Where do Comets come from?

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Module 18 Solar System Debris
Activity 1
Where Do Comets Come From?
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Summary
In this Activity the main topics covered will be
(a) are comets bound to the Solar System? (b) the
origin of long-period comets and the Oort
Cloud, (c) the origin of short-period comets and
the Kuiper Belt, and (d) the Real Puzzle -
the Origin of the Oort Cloud.
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Are comets bound to the Solar System?
Newtons laws of motion determine the orbit of an
object around the Sun and are used to predict
comet trajectories. Comets follow paths on one of
the family of curves known as conic sections.
Parabola
Ellipse
Conic sections include closed circles and
ellipses as well as two open curves called
parabolas and hyperbolas.
Circle
The periodic comets we have examined follow
trajectories on closed elliptical curves.
Hyperbola
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So - where do the dirty snowballs come from?

• Is there some alien snowball factory
  manufacturing comets, out beyond Pluto, and
  hurling them in towards the Sun to melt?
• Our telescopes arent powerful enough to track
  the comets much beyond Neptune, but we can take
  careful note of their orbits closer in and
  extrapolate out to the rest of their trajectory.
• One possibility is that they came from
  interstellar space. Trillions of snowballs might
  be wandering around our galaxy, just like the Sun
  does.
• In this case, we should see some very fast moving
  comets the Sun is orbiting our galaxy at 200
  km/s and some of the comets should be coming in
  at comparable relative speeds.

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Anything coming in at these enormous speeds will
skim through the Solar System, barely changing
direction (on a hyperbolic orbit).
No such comet has ever been seen.
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So the comets are moving around the Galaxy with
our Sun - they are members of our Solar System
after all. They all seem to be moving in
elliptical orbits around the Sun this means we
can extrapolate out to where they come from and
where they are going.
We observe this part of the orbit...
and by extrapolation we can work out the full
trajectory of the comet
Limit of our telescopes
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As we have seen, comet orbits break up into two
classes long period comets and short period
comets.
Most comets have long periods their orbits seem
to stretch an enormously long way out.
orbit of a long period comet
These comets take hundreds of thousands of years
to complete even one orbit - so well never see
them again. Hence the name Long Period Comets.
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The Origin of Long Period Comets

• Long period comets are the most common ones, with
  at least 80 of comets having long periods.
  Hyukatake and Hale-Bopp were both examples of
  long period comets.

