What makes a Planet a Planet?

Question

In 2006 the IAU demoted Pluto from the status of ‘Planet’ and created a new class of objects call ‘Dwarf Planet’. This new classification didn’t seem to make anyone happy. In the next couple of weeks there will be a conference which will reopen this debate. This should be interesting. So, what exactly should the definition of a ‘Planet’ be? Big enough to be round? Cleared it’s orbit of other objects? Have an atmosphere? Not be orbiting another ‘Planet’? Or something more arbitrary? All these definitions create a grey area. So I ask for wisdom from Yahoo Answers; What is it that makes a Planet, a Planet?

http://www.space.com/scienceastronomy/080805-st-planet-debate.html

Answer 1

I'd prefer a definition that is free of dynamic context. It should work in our solar system, and in every other solar system.  It should be easy to tell what is and isn't a planet.

It's mass should be large enough that it will be held together gravitationally in hydrostatic equilibrium.  Smaller than this is a minor planet, sometimes called an asteroid.

It's mass should be small enough that it never undergoes significant fusion.

That's it.  So Ganymede is a planet.  The Moon is a planet. Ceres is a planet. There are lots of them.  Get over it.  You could also have rogue planets.  It is currently thought that the process of star and planet formation should typically result in a significant sized planet getting ejected from the system.  There should lots of rogue planets.

We also need a definition for Moons.  They keep finding smaller and smaller ones.  Does Saturn have trillions of Moons?  In any case, Ganymede could be both a planet and a moon.  Phobos could be a moon and a minor planet. But i can't think of a good arbitrary lower bound on the size of a moon.  You could just say 10 km.  But i'd want something that has to do with it's physical characteristics.

Answer 2

I really feel public outcry has much more to do with sentimentality  than science. The astronomy community (IAU) made a correct decision the first time around.From the web:The 2006 definition of "planet" by the International Astronomical Union (IAU) states that in the solar system a planet is a celestial body that: (1) is in orbit around the Sun, (2) has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape, and (3) has "cleared the neighbourhood" around its orbit.

A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet", which is not a type of planet, while a non-satellite body fulfilling only the first criterion is termed a "small solar system body" (SSSB). Initial drafts planned to include dwarf planets as a subcategory of planets, but because this would have led to the addition of several dozens of planets in the Solar System, this draft was eventually dropped. The definition was a controversial one, and has drawn both support and criticism from different astronomers but remained in use.

According to the definition, there are currently eight planets and four dwarf planets known in the solar system. The definition distinguishes planets from smaller bodies, and is not used outside the solar system, where smaller bodies cannot be found yet. Exoplanets are covered separately under a complementary 2003 draft guideline for the definition of planets, which distinguishes them from dwarf stars, which are larger

Answer 3

The current IAU definition of "planet" has been criticized for several reasons.  First, the requirement that a planet should have fully cleared its orbital zone would disqualify Earth, Mars, Jupiter, and Neptune from being planets.  Second, the dividing line between "planet" and "dwarf planet" has been deemed as arbitrary by some critics.  Third, the definition does not cover extrasolar planets.

The IAU definition will most likely be revisited and challenged during its next Congress in 2009, to be held in Rio de Janeiro, Brazil.

To borrow a little tune from the WITTENBURG DOOR magazine, the mechanism for determining the current IAU definition could be summed up as this:

"Pluto may be a plutoid, halle-luuuuu-jah,

Pluto may be a dwarf planet, halle-luuuuu-jah,

Besides, Eris is larger, halle-luuuuu-jah,

We'll take a vote, and then we'll see!  Halle-luuuuu-jah!"

Answer 4

The IAU definition of a planet is fine.

Pluto has many traits that make it different from the planets, so having a way to define it as something else is actually useful, because it really helps with teaching solar system science.

If we do decide that dwarf planets should be regular planets it will be just as annoying as the demotion of Pluto. Suddenly we will have Eris and Ceres as planets and then people will argue for Sedna, Quaoar and a whole host of other Kuiper belt objects to be called planets...