Moons orbit PLANETS, and planets orbit the SUN (or star), for crying out loud.

[quote]Moons orbit PLANETS, and planets orbit the SUN (or star), for crying out loud.

[quote]Moons orbit PLANETS, and planets orbit the SUN (or star), for crying out loud.

A lot more then just planets orbit the sun... I don't know if that would work as a definitve awnser for what is a planet. As I said in the other thread, my vote would be to use gravity as the yard stick to mesure what is and isn't a planet. It is one of the fundimental forces behind the universe so why not use it to define planethood

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The reason Charon gets the nod as a planet and Titan does not is that the center-of-gravity of the Pluto-Charon system is between the surfaces of the planets. The center of gravity of the Saturn-Titan system is far beneath the surface of Saturn.

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In the case of Ceres, if any body in the Asteroid Belt would qualify, then Ceres would.

"I'm trying to maintain a shred of dignity in this world." - Me

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The reason Charon gets the nod as a planet and Titan does not is that the center-of-gravity of the Pluto-Charon system is between the surfaces of the planets. The center of gravity of the Saturn-Titan system is far beneath the surface of Saturn.

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In the case of Ceres, if any body in the Asteroid Belt would qualify, then Ceres would.

Where is the center-of-gravity between the Earth and our Moon?

What about Pallas and Vesta, they are pretty big too?

Moons orbit PLANETS, and planets orbit the SUN (or star), for crying out loud. Size or composition have nothing to do with it.

I think any solid body large enough to collapse into a spheroid should be called a planet. This would include the moon, Europa and Titan among others. This is a clear lower limit that eliminates smaller irregular bodies, in fact one of the few really clear thresholds of transition from one thing to another that exist in astronomy.

On the high end I think gas giants should be eliminated as planets and have their own classification, which they deserve. They are closer to brown dwarfs than to Earth-like bodies. If we dont consider brown dwarfs orbiting stars to be planets, why should we call gas giants planets? They would be eliminated by defining a planet as having a surface that is at least in principle an object on which humans could land - a surface that is observable in visible light or radar and is solid, liquid or a combination.

Since we humans are defining what a planet is, it should include only objects that are a little reminescent of Earth. Humans might someday mine the upper atmosphere of gas giants, but their potential as a home for humans is zero.

This definition would include the vast number of true planets ejected from solar systems during their formation - likely far more than the number remaining in orbit around stars.

So, have they made a decision yet?

Jim, you've brought up a number of ideas!

Re: any spherical object... Since many "moons" around the solar system undoubtedly started life as independent (if I can call it that) planets, and were captured by the gravitaional pull of larger planets, I believe your idea has good merit.

Jupiter may indeed be a brown dwarf, but I think composition should define a brown dwarf...the fuel of stars is hydrogen, and with a brown dwarf being a "failed star" which failed to reach critical mass, I would define it as being mostly hydrogen. While I am unaware of the composition of Jupiter and Saturn, I know Uranus and Neptune both contain large amounts of methane, a carbon based compound. Carbon is produced by very old stars. IMHO, this would disqualify them from "brown dwarfhood". (Sorry for the made-up words here)

Defining a planet as an object that is safe for humans to live on is a little narrow, but leaves some room for variation. I believe "planet" as a term could still include a wide class of objects. What would we call objects like Mars or Venus, for instance?

The planets ejected from solar systems are called "Planemos" (Planetary Mass Objects). Too little is known about these objects as yet, as only the most massive have been discovered. My feeling is that these objects will also be found to have an enormous variety. They will likely range from little more than wandering comets to brown dwarfs, hundreds of times the mass of Jupiter. Who knows? maybe some have a breathable atmosphere? ( A little fantastic, I know)

I have no doubt all these definitions will change many times as our race learns more about the cosmos.

Actually brown dwarfs are already defined at the low end as being massive enough to fuse deuterium, which happens around 19 Jupiter masses. At around 80-85 Jupiter masses, hydrogen fusion occurs and defines the upper limit of brown dwarfs.

I don't think a planet should be defined as being human habitable - just capable of being landed on by a spacecraft (in principle only). Which further requires its surface to be observable in visible light or radar.

Pluto would keep its planethood while three other bodies would be added, including Pluto's moon Charon, the asteroid Ceres and Brown's object 2003 UB313, which he nicknamed Xena".

Pluto would keep its planethood while three other bodies would be added, including Pluto's moon Charon, the asteroid Ceres and Brown's object 2003 UB313, which he nicknamed Xena".

For me there will always be only 8 planets.

For me there will always be only 8 planets.