Pluto As A Planet or Not.....

Hello all,

As the designated "company astronomer" (as many of you also are),
whenever something astronomical creeps onto CNN, I usually field a
bunch of questions from colleagues.

I've been asked on more than one occasion about if Pluto is really a
planet or not. I tend to lean towards it not being a planet, but I
really don't know all the sides of the issue.

What I do know, which leads me to believe that it's not a planet:

Pluto isn't much bigger than our moon (but then again neither is
Mercury)
It has an inclined orbit around the Sun that is decidedly non-planetary

But then I'm a little confused about the atmosphere issue. Pluto does
have an atmosphere....sometimes....right? Doesn't it freeze and fall
to the surface depending on where it is in its orbit? Are there any
asteroids that have an atmosphere? How about any asteroids that are of
similar size that have a similar orbit to Pluto?

I just can't really make up my mind on the topic and was hoping those
of you more "in-the-know" could help out.




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On Tue, 16 Dec 2003 15:22:18 +0000 (UTC), "Kevin Rehberg"
wrote:

I've been asked on more than one occasion about if Pluto is really a
planet or not. I tend to lean towards it not being a planet, but I
really don't know all the sides of the issue...

There is no answer to this question. It all comes down to how you want to define
"planet" and there is no universally accepted definition. If you prefer "form
and function" than Pluto is a planet: it is of the right general size (perhaps a
bit small), shape, and orbit. If you prefer "origin" than it would seem that
Pluto is more likely related to KBOs.

Eventually, perhaps, a definition will be agreed on by all, and this will
determine whether Pluto is a planet or not. However, I predict that Pluto will
continue to chug away in its orbit regardless of the category chosen!

_________________________________________________

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

Kevin Rehberg:
...What I do know, which leads me to believe that it's not a planet:
Pluto isn't much bigger than our moon (but then again neither is
Mercury)
It has an inclined orbit around the Sun that is decidedly non-planetary

I just can't really make up my mind on the topic and was hoping those
of you more "in-the-know" could help out.

I think that this is more a question of semantics and opinion than a
question of science.

I come down on the side of Pluto being a planet because:

-- It has been called a planet since its discovery.
-- I'm a softie, and I'm very troubled by the wave of dismissals that
come 'round every Christmastide.
-- Pluto is a sizeable body that orbits the sun. It has a moon and an
atmosphere.
-- "Planet," we all know, means "wanderer." Pluto's odd orbit makes it
even more a wanderer than the other planets. On that basis we could
call Pluto a planet and find a new name for the other eight.
-- Most importantly, "planet" is almost certainly a better word than
any contrived, compound-adjective description of Pluto (planet-like
body, quasi-planet, planetoid, planetisimal, non-planet, etc.) would
be. Her Majesty's Own Language doesn't need more ugliness just now, and
Pluto doesn't need a new description.

Davoud

--
usenet *at* davidillig dawt com

Hi Kevin,

"Kevin Rehberg" wrote in message
news:34d91f2d4673afe12dc35642c536c59a.61639@mygate .mailgate.org...
Hello all,

As the designated "company astronomer" (as many of you also are),
whenever something astronomical creeps onto CNN, I usually field a
bunch of questions from colleagues.

I've been asked on more than one occasion about if Pluto is really a
planet or not. I tend to lean towards it not being a planet, but I
really don't know all the sides of the issue.

Even if you did, you won't get a consensus among Astronomers, even when
they know all the facts. Part of it is emotion driven, certainly for me. I
met Clyde Tombaugh once, and I loved the man's humor and uplifting spirits.
If we demote Pluto to a non-planet, then Mr. Tombaugh loses his distinction
of being the last discover of a planet within the Solar System.

But now, getting to your specific questions . . .

-------------------------------------------------

What I do know, which leads me to believe that it's not a planet:

Pluto isn't much bigger than our moon (but then again neither is
Mercury)

Actually, it's 2/3rds the size of our Moon, but size is a tricky issue.
Jupiter's moon Ganymede and Saturn's moon Titan are larger than Mercury, so
by size standards they qualify as planets, but they don't circle the Sun as
Pluto does, so we don't call them planets.

---------------------------------------------------

It has an inclined orbit around the Sun that is decidedly non-planetary

Both Mars and Mercury also have orbits tilted significantly more than the
other planets. I believe Mercury's is close to 7 degrees, Pluto's orbital
tilt is 17 degrees, but it's still clearly closer to the plane of the Solar
System than not.

----------------------------------------------------

But then I'm a little confused about the atmosphere issue. Pluto does
have an atmosphere....sometimes....right? Doesn't it freeze and fall
to the surface depending on where it is in its orbit? Are there any
asteroids that have an atmosphere? How about any asteroids that are of
similar size that have a similar orbit to Pluto?

Charon, an asteroid, develops an atmosphere at times. Atmosphere is not a
qualification for a planet; Mercury has almost none. Titan has a
significant atmosphere, but then there's that problem that it's orbiting
Saturn and not the Sun.

------------------------------------------------------

I just can't really make up my mind on the topic and was hoping those
of you more "in-the-know" could help out.

