Definition of a planet

On Tue, 13 Jun 2006 15:47:06 +0100, (Gareth Slee)
wrote:

How about?
Greater than 1000Km - Major Body
Between 20 and 1000Km - Medium Body
Less than 20 Km - Minor Body

Although you'd probably need more ranges.

Maybe something along those lines. But also you would consider
composition, origin, type of orbit, and possibly many other relevant
things.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On 13 Jun 2006 07:49:14 -0700, "Don't Be Evil"
wrote:

To me it's easy: There are eight planets, Mercury through Neptune...

Doesn't work. That leaves out Pluto. And in common usage, Pluto _is_ a
planet and is likely to remain one. So why confuse the issue by trying
to come up with a technical definition of "planet" at all. That
definition will almost certainly have to be adjusted over time, and will
still need to be qualified in most cases to avoid confusion with the
popular usage.

In my view, it's even easier. There are nine planets, Mercury through
Pluto. That's all there will ever be. There are also more objects,
farther out, that share some characteristics with the planets. We will
probably continue discovering more of them, and just what they actually
are will require something more than simply "planet" to make clear.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

steve wrote:
With this set to be finalised fairly soon in September 2006 I think it
is essential that a few sensible parameters are added.

1) A body in orbit around a star.

2) Size must be larger than Pluto ( or another agreed specified size
and mass) to give a minimum size UNLESS extra-terrestial life is found
on the planet when a much smaller size would be allowed.

3) If the size of a moon is similar (to a stated percentage e.g. 80%)
to the larger body in its group it to can be called a planet if it
meets (1) and (2)
Thus it is possibe to have binary planets.

4) It would probably be necessary to specify a maximum distance from
the star in which the planet is in orbit to dis-allow wandering rocks.


Should there be a maximum size for a planet?

What about brown dwarfs in orbit around a star?

Brown dwarfs give light, but they do not have fusion reactions
occurring as is necessary in the usual definition of a star.

What about the continuum of objects between the size of Jupiter and
brown dwarfs?

Where do we draw the line?

And on the low end, why shouldn't Ceres be the minimum planet size?

It has almost half the diameter of Pluto.

It is spherical.

It contains 25% of all the mass in the asteroid belt.

It fills a gap in the planet spacing predicted by Bode's Law.

It probably has more water (ice) than all the fresh water on Earth.

And it orbits the Sun and has a 9 hour day.


http://www.astrobio.net/news/modules...cle&sid= 1711

Ceres for 5th planet!

Dobule-A

Chris L Peterson wrote:
On 13 Jun 2006 07:49:14 -0700, "Don't Be Evil"
wrote:

To me it's easy: There are eight planets, Mercury through Neptune...

Doesn't work. That leaves out Pluto. And in common usage, Pluto _is_ a
planet and is likely to remain one.

Pluto has been a "planet" for only 75 years. I don't see a
problem with changing the designation. We've learned a
lot about the solar system since Pluto was discovered.
If we stop calling it a "planet", the common usage will
adjust over time until almost no one remembers that it
ever was called a "planet".

I like the orbit-based definition. Something based on
eccentricity or orbit-crossing seems to work. It gives us
eight major planets.

- Ed Kyle

Chris L Peterson wrote:

To me it's easy: There are eight planets, Mercury through Neptune...

Doesn't work. That leaves out Pluto. And in common usage, Pluto _is_ a
planet and is likely to remain one. So why confuse the issue by trying
to come up with a technical definition of "planet" at all. That
definition will almost certainly have to be adjusted over time, and will
still need to be qualified in most cases to avoid confusion with the
popular usage.

In my view, it's even easier. There are nine planets, Mercury through
Pluto. That's all there will ever be. There are also more objects,
farther out, that share some characteristics with the planets. We will
probably continue discovering more of them, and just what they actually
are will require something more than simply "planet" to make clear.

Planets are characterized by sensationalism.

If it isn't sensational, it's not a planet.

By that definition, Pluto easily makes it.

Xena still has a way to go, obviously.

But it is a promising start, right?

http://cosmic.lifeform.org

Don't Be Evil wrote:

To me it's easy: There are eight planets, Mercury through Neptune.
They are all in circular orbits.

Er ... no, they aren't.

Do you even know anything about space?

http://cosmic.lifeform.org

Chris L Peterson wrote:
In my view, it's even easier. There are nine planets, Mercury through
Pluto. That's all there will ever be. There are also more objects,
farther out, that share some characteristics with the planets. We will
probably continue discovering more of them, and just what they actually
are will require something more than simply "planet" to make clear.

That's an attractive option, but I don't think it will work in practice.
If there's a substantial body out there--say, Earth-sized--there will be
an almost irresistible inclination to call it a planet, whether by that
time it's the ninth planet, tenth, eleventh, whatever. I don't think
that excluding it on the basis of history will work. Some technical
definition will probably be needed.

I do think that a definition ought to include at least some aspect of
the dynamic behavior of the object. Sphericity, though it feels right,
still is arbitrary. David Knisely is a proponent of this definition,
but when pressed to identify a precise definition of spherical, failed
to do so (at least, to my satisfaction). Including a dynamic component
allows us to exclude the asteroids, which I think most agree is right.

As an example, we call Pan a satellite of Saturn, and not one of its
countless ring particles. Does Pluto play a role similar to Pan, with
respect to the rest of the Kuiper belt? Or is Pluto's role there not
significantly different from that of Ceres in the asteroid belt? Do we
have enough information yet to say?

Personally, I am not in favor of the "orbit crossing" criterion. After
all, Neptune's orbit also crosses Pluto--does that mean that we should
exclude Neptune? No, of course not, we say, because Neptune is so much
larger than Pluto. But this means that the criterion will be arbitrary
again, because surely if Pluto were, say, 6 Earth masses (as was once
suggested, long ago), we would certainly not exclude it on the basis
that the orbits crossed. Would we cross it out if it were Earth-sized?
Mars-sized? Moon-sized?

I don't envy the IAU this issue.

--
Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html

Brian Tung wrote:
Chris L Peterson wrote:
In my view, it's even easier. There are nine planets, Mercury through
Pluto. That's all there will ever be. There are also more objects,
farther out, that share some characteristics with the planets. We will
probably continue discovering more of them, and just what they actually
are will require something more than simply "planet" to make clear.

That's an attractive option, but I don't think it will work in practice.
If there's a substantial body out there--say, Earth-sized--there will be
an almost irresistible inclination to call it a planet, whether by that
time it's the ninth planet, tenth, eleventh, whatever. I don't think
that excluding it on the basis of history will work. Some technical
definition will probably be needed.

Why? Because a "*planetary* geologist" is legally barred from studying
a "large circum-solar body"?

Thomas Lee Elifritz wrote:
Don't Be Evil wrote:

To me it's easy: There are eight planets, Mercury through Neptune.
They are all in circular orbits.


Er ... no, they aren't.

Do you even know anything about space?

http://cosmic.lifeform.org



Sure he does. It's that empty volume between his ears.

Anonymous AtWork wrote:
Ed Kyle wrote:
Chris L Peterson wrote:
On 13 Jun 2006 07:49:14 -0700, "Don't Be Evil"
wrote:

To me it's easy: There are eight planets, Mercury through Neptune...
Doesn't work. That leaves out Pluto. And in common usage, Pluto _is_ a
planet and is likely to remain one.

Pluto has been a "planet" for only 75 years. I don't see a
problem with changing the designation.

Won't someone think of the children: "When it was learned that a new
model of the solar system at the American Museum of Natural History in
New York leaves out Pluto, kids began a letter-writing campaign urging
the museum to reinstate Pluto."

http://www.cbsnews.com/stories/2006/...n1294926.shtml

Re-identifying Pluto to something other than major planet
could be an excellent chance to get kids fired up about
astronomy and physics, etc. The letter writing campaign
above provides evidence of just that. I think it would be
a terrific worldwide educational opportunity.

- Ed Kyle