Their extrapolated orbits go out to phenomenal
distances from the Sun, typically between 20,000
and 150,000 AU. (Remember that Jupiter is 5 AU
from the Sun and Pluto averages 40 AU.) To put
this in perspective, if the Earth were one
centimetre from the Sun, Pluto would be an arms
length away. These long period comet orbits would
loop out to a kilometer away and back. No wonder
their periods are so long!
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These long period comets seem to come in from all
directions equally (or so we thought until late
last year). They are not confined to the plane
in which the planets orbit. Their orbits are
always very elliptical, and equal numbers orbit
the Sun clockwise and anticlockwise.
Orbits of the planets
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This led Dutch astronomer Jan Oort to propose
that our Solar System is surrounded, far far out,
by a vast sphere of ice worlds, called the Oort
cloud.
20,000 AU
Note this picture is not to scale. The Solar
System would be invisibly small if drawn to scale
150,000 AU
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The Origin of Short Period Comets
The other 20 of comets are quite different. As
we have seen, these short period comets have
orbits that only carry them a little way beyond
Plutos orbit, and sometimes not even that far.
These comets only take a hundred years or so to
complete an orbit.
Plutos Orbit
Unlike the long period comets, the orbits of the
short period comets all lie within about thirty
degrees of the orbital plane of the planets. Most
of them orbit the Sun in the same direction as
the planets.
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At first it was believed that the short period
comets were simply long period comets that got a
bit close to Jupiter (or some other planet) which
warped their orbits, shortening their periods.
Unfortunately, the details of this theory dont
work (it cannot explain their orbits). Instead,
Dutch-American astronomer Gerard Kuiper proposed
that they come from a new belt of comets, now
called the Kuiper Belt.
Plutos Orbit
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The Kuiper Belt is 3,000 times smaller and closer
in than the Oort cloud. The ice worlds in the
Kuiper Belt are more like an extension of the
planets they are orbiting roughly in the same
plane, and roughly in the same direction as the
planets.
So - this is getting pretty silly. We now have
not one but two mysterious, unobserved clouds of
ice-worlds beyond Pluto! And we still have no
idea why these clouds are out there, why they
have the shapes they do, and why comets ever
decide to leave these clouds and come down and
visit us, huddled down close to the Sun. This was
the situation for decades. But slowly telescopes
were getting better, and in the late 1980s, Jane
Luu and David Jewitt decided that it was now just
about possible to directly spot objects in the
Kuiper Belt.
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They succeeded! Kuiper Belt objects are REALLY
faint - very uninteresting to look at even in
Hubble pictures, like that on the previous page.
But they were there. Hundreds have now been
found, and were probably only finding the few
nearest, larger ones.
The ones found to date are relatively small
generally a few hundred kilometres across (though
they would make awesome comets if they ever came
any closer!), and typically between 40 and 50 AU
out from the Sun, though there are a few as large
as Plutos satellite Charon. There are probably
several million smaller ice worlds out there in
the same region, too small to see with current
telescopes.
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When the orbits of these Kuiper Belt Objects
(KBOs) were measured, something very curious was
found the orbits very carefully avoid any close
encounters with Neptune. The KBOs are said to be
on resonant orbits, so for example, they go
around the Sun three times for every four Neptune
orbits, or five times for every six. This way
they never get close.
Why is this? Perhaps all these ice worlds formed
in the protostellar disk, just like the rocks
that eventually formed the planets. The Suns
protostellar disk probably extended far beyond
Pluto, just as we see disks stretching out to
enormous distances around other new-born stars.
In the outer regions of the disk, the rocks
should have been mostly made of ice. Perhaps
trillions of ice worlds formed out here
originally. They may have weighed far more (in
total) than all the inner planets combined.
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Pictures of protoplanetary disks around new-born
stars. Notice that they are more than 500 AU in
size (Pluto is 40 AU from the Sun)
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Because there is so much space out here, these
ice blocks would not have collided together as
rapidly as rocks closer in. The gas giant planets
might have finished forming while the ice worlds
were still only tens or hundreds of km in size.
This would have been a dangerous time to be an
ice world! If you got too close to a gas giant
(particularly Neptune), its gravity would have
either flung you out into deep interstellar
space, or into the Sun. The vast majority of the
ice worlds within reach of Neptune would have
been destroyed in this way. Only a relative
handful would remain the Kuiper Belt as we see
it today. Though maybe, out beyond 50 AU where
Neptunes gravity cannot reach, the full density
of ice worlds might still remain. Perhaps, out
there where we cannot yet see them, they kept on
colliding, getting bigger and bigger until huge
ice-planets formed.
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Astronomers are involved in several searches for
these possible giant ice worlds. They would be
pretty faint, and there may only be a few close
enough to see. That means you have to survey most
of the sky, looking for really faint objects much
like stars, but which move slowly in their orbits.
In January 2000, a five year search for these
objects began on the Great Melbourne Telescope at
Mt Stromlo, Australia. The Great Melbourne
Telescope is an historic telescope which has now
been converted to an ultra-modern robotic
telescope, capable of running itself and the
whole survey completely automatically. Another
search, in collaboration with astronomers from
Taiwan, aims to look for the shadows these KBOs
cast when they block the light from distant
stars. If it works, this survey should be
sensitive to much smaller KBOs than any other
search.
The Great Melbourne telescope was tragically
destroyed in bush fires in February 2003.
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Why do KBOs occasionally leave their orbits in
the distant Solar System and come and visit us in
the inner Solar System? Why, in short, do they
occasionally turn into short period comets?
This is basically an unsolved problem. Collisions
between KBOs may occasionally knock them out of
their orbits and inward, but these collisions
should be fantastically rare (since there is a
LOT of space out there!).
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The current favourite theory blames the other gas
giants.
Remember that some of the KBOs are on very
special orbits that avoid ever getting close to
Neptune. If there were no other planets, theyd
stay in these orbits for ever. The other gas
giants, though further away, still exert their
gravitational pull, slowly warping the orbits of
the KBOs. Normally this warping is too small to
matter, but occasionally it builds up enough to
nudge a KBO out of its safe resonant orbit.
Neptune then grabs it (maybe taking a few million
years to do so), and either flings it out into
space, or in towards us.
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The Real Puzzle - the Origin of the Oort
Cloud

• So far, weve been discussing matters we think we
  know something about. Now it is time to get on to
  the real puzzle the nature and origin of the
  Oort cloud.