The only real criteria before the discovery of the Kuiper Belt objects is
that the body orbits the Sun and not another Solar System body, and that it
be larger than the largest asteroid Ceres, as far as I can ascertain. The
discovery of relatively large Kuiper Belt objects (although none are as
large as Pluto) was the thing that recently called into question Pluto's
status as a planet. The debate will continue.

Kevin Rehberg wrote:
As the designated "company astronomer" (as many of you also are),
whenever something astronomical creeps onto CNN, I usually field a
bunch of questions from colleagues.

Hey, this is one of MOPFAQ!

Q. Is Pluto still a planet?

A. Yes, it's still a planet. To be more precise, it is still
considered a major planet by the International Astronomical Union (IAU).

Pluto was discovered by Clyde Tombaugh in 1930, but people first began
searching for it in the 19th century after it was noted that Uranus and
Neptune weren't behaving quite as they ought to. Instead, they seemed
to be under the gravitational influence of some unseen body further out
than Neptune. Based on the perturbations seen in the orbits of these
two planets, the ninth planet was estimated to be smaller than either
of them, but still significantly more massive than the Earth. (Both
Uranus and Neptune are about 16 times as massive as the Earth.)

Soon after Pluto was discovered, it was found to be smaller than
expected. In fact, every new measurement of Pluto seemed smaller than
the last! At first, it was thought to be about the size of the Earth,
about 13,000 km across. By 1960, based on visual appearance and
assumptions of actual brightness, its estimated size had dropped to
about 6,000 km.

The final straw came in 1978, when Pluto's satellite, Charon, was
discovered. By observing the motion of a planet's satellite, scientists
can determine the mass of the planet. Pluto turned out to be a whopping
500 times *less* massive than the Earth. It is only 1/6 as massive as
our own Moon, in fact. The current best estimate of Pluto's size is
about 2,300 km in diameter. More recently, the observations of Uranus
and Neptune have been re-examined in light of better mass figures for
those two planets, and with those corrections, the discrepancies have
been completely explained. No further planet is necessary.

Back to Pluto. As a result of its diminutive size--Mercury, the next
smallest planet, is still about 4,900 km in diameter--and its unlikely
location out beyond the gas giants, many astronomers proposed changing
Pluto's status. The group of planets includes not only the familiar
nine from Mercury to Pluto, but also asteroids and comets. The big nine
are called major planets, and the rest are called minor planets.

Some people, then, were proposing to declare Pluto a minor planet. This
proposal became stronger as it became evident that there were a bunch of
similarly composed objects beyond Neptune, at around Pluto's distance
from the Sun. None of them was as big as Pluto, but it might just be a
matter of time until another body that large was discovered. One
version of the proposal suggested that Pluto be labelled minor planet
number 10,000 (it would retain its proper name, too, of course, just
like the asteroids).

The matter never got to a vote. Once word of the proposal got out,
there was such a tremendous backlash that the IAU promptly sent out a
statement that Pluto's status was unchanged. Apparently, there is still
a sense of pride that someone in the 20th century should have discovered
a planet.

My personal feeling on the matter is that Pluto is currently considered
a major planet, and we have no hard evidence yet that that consideration
is flat out wrong. Until we understand the nature of the outer solar
system in sufficient detail to comprehend how Pluto figures in it, we
should defer any changes in its status.

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.txt

Kevin Rehberg wrote:
I've been asked on more than one occasion about if Pluto is really a
planet or not.**I*tend*to*lean*towards*it*not*being*a*planet ,*but*I
really don't know all the sides of the issue.

The question really is: what are your friends and co-workers actually asking
you? Whether scientists call Pluto a planet? If Pluto is like Earth? How
big Pluto is?

It seems to me that which bodies we classify as planets and which we don't
is very largely a matter of custom. Mercury, Venus, Mars, Jupiter, and
Saturn are definitely planets because that's what they have been called
since Babylonian times. Nobody questions whether Uranus and Neptune are
planets because they are so obviously in a class with Jupiter and Saturn.
Earth itself is a planet because it was discovered that those bright lights
in the sky were also worlds orbiting the Sun. Pluto is referred to as a
planet because it was expected that another planet would be found - and
sure enough, there it was.

But what do all these bodies actually have in common? Well, basically they
are roughly spherical and have fairly circular orbits about the Sun (at
least in comparison with say, the comets). In most other respects -
composition, orbital period, magnetic field, number of satellites - they
vary wildly. About the only thing everybody now agrees on is that a planet
must orbit the Sun only, and even *that* is recent - not so long ago in
history the Sun itself was called a "planet"!

I wonder: Would aliens arriving in our solar system put Neptune and Mercury
into a class, but leave out the asteroids and Kuiper Belt objects, as we
have done? I very much doubt it! Perhaps they would note "4 planets [the
gas giants] and assorted debris", as Asimov put it. Or perhaps they would
call Ceres and Chiron (and the comets!) "planets", too. In any case, they
would not be likely to come up with the same 9 planet classification that we
use.

So what's the problem? I think it's that if we continue to call Pluto
"planet", then we'd logically have to call Quaoar and Eros "planets" too,
and we don't want to do that because it messes up our nice little ideals of
order. But the nomenclature we have now does not make much sense, so who
says we have to be logical?

