Message-Id: <v01530500af610ff532ee@[207.90.162.217]>
Date: Fri, 28 Mar 1997 00:26:44 -0800
To: pinhole@exploratorium.edu
From: ronwong@unleashed.net
Subject: Re: origin of comets
Dan Grey made the following comments:
>>The problem I have with comets being located out of the plane of the solar
>>system is that this indicates that they are not of solar origin.
Well, maybe yes but probably no.
>>The sun and everything else (just about) in the solar system revolves
>>counterclockwise in a plane perpendicular to the sun's axis of rotation
>>indicating that everything spun outward from the sun. Since comets don't
>>follow this at all,
This last remark isn't quite true. It depends on whether you are talking
about short period comets or long period comets. The short period comets
are found to orbit in planes not unlike that of the planets. Whether they
orbit one way or the other depends on how they were perturbed by the
planets into their observed path.
>>that leaves me with three guesses about what's going on
>>with them:
>>
>>1. Comets are bodies from interstellar space captured by the sun's gravity
>>
>>2. Comets formed beore the sun condensed and started spinning, meaning that
>>they are remnants from the nebula that condensed to form our sun.
These two comments may be true for comets in the Oort cloud but more about
this later.
>>This might also explain why the Oort coud (If such a thing exists) orbits so
>>far out in space.
>>
>>3. Some event in the formation of the sun, perhaps the initial ignition of
>>the solar furnace, blasted material far out into space that later condensed
>>into the comets. ....
Let's start at the beginning.
*Current* thinking is constantly undergoing change, but basically the
material that was to later become the comets we see was believed to have
developed within the same accretion disk out of which our star and its
planets were to develop.
Both the sun and its planets cleaned up much of the early debris but much
of the material that we associate with comets remained in a disk-shaped
cloud that still exists beyond the orbit of Neptune. Planetary influences
cause some of this debris to fall into an orbit around the sun that crosses
the paths of the planets. These comets orbit more or less in the plane of
the planets and have periods of the order of a few hundred years or less
and are referred to as short period comets.
The planets also fling material in the cloud out of the planetary system.
It is from this material that the Oort cloud is formed (actually, there are
other theories regarding how the cometary material got to the Oort cloud
but I'm trying to keep this article interesting). Jan Oort was able to
show that over our solar system's lifetime the stars in our galaxy would
gravitationally alter the orbital characteristics of the comets in this
region so as to make them random in their distribution. What started as
material orbiting in the ecliptic plane at a great distance from the sun
ended up as a prolate spheriod centered about the sun. Thus the random
inclination of long period comets.
The vast bulk of comet material is in the disk shaped cloud orbiting beyond
Neptune (called the Kuiper belt, I believe). It supposedly expands out to
what is called the inner Oort cloud, gradually changing from a disk to the
spherical shape of the outer Oort cloud.
It has been postulated by some that as our solar system has moved through
the galaxy it has encountered gigantic molecular clouds which have stripped
the Oort cloud of its comets and replaced the original comets with its own.
Thus, there may be some truth to Dan's initial point. Some of the long
period comets may in fact be galactic in origin. The jury is still out on
this one.
In fact the jury is out on a lot of what has been said about comets. The
existance of the Oort cloud is more or less taken for granted right now.
Questions remain about the nature of the Kuiper belt as well as the inner
Oort cloud. A lot of other little details remain in question as well, but
that's what makes it all interesting.
Ron Wong
Lowell High School
San Francisco, CA