it wouldn't gravitationally dominate its orbit either. If you put Earth in orbit around Jupiter it would be demoted to a moon. Ceres could possibly be an icy dwarf that migrated to the asteroid belt.

That's where Stern's definition becomes all-encompassing. He says that big spherical moons should be considered planets too. Therefore the magnificent major moons of the solar system such as Europa, Ganymede, Callisto, and yes, even our moon should be called planets, according to Stern. He's even come up with a new term: Satellite Planet.

In other words, the object's location shouldn't matter. If you move from the United States to Australia, you are still a human. In fact Neptune's moon Triton is probably a captured dwarf planet from the Kuiper belt.

Conversely, Saturn's giant moon Titan is sort of a "Pandora on ice" because its surface is a chilly -300 degree Fahrenheit. But if you moved Titan to the inner solar system it would be a full-fledged terrestrial planet, larger than Mercury.

Titan is the most Earth-like world in the solar system. It has giant lakes, rivers, rainfall, drainage valleys, dune fields and volcanoes. It is the first place other than Earth where we've ever seen sunlight glinting off a body of liquid.

But because Titan is a moon, it doesn't have the same sex appeal for the public or Congressional lawmakers who must decide where NASA should spend its planetary exploration resources. If scientists were proposing a mission to the planet Titan, it would get boosted to a higher visibility and carry more gravitas.

Moons also must outnumber planets in our galaxy, so they are probably the most abundant locations for the genesis of life. We have minimally two such candidates for life in our solar system: Europa and Enceladus. The setting of the film "Avatar" captures what may be a common reality across the Galaxy.

Stern's planet definition would make life easy for interstellar explorers. But the IAU definition for a planet would befuddle even Captain Kirk:

(External scene of Enterprise)

(Cut to flight deck interior)

Sulu: "We're coming out of warp Captain, Gamma Draconis system dead ahead."

Kirk: "Spock, what's that planet on the forward view screen?"

Spock: "I don't know if it is a planet, Captain"

Kirk: "Spock, it's right there in front of us!"

Spock: "Before I can call it a planet captain I must conduct a survey of the entire Gamma Draconis system and assemble an ephemeris of the motions of all substellar bodies."

Kirk: (puzzled) "Explain, Spock"

Spock: "We need to know if this object on the view screen dynamically dominates its orbit."

Kirk: "What?!"

Spock: "Once a survey of all bodies in the systems is collected we'll need to run a dynamical simulation modeling planet migration and debris belt evolution over the past 4 billion years. I'll have an answer for you in 6.2 hours."

Kirk: "Spock, this is a planet! It's round, it has an atmosphere, oceans, I can even seen volcanoes! McCoy, what do you think?"

McCoy: "Damn it, Jim! I'm a doctor not a grammarian!"

Spock: "Respectfully Captain, If you review the Enterprise's memory banks, the science history log chronicles the definition for a planet set forth in by the International Astronomical Union in the Old World year 2006."

Kirk: "That document is nearly 300 years old! Hell, it was written when our ancestors hardly knew of any planets in the galaxy!"

Spock: "The planet definition has never been updated, it is still the reference guide as prescribed by Starfleet."

Kirk: "Unbelievable! Well if we can beam down onto it, it's a planet!"

Spock: "Excuse me Captain, I've just discovered that apparently the IAU definition only applies to our solar system... never mind."