How did comets get out all that way? The Oort
cloud is far far too large to have been formed
from some extension of the protoplanetary disk -
and gas out there would have been far too tenuous
to have formed ice worlds. Why is the Oort cloud
a sphere, and not a disk like everything
else? Once you get comets out there, why do they
decide to come in and visit the distant Sun? And
why wasnt the Oort cloud destroyed long ago by
some passing star or giant molecular cloud?
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For a long time, astronomers thought that the
Oort cloud was disturbed by passing stars. Their
gravity would stir up the ice-worlds and cause
some of them to fall in towards the Sun (and
others to fly out into space)
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Suitable stars pass nearby every million years or
so (the next one due past is Gliese 710, which
will pass within a light-year of the Sun one
million years from now). As some comets will take
a million or more years to reach us, this could
explain the steady stream of comets we see.
This method of stars sending comets in towards us
is very inefficient, as most get scattered in
some other direction. For this to work, there
would need to be a trillion comets
(1,000,000,000,000) in the Oort cloud! Two rival
theories have been proposed to explain why the
long period comets leave the Oort cloud.
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One theory comes from Geology. Some claim that
the mass extinctions in the fossil record occur
regularly every twenty million years. The
evidence is pretty thin, but enough to convince
some people.
Another relevant fact is that most stars are
binary stars - two stars orbiting each other. Our
Sun is highly unusual in living all by
itself. Perhaps, these astronomers reasoned, the
Sun really is a binary. The other star orbiting
it would have to be very small and faint, and in
an extremely eccentric orbit.
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Nemesis!

• This other star was named Nemesis. At present, it
  must be at the far end of its elliptical orbit,
  or we would have seen it.
• Once every twenty million years, goes the theory,
  Nemesis sweeps through the Oort cloud. As it
  passes through, it scatters comets in all
  directions. A tiny fraction of these comets fall
  into the inner Solar System. All the planets are
  bombarded. The bombardment of the Earth kills
  large numbers of species.
• This theory has been around for ten years now,
  but it is not widely accepted, basically because
  the evidence is so ratty. However, recently two
  groups proposed a novel twist on this idea...

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Planet X

• In 1999, John Murray of the Open University in
  the UK was looking at the orbits of long period
  comets in detail. He was basically going through
  records of historic comets and the orbits that
  had been measured for them. As he studied them,
  he noticed a pattern an awful lot of them were
  coming from one particular arc on the sky. All
  these comets seemed to be coming from one
  particular region and from a distance of between
  30,000 and 50,000 AU away.

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• He suggested that there must be something causing
  this some giant planet, 40,000 AU away, moving
  slowly through the Oort cloud. This planet
  would have between 1 and 10 times the mass of
  Jupiter. As it drifts through the cloud, it
  scatters comets in all directions - some of them
  falling in towards the Sun.

For the science announcement, seehttp//www.ras.o
rg.uk/html/press/pn99-32.htm
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• John Matese, in the USA, had noticed the same
  thing independently. He is also claiming that
  something big is moving through the Oort cloud,
  between 30,000 and 50,000 AU away. He thinks that
  this thing is bigger than a planet - maybe a
  brown dwarf (a failed star).

Both groups agree on the distance, but disagree
on how heavy this thing is, and which way it is
moving. If correct, this is a great puzzle how
did something so big form so far out? This is far
beyond the edge of the protoplanetary disk no
planets should have formed out there. Could it be
some interstellar wanderer, just drifting through
the Oort cloud in passing?
Visit John Mateses website at http//www.ucs.lou
isiana.edu/jjm9638/matese.html
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• Another twist to the story was announced in March
  2000. A group of astronomers in the USA have
  been trying to determine how often the Moon has
  been hit by meteorites.

One way to do this would be to send hundreds of
Apollo missions, one to every crater in the Moon,
and bring back samples from them all to work out
the ages. You could then say when each crater was
formed, and hence estimate how the rate at which
the Moon was bombarded has changed with
time. These people, however, came up with a much
sneakier method. Whenever a meteorite hits, it
melts vast amounts of rock, which fly up into
space as tiny molten droplets. Eventually they
solidify and fall back to the Moon, where they
build up as great drifts of dust.
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• This is the moondust the astronauts were walking
  through the debris splatted out be millions of
  years of meteorite impacts.

So - any lunar soil sample should contain
solidified droplets of molten rock from hundreds
of different meteorite impacts. And indeed, the
samples taken home by the Apollo astronauts
contain many spherules, or solidified droplets.
By measuring the ratio of Argon-40 to Argon-39,
the team were able to identify spherules from 149
different craters, and date all these impacts.
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• What they discovered was surprising. As expected,
  they found that most meteorites hit the Moon when
  it was very young more than four billion years
  ago. The rate of impacts then dropped way down.

