Definition of a planet

In article . com,
wrote:

"Purely historical. Mercury, Venus, Earth, Mars, Jupiter, Saturn,
Uranus, Neptune, and Pluto are planets. Nothing else in the solar
system is a planet. This definition is definitely historically valid,
but fails miserably under scientific meaning. What if a new object
larger than Pluto is found? What is it? Why is Pluto a planet but an
object 3/4 its size, like Sedna, is not? This definition, completely
lacking in scientific motivation, makes the word 'planet' meaningless
as a scientific description."

Does it matter that the term planet is "meaningless as a scientific
description"? Are scientists suffering from any confusion as a
result? Is research being held back?

It seems to me that if there were a need for a precisely defined term,
it would be clear what the required definition was. The fact that
there is no consensus on a definition suggests that we have no real
need for one.

-- Richard

Richard Tobin wrote:
It seems to me that if there were a need for a precisely defined term,
it would be clear what the required definition was. The fact that
there is no consensus on a definition suggests that we have no real
need for one.

I think you are correct that there is no technical need. I suggest
that there is a public relations need. Maybe it can be obviated by
consistent avoidance of the term, but I'm doubtful of that.

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

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.


The definition of 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.


Why should the existence of life be a criterion? Might not life also
exist on moons?


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.


Double-A

wrote:

: Eric Chomko wrote:
:
: : 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.
:
: The problem with 4 is that you have asteroids in between planets right
: now.

: By maximum distance, I am thinking big , something way past Pluto. I am
: not trying to keep out comets and asteroids but objects that may have
: formed around another star and then have been kicked out and are
: effectively in orbit round the sun but at such a distance that it will
: not hold them permanently.

: This distance could be something of the order of 1/4 lightyear.

: I think the definition we come up with for a planet should apply to
: objects in orbit around all stars and not just the Sun.

Agreed, but haven't ALL extrasolar "planets" discovered thus far been on
the order of gas giants and larger? IOW, only comets, Kuiper Belt objects
and the like will be found in our own solar system as they are simply too
small to be seen or sensed from 4+ LRs away.

I understand that your definition is to be generic and apply to all stars
including our sun, but there are practical limitations making all known
extrasolar orbitting bodies, planets, by every definition.

That said, I still believe that we have planets, asteroids and Kuiper Belt
Objetcs which are distinct from one another and comets as well (Oort Cloud
Objects?).

Eric

Astronomy is a science. Formal definitions are what it's based on.
Let the IAU figure it out.

Saul Levy


On Thu, 15 Jun 2006 04:34:54 GMT, Chris L Peterson
wrote:

On 14 Jun 2006 19:57:25 -0700, wrote:


Prior to his discovery of 2003UB313, Mike Brown of Caltech wrote a
lengthy opinion about what's a planet...

...In fact, this is the best analysis of the issue I've seen. Everybody
should read this before continuing the debate. I think he got it
right, though he changed his position with the discovery of 2003UB313.

Indeed, he did change his mind. I think most of what is fueling the
current IAU drive for a definition is the egos of people who are now
discovering new "planets".

In my usage, the nine historical planets (and I do consider Pluto to be
a historical planet) will always be called planets. Everything else will
be qualified in some way, regardless of what the IAU does. And I expect
that will usually hold true even in scientific publications, because
"planet" is simply too contaminated by non-scientific usage ever to be
used unambiguously. All the more reason that no attempt at a formal
definition should be made.

_______________________________________________ __

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

Brian Tung wrote:
Double-A wrote:
The definition of a star?

People often say "fusor": that is, an object that fuses hydrogen for
energy (or did, previously, if it's a "dead" fusor). Sometimes a
distinction is made between stars like our Sun that fuse ordinary
hydrogen, as opposed to brown dwarfs that fuse only deuterium. Since
deuterium is much scarcer than ordinary hydrogen, a brown dwarf's supply
of it doesn't last very long.

