*PLUTO* - from Gas Giant to Comet Wannabee [004]

Hopefully, one of the ideas i'm conveying here is that
gradually, over a long period of time, the planet Pluto
has decreased in size. ahem Well, not exactly has
it decreased in size, of course, but astronomers'
estimates and measurements of Pluto's size, in terms
of its mass, its diameter and its density have gone from
that of a huge gas giant of from 6 to 10 Earth-masses
down through a no-atmosphere rocky planet just over
one Earth-mass, to its present rather tiny size.

Pluto's orbital inclination, or the angle off of the plane of
Earth's orbit, the ecliptic, was another nail in the coffin
of its perceived large size. Generally, the inclinations of
the planets are small. They circle the Sun in so nearly
the same plane that if you make a small enough three-
dimensional representation of the planetary system out
as far as Neptune, it would all fit comfortably into a pizza
box.

Once more the smallest planet is a bit of an exception.
While inclinations are usually 3 degrees or less, small
Mercury's is 7 degrees. And if a high orbital inclination
implies a small mass, then what are we to make of
Pluto's orbit that has an inclination of some 17 degrees?

Okay, so there may or may not be a connection between
inclination and mass. Pluto's high inclination might not be
significant. But once again we are forced to think.

Pluto's rotation period also puts its size in question. It has
been found to rotate once every 6.39 days. On the whole,
it seems that the larger a planet is, the faster it rotates on
its axis. With most of the planets' rotational periods being
measured in hours (Venus and Mercury are notably much
slower, but the Sun's tidal influences are at play), here is
the ever shrinking Pluto weighing in at almost a week!

This seems to be the mark of a very small planet. Again,
this may be a coincidence, but we now have three
parameters--orbital eccentricity, orbital inclination, and
rotational period--all of which appear to mark Pluto as
very small. How far can coincidence stretch?

What was needed was a direct measurement of Pluto's
diameter. As far away as it was, this was not an easy
task. It was tackled in 1950 by none other than the KBO
man himself, the astronomer Gerard Peter Kuiper
(1905 - 1973). Using the then-new 200-inch Palomar
telescope, Kuiper turned it on Pluto and tried to estimate
the width of the tiny dot of light. One thing that made it
harder was because Pluto's tiny orb twinkles a bit and
magnifying its size by telescope also magnifies the
twinkles.

The best Kuiper could do was to estimate that Pluto's size
was 0.23 seconds of arc. Neptune by comparison is never
seen as less than 2.2 seconds of arc. Pluto's apparent
width, then, is about one-tenth that of Neptune.

An apparent width of 0.23 seconds of arc, allowing for
Pluto's distance, would mean that its diameter would be
something like 6,100 kilometers (3,800 miles). This would
make our incredible shrinking planet considerably smaller
than Earth. It would make Pluto smaller even than Mars,
which has a diameter of 6,790 kilometers (4,220 miles).

So now, instead of being the fifth largest planet, Pluto is
the eighth largest with only Mercury, among the major
planets, smaller. Oops! Not everyone accepted Kuiper's
figure. Many scientists wanted to hold on to the "Pluto is
bigger than Earth" idea, and measuring the diameter the
way Kuiper did it was just not too convincing. However,
there *is* another way to skin this cat.

In our next article...

*PLUTO* - from Gas Giant to Comet Wannabee [005]

....we will see how every once in a while, Pluto, as it moves
slowly across the sky, passes near a dim star. If it happens
to move directly in front of the star (an "occultation"), the
star will wink out for a period of time. Using this method of
measuring the diameter, it can be worked out with a fair
amount of accuracy.

--
happy days and...
starry starry nights!

Painius

"Jonathan Silverlight" wrote in message...
...

Painius writes in message...


Pluto's rotation period also puts its size in question. It has
been found to rotate once every 6.39 days. On the whole,
it seems that the larger a planet is, the faster it rotates on
its axis. With most of the planets' rotational periods being
measured in hours (Venus and Mercury are notably much
slower, but the Sun's tidal influences are at play), here is
the ever shrinking Pluto weighing in at almost a week!

