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I was surprised that, in order to reach a concencus, they had to a) define that a planet must orbit a star and b) that the star is explicitly stated to be the Sun.

This means that all the extra-solar planets that have been found over the last decade are not planets. It also leaves out the possibility of rogue-planets.

It's not that they excluded the two groups that I'm questioning; rather, why did have to narrow the definition so much in order to reach a concensus? What was there about extra-solar planet and rogue-planets that bothered them so much that they had to implicitly exclude them from the definition?

Any ideas?

Odds are that this is just a result of their thoughts being so tightly focused on objects in our solar system. No doubt that if this proves to be a serious point of contention, then they'll revise the definition.

...John...

One of the requirements in the planet definition can't be confirmed for any extrasolar planets. That is, has the object cleared t's orbital neighbourhood?
So narrowing on the Solar System is just putting the definition where the evidence is. Presumably referring to objects around other stars as "exoplanets" is still acceptable, because the term hasn't been officially defined in the way "planet" and "dwarf planet" have been.

As for "rogue planets", is'nt the problem to do with a theory of formation? Were they in a protoplanetary disc and ejected, or have they formed in some tiny nebula, like a sort of failed star.

Yeah, on further thought this definition is very poorly defined and full of holes...

As I said elsewhere though a revolt is brewing and I think the definition may get toppled. Only about 5% of the IAU (those actually present in the room where the vote was cast) were actually allowed to vote, and the rest are a bit miffed...

Malenfant wrote:Yeah, on further thought this definition is very poorly defined and full of holes...

I did'nt think restricting to the solar system was a problem. Would have been difficult to do anything else if "clearing the orbital neighbourhood" is a criterion for planethood.
A proper definition of "clearing" is needed though. Presumably objects in resonances, including 1:1 resonances are included as being "cleared". Otherwise the Trojans would disqualify Jupiter.
So it's more like gravitational "tidying" than "clearing". Like the difference between tidying your desk and clearing your desk.

I think "clearing of an objects orbital path" would mean of all bodies within a certain percentage of said objects own size...exactly what that percentage would be, I wouldnt like to be the one deciding

It would be really hard to apply this to other star system I reckon - take Tau Ceti for instance, the entire system is full of debris. Any planets orbitting there would no doubt have a huge amount of stuff in their orbits...

Neethis wrote:...Any planets orbitting there would no doubt have a huge amount of stuff in their orbits...

I doubt it. For a star of that age, I would find it absolutely unbelievable that a planet even the size or mass or Mercury would not have, by now, cleared its orbital region of substantial debris. I'm not talking about objects in resonance orbits or at the LaGrange points, or knocked into planet crossing orbits by a long series of gravitational encounters with other bodies, either. I mean objects that move in the same orbital path, give or a take a few tenths of an AU, as the planet.

...John...