This continued, as expected, until 600 million
years ago. At this time, the rate of impacts came
back up again somewhat, and has stayed high ever
since! So something happened to increase the
number of comets in the inner Solar
System. Perhaps this is when we acquired this
mysterious object orbiting in the Oort cloud? Or
this is when some passing star perturbed Nemesis
(if it exists) into a more eccentric orbit - one
that passes through the Oort cloud?
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• Either way, 600 million years ago is a suspicious
  date - it is the beginning of the Cambrian
  period, the time when evolution on Earth really
  kicked into top gear.

Perhaps life-forms on Earth had to start evolving
more rapidly to cope with constant meteorite
bombardment. If life was easy there would be no
selective evolution to destroy weak species. Who
knows? Another possibility is the comet
bombardments dump organic chemicals on Earth.
Perhaps these chemicals had something to do with
the incredibly rapid evolution that started 600
million years ago. Or maybe the whole thing is a
mass of wild speculation based on pitifully
inadequate data...
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Where did the Oort Cloud come from?

• And now for the final mystery where did this
  Oort cloud come from in the first place? It is
  much too far out for planets and comets to have
  formed in the normal way (and besides, it isnt
  disk shaped).

Some believe that it is made up of comets that
formed in the inner Solar System but were then
flung out by the gravity of the giant planets
(these are all the missing objects from the
Kuiper belt). But there is another problem the
Oort cloud cannot last for very long. Every ten
or so million years, well have a particularly
close encounter with another star, or we will
pass through a giant molecular cloud. In either
case, the Oort cloud should be stripped away from
the Sun (since it is so far away form that Sun
that the Suns gravity cannot hold it).
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A star passes particularly close and scatters the
Oort cloud.
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Or the Sun passes through a giant molecular cloud
and loses the Oort cloud.
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so, weve probably lost the Oort cloud hundreds
of times since the Solar System formed.

• If true, this means that it must be replenished
  somehow.
• Could we pick up a new Oort cloud somewhere?
  Perhaps giant molecular clouds are full of comets
  waiting for us to pass through and snaffle some?
• The trouble with this is that the Sun is moving
  through the clouds rapidly anything we picked up
  would be on a hyperbolic orbit. We never see
  comets like this.
• So where can they come from?

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An Inner Oort Cloud?

• We dont know. But for what it is worth, here is
  one theory currently doing the rounds.
• Perhaps there is a third cloud of comets, mid-way
  between the Kuiper Belt and the Oort cloud (about
  10,000 AU out).
• This inner Oort cloud must contain a staggering
  1013 ice worlds that is 10,000,000,000,000 of
  them.
• Together, they would weigh more than all the
  planets (yes, even including Jupiter) combined!
  They would be the real solar system - everything
  weve talked about so-far would be just the tiny
  junk in the middle.
• These comets are too close to the Sun to be
  stripped away by passing stars or giant molecular
  clouds.

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Comet Re-supply Station

• Whenever something happens to strip away the
  normal Oort cloud, however, it also stirs up this
  hypothetical inner cloud. Some of its comets fall
  towards the Sun (producing a brief burst of
  middle-period comets), but the others wander
  outwards, replenishing the normal Oort cloud.
• Between close encounters with stars and molecular
  clouds, we wouldnt see medium-period comets a
  star has to come VERY close to stir these tightly
  bound ones up, and they are too far out for the
  planets to bother them.
• Where could this massive inner cloud have come
  from? Perhaps it is made up of all the comets
  that formed in the outer protoplanetary disk, or
  ones flung out from closer in by the giant
  planets?

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Bizarre Picture

• So we are left with a complex and somewhat
  implausible picture.

The Kuiper Belt. Source of short period comets
(pulled out by the gravity of the planets).
Formed with the planets.
Inner Oort cloud. Unobserved and does not supply
comets at present. Supplies comets to the Oort
cloud? No idea how it formed.
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Part of Comet Halleys ion tail breaking off
Comets have been seen to be transient objects in
our evolving Solar System, evaporating away part
of their mass each time they pass the Sun.
In the next Activity we will examine the
meteoritic swarm caused by dust and rock
fragments of comets and the impact of meteors on
Earth.
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Image Credits
Comet Halley detachment event -
NASA http//pds.jpl.nasa.gov/planets/gif/smb/halde
t.gif Candiate Kuiper Belt