What I've Learned

I want 900 planets for Christmas.

To help you shop--lest you buy me something that is not a planet--let's go over a few basics.

What makes a rock a rock?

Chemical bonding.

What makes a star a star?

Fusion at its core.

What makes a planet a planet?

Well, we can eliminate two things. Planets are not held together by chemical bonding, so planets, though they can contain rocks, are not rocks. And planets do not have fusion at their core, so planets are not stars.

A planet is an object in space that is not a rock and not a star.

The Hubble Telescope is an object in space that is not a rock or a star, so is Hubble a planet? What about comets? Are they planets?

Let me refine my definition by adding a few more qualifiers. To be a planet, an object must be held together and forced into a roundish shape by its own gravity. And to be a planet, an object must be in orbit around a star.

To this I would add, "and is not a moon."

Let's compare my definition of a planet with the one adopted in 2006 by the International Astronomical Union [AIU]. They say:

A planet is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

Around the Sun? So no other stars can have planets? Pardon me while I duh.

By my definition, Pluto is a planet.

By their definition, Pluto is not a planet. Neither is the Earth.

Why?

Because our home planet has about 10,000 near-Earth asteroids around its orbit. But that's okay, Jupiter's not a planet either. It has some 100,000 Trojan asteroids in its orbital path.

Since Earth has not cleared its orbital space, it is not a planet, at least according to the IAU's own definition. Yet the IAU claims Earth is a planet. Again, I duh.

The further out a planet is, the more objects there tend to be in its orbital path, the longer it takes to orbit the Sun, and the slower it tends to move. If Mercury, which the IAU says is a planet, were to swap locations with Pluto, Mercury could no more clear that orbital neighborhood than Pluto can now. Heck, if Earth were in Pluto's orbit, we couldn't clear that far-out path either. We can't even clear the one we've got.

So, using the IAU's definition--if any one were to take it literally, which one should be able to do with scientific definitions--our Solar System has only four planets: Mercury, Venus, Saturn, and Uranus.

Using my definition, Mercury is a planet, Earth is a planet, and Pluto is a planet. So are Venus, Mars, Jupiter, Saturn, Uranus, and Neptune. And so are about 900 other objects further out.

What? We can't have 900 planets. Children could never learn them all.

First of all, I think you underestimate the mental capacity of children.

Second of all, who says they would have to learn them all? Do children learn the names of all the rivers on earth? No, just the major ones and the ones nearby.

Third of all, scientific definitions should not hinge on concerns about what children can and can not learn, but on scientific sense and consistency.

And fourth of all, what's wrong with having 900 planets? Wouldn't that be enormously exciting?

Alan Stern, a planetary scientist at the Southwest Research Institute and leader of NASA's New Horizons mission to Pluto, said this in a recent interview on space.com:

"I like to tell audiences that we'd be better off with the "Star Trek" test for planethood than what the IAU adopted.

"Whenever a starship on Star Trek pulls up to something in space and turns on the viewfinder, the audience and the ship's crew know, within about a second, whether it's a planet, or a star, or another spaceship, or a comet, or a nebula or an asteroid. Whatever. They don't need to know what else is nearby, they don't need to conduct a survey of the solar system and integrate orbits to determine what objects have cleared their zone, they don't even need a Ph.D astronomer to advise them.

"They just know by looking. They know, because it's not all that hard to tell a planet when you see one, and they know that it doesn't matter if it's alone or in a flock. If it's big and roundish, not on fire with fusion, and not a spaceship, it's a planet."

To which I say amen.

What I want for Christmas is 900 planets. Check with Stern. He can point them out to you.