For now at least, Pluto is a planet because that's what we call it.

Chris L Peterson wrote in message . ..
On Tue, 16 Dec 2003 15:22:18 +0000 (UTC), "Kevin Rehberg"
wrote:

I've been asked on more than one occasion about if Pluto is really a
planet or not. I tend to lean towards it not being a planet, but I
really don't know all the sides of the issue...

There is no answer to this question. It all comes down to how you want to
define
"planet" and there is no universally accepted definition. If you prefer "form
and function" than Pluto is a planet: it is of the right general size
(perhaps a
bit small), shape, and orbit. If you prefer "origin" than it would seem that
Pluto is more likely related to KBOs.

Eventually, perhaps, a definition will be agreed on by all, and this will
determine whether Pluto is a planet or not. However, I predict that Pluto will
continue to chug away in its orbit regardless of the category chosen!

True! :-)

And let's not forget Pluto has a moon. Additionally, a friend (and the wife of
the CTO of my company) recently completed some Pluto research using the Hubble
reported in ICARUS and mentioned in SKY & TELESCOPE where we find on page 27 of
the November 2003 issue:

ROCK AND ICE IN PLUTO AND CHARON. The Hubble Space Telescope's Fine
Guidance Sensors, whose main job is to keep the telescope pointed
steadily, continue to show off in their secondary role as the world's
most precise astrometric instruments (April issue, page 21).

Catherine Olkin and two colleagues at Lowell Observatory have used
them to measure, to a new level of accuracy, the separate motions of
Pluto and its satellite, Charon, around their common center of gravity.
The team finds that Charon contains 12 percent of the mass of Pluto and
has a density of 1.6 to 1.8 grams per cubic centimeter, compared to
Pluto's density of 1.8 to 2.1. The rock-to-ice ratios that these
numbers imply in both objects "are higher than expected for bodies
forming in the outer solar nebula," the group writes in the July ICARUS.
This suggests that both Pluto and Charon have somehow lost large amounts
of their volatile ices since they formed.

"Kevin Rehberg" wrote in message
news:34d91f2d4673afe12dc35642c536c59a.61639@mygate .mailgate.org...
Hello all,

As the designated "company astronomer" (as many of you also are),
whenever something astronomical creeps onto CNN, I usually field a
bunch of questions from colleagues.


Yep, I know the feeling.

I've been asked on more than one occasion about if Pluto is really a
planet or not. I tend to lean towards it not being a planet, but I
really don't know all the sides of the issue.

What I do know, which leads me to believe that it's not a planet:

Pluto isn't much bigger than our moon (but then again neither is
Mercury)
It has an inclined orbit around the Sun that is decidedly non-planetary

Well, I'm not sure I'd say its orbit is *that* non-planetary, though it's
certainly different.


But then I'm a little confused about the atmosphere issue. Pluto does
have an atmosphere....sometimes....right? Doesn't it freeze and fall
to the surface depending on where it is in its orbit? Are there any
asteroids that have an atmosphere? How about any asteroids that are of
similar size that have a similar orbit to Pluto?

Not sure about asteroids, though, IIRC, they're all significantly smaller.
The big issue with Pluto are the Kuiper Belt Objects (KBO), which appear to
be darned similar to Pluto and Charon. I personally believe Charon is
definitely a gravititationally-captured KBO. Pluto? Guess it depends on
which its composition, formation, geography, etc. more closely resembles -
inner rocky planet or KBO? I think that'll tell us whether Pluto is the
littlest planet or the heavyweight champ among the KBOs.


I just can't really make up my mind on the topic and was hoping those
of you more "in-the-know" could help out.


Unfortunately, those "in the know" can't seem to decide, either.

--Jason

"Brian Tung" wrote in message


"Back to Pluto. As a result of its diminutive size--Mercury, the next
smallest planet, is still about 4,900 km in diameter--and its unlikely
location out beyond the gas giants, many astronomers proposed changing
Pluto's status. The group of planets includes not only the familiar
nine from Mercury to Pluto, but also asteroids and comets. The big nine
are called major planets, and the rest are called minor planets."

-----------

Thanks for the great replies, everyone. It's nice to see the group
back on track and constructive again.

May I be so bold as to think of the possibilities of a modern day
astronomer discovering a minor planet well beyond the orbit of Pluto
AND slightly larger than Pluto - in essence....discovering a 10th
planet?? We know now that many objects with extremely low surface
brightness lurk in this neck of the woods. Could it be that the next
Clyde Tombaugh is among us right now, waiting for his/her big discovery?
Obviously as our technology improves, our chances improve - that is if
it IS out there.

Or maybe we'll never add a "10th planet" to our list, even if an object
slightly larger than Pluto IS discovered; as the same hubris that lead
to the no-questions-asked classification of Pluto as a planet currently
has us in the frame of mind that "our solar system has already been
mapped out, and the case is closed for any further planets". In other
words, if there was something significant out there,our superior
technology would have found it already.




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Thad Floryan wrote:
And let's not forget Pluto has a moon.

Mmm. But so does the asteroid Ida!