Planets and Plutons - IAU Draft Definition

I think Pluto should either be demoted to a KBO object, or, as is the current situation, a class definition be made, thus raising the status on some other objects out there. There could still be a big Planet X out there that may even defy the current norm of planet charateristics, such as a highly elliptical orbit, even a highly inclined orbit, or perhaps even a double planetary system - that will just open a can of worms about satellites! (seeing as Charons status was about to be raised to planet).

Seanie.

The part of "IAU Resolution 5 for GA-XXVI" that describes the planet definition, states "A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet." Member of the Planet Definition Committee, Richard Binzel says: "Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet."

What in there stops Pluto from being a planet? From that definition it seems clear that Pluto is a planet and charon is a satellite.

The 'overcome rigid body forces to achieve hydrostatic equilibrium' is clever. Thats the more scientific approach and is better than setting an arbitrary size limit.

I think that definition sounds fine, though if they start coming up with plutons and other silly definitions then I'll start complaining again!

Cheers,
~Al

The Charon designation is strange given that as Bart says, flies in the face of the proposed definition. Its a bit odd - perhaps the article is incorrect.

As for defining a body by its size, suppose that a KBO is discovered with a diameter of approximately 2000 km. Is it a planet or not? Wewon;t know until the diameter is accurately known. If the diameter is later found to be 1999 km, then the body is not a planet. But if some years later a better result from the latest orbiting observatory is 2001 km, then the body becomes a planet. What if the body is not perfectly spherical and has a minor diameter of 1950km and a major diameter of 2056km... My instinct tells me actual dimensions within the definition could cause problems.

Anyhow, as regards the debate going on in Prague, I guess we have no say and our opinions will have no effect - other than hopefully, folks with similar opinions in Prague will express them and then vote on the acceptance/rejection of the definition.

Then we will live with the consequences.

Oh yes, here's another quote:

"Astronomers also were being asked to get rid of the term 'minor planets', which long has been used to collectively describe asteroids, comets and other non-planetary objects. Instead, those would become collectively known as 'small solar system bodies."

So the MPC will become the SSSBC....

Dave

Here's another comment:

Earth will no longer be a planet.

The IAU draft definition says an object is a planet if it is in hydrostatic equilibrium.

The Earth isn't quite at hydrostatic equilibrium as its oblateness is still in recovery from the loss of the more extensive ice caps of the last ice age. This is an observation from laser tracking of satellites and the observed rate of change is what would be expected in the circumstances.

So, we'll have 'Solar System Body Hollywood' from now on I guess....

As for defining a body by its size, suppose that a KBO is discovered with a diameter of approximately 2000 km. Is it a planet or not? Wewon;t know until the diameter is accurately known. If the diameter is later found to be 1999 km, then the body is not a planet. But if some years later a better result from the latest orbiting observatory is 2001 km, then the body becomes a planet. What if the body is not perfectly spherical and has a minor diameter of 1950km and a major diameter of 2056km... My instinct tells me actual dimensions within the definition could cause problems.

It also brings the density into play. So a planet might not be a planet just because it has a different density. Another reason why the hydrostatic equilibrium bit is good.

If Charon is included then by that logic so should Titan, Ganymede, Callisto, Europa, Io, Rhea, Triton and a whole host more, including our own Moon!

What about other asteroids - are any of them spherical other than Ceres? I can't remember.

Apart from that I don't really have any problem with the concept - but i prefer ice dwarfs to plutinos. Just like I don't like the term cubewanos or however you spell it!