This seems to be the mark of a very small planet.

I think you're reaching a bit there :-)
Granted, the giant planets spin very fast because there's no way for
them to lose the angular momentum they had when they were huge gas
clouds, but isn't Pluto's rotation period the result of being in tidal
lock with Charon?
If the Earth's moon was as big in proportion the day would be a week or
more, and we probably wouldn't be having this discussion.
--
"It is written in mathematical language"
Remove spam and invalid from address to reply.

Excellent point, Jonathan! While i was writing this part,
i thought of another reason to think that rotational period
is *not* connected with size.

While the periods of the gas giants follow the rule of "the
smaller, the slower"...

EARTH
PLANET MASSES ROTATION PERIOD
Jupiter 318 9 hours, 50 minutes
Saturn 95 10 hours, 14 minutes
Uranus 15 17 hours, 15 minutes
Neptune 17 16 hours, 48 minutes

....and Mars, considerably smaller than Earth, rotates slightly
slower than Earth (24 hours, 37 minutes)...

....the belt asteroids break the rule dramatically. The largest
of them, Ceres, rotates on its axis every 9 hours, 4 minutes,
and Ceres is a bit smaller than Pluto. A glance at the table
toward the bottom of this page...

http://history.nasa.gov/SP-345/ch9.htm

shows rotational periods of the asteroids to be on the same
order as the gas giants, all of them quite a bit smaller than
Ceres and Pluto, and most of them spinning about the same
speed as--or faster than--Jupiter!

So it does appear that, at least in the case of rotational
period, coincidence wins.

happy days and...
starry starry nights!

--
Do you have yourself a dream?
Are you burning with desire?
If no dream, you have no steam
To fan your ember into fire!
Do you have yourself a dream?

Paine Ellsworth

"Jonathan Silverlight" wrote in message...
...

Painius writes in message...


Pluto's rotation period also puts its size in question. It has
been found to rotate once every 6.39 days. On the whole,
it seems that the larger a planet is, the faster it rotates on
its axis. With most of the planets' rotational periods being
measured in hours (Venus and Mercury are notably much
slower, but the Sun's tidal influences are at play), here is
the ever shrinking Pluto weighing in at almost a week!

This seems to be the mark of a very small planet.

I think you're reaching a bit there :-)
Granted, the giant planets spin very fast because there's no way for
them to lose the angular momentum they had when they were huge gas
clouds, but isn't Pluto's rotation period the result of being in tidal
lock with Charon?
If the Earth's moon was as big in proportion the day would be a week or
more, and we probably wouldn't be having this discussion.
--
"It is written in mathematical language"
Remove spam and invalid from address to reply.

Excellent point, Jonathan! While i was writing this part,
i thought of another reason to think that rotational period
is *not* connected with size.

While the periods of the gas giants follow the rule of "the
smaller, the slower"...

EARTH
PLANET MASSES ROTATION PERIOD
Jupiter 318 9 hours, 50 minutes
Saturn 95 10 hours, 14 minutes
Uranus 15 17 hours, 15 minutes
Neptune 17 16 hours, 48 minutes

....and Mars, considerably smaller than Earth, rotates slightly
slower than Earth (24 hours, 37 minutes)...

....the belt asteroids break the rule dramatically. The largest
of them, Ceres, rotates on its axis every 9 hours, 4 minutes,
and Ceres is a bit smaller than Pluto. A glance at the table
toward the bottom of this page...

http://history.nasa.gov/SP-345/ch9.htm

shows rotational periods of the asteroids to be on the same
order as the gas giants, all of them quite a bit smaller than
Ceres and Pluto, and most of them spinning about the same
speed as--or faster than--Jupiter!

So it does appear that, at least in the case of rotational
period, coincidence wins.

happy days and...
starry starry nights!

--
Do you have yourself a dream?
Are you burning with desire?
If no dream, you have no steam
To fan your ember into fire!
Do you have yourself a dream?

Paine Ellsworth