--
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


Saying that a star is a current or former fusor certainly covers most
bases. But I can still think of some exceptions. For instance, if
Jupiter were adrift in space, would its satellites be called moons or
planets?

Double-A

Double-A wrote:
Saying that a star is a current or former fusor certainly covers most
bases. But I can still think of some exceptions. For instance, if
Jupiter were adrift in space, would its satellites be called moons or
planets?

That is a reasonable question, but the answer has nothing to do with the
definition of a star; Jupiter is far too small to fuse even deuterium.
A body with the composition of Jupiter would have to be on the order of
13 times as massive to begin deuterium fusion. Approximately 80 Jovian
masses are needed to start the fusion of ordinary hydrogen.

If Jupiter were free-floating with its satellites, I think they would
just be called satellites (as they are now); the bigger question is what
we would call Jupiter. They are sufficiently exotic (in terms of public
awareness, not in terms of their physics) that I think one could get
away with calling them free-floating non-fusors, or something like that.

--
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

In article .com,
Double-A wrote:

Brian Tung wrote:
Double-A wrote:
The definition of a star?

People often say "fusor": that is, an object that fuses hydrogen for
energy (or did, previously, if it's a "dead" fusor). Sometimes a
distinction is made between stars like our Sun that fuse ordinary
hydrogen, as opposed to brown dwarfs that fuse only deuterium. Since
deuterium is much scarcer than ordinary hydrogen, a brown dwarf's supply
of it doesn't last very long.

--
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


Saying that a star is a current or former fusor certainly covers most
bases. But I can still think of some exceptions. For instance, if
Jupiter were adrift in space, would its satellites be called moons or
planets?

Double-A

The definition of a star we were given involved spherical symmetry,
thermal equilibirum, fusion, hydrostatic equilibirum, constant mass
yadda yadda with the proviso that many of these could be not relevant
in certain cases.

I like - "Big shiny thing" myself.


--
The greatest enemy of science is pseudoscience.

Jaffa cakes. Sweet delicious orangey jaffa goodness, and an abject lesson why
parroting information from the web will not teach you cosmology.

Official emperor of sci.physics. Please pay no attention to my butt poking
forward, it is expanding.

The definition of a star we were given involved spherical symmetry,
thermal equilibirum, fusion, hydrostatic equilibirum, constant mass
yadda yadda with the proviso that many of these could be not relevant
in certain cases.

I like - "Big shiny thing" myself.

How big? How shiny? How thing?

Greg

In article . com,
wrote:
The definition of a star we were given involved spherical symmetry,
thermal equilibirum, fusion, hydrostatic equilibirum, constant mass
yadda yadda with the proviso that many of these could be not relevant
in certain cases.

I like - "Big shiny thing" myself.

How big? How shiny? How thing?

Why must it always be about size? Any choice on size by itself is
always going to be completely arbitrary.

Even the "spherical compaction" idea, while at least vaguely more
interesting, is still arbitrary: these two objects are identical
in all respects, except this one had enough mass to become quite
spherical, whereas this other one is so very lumpy. But if they
are, in their formation, composition, and general dynamics, the
same, why would you bother distinguishing between them on an otherwise
unimportant feature?

The definition of stars and planets should tell us something about how
they formed, and how they act. Not how big they are. Who cares how
big a star is. What does is fuse now? What did it fuse when it was
born? What was it born from? What composition? Same for planets.
How did they form? Early or late? Close in, or far out? What are
they composed of?

Of course, the big problem here is, what is the purpose of defining
a planet? Is it a purely scientific purpose, or a popular (educational)
purpose, or both? Ideally, we want a definition that is scientifically
meaningful, and at the same time useful for elementary school astronomy.

A definition of planet that results in Jupiter, Earth, Mercury, Pluto,
"Xena", and Ceres all being called planets is not useful to
scientists. And when we get up to 15 or 20 planets that way, it stops
being useful in elementary school also.

So I'm not sure exactly what the definition should be, but I'm pretty
sure that eight planets is more useful, both scientifically, and
pedagogically, than 15-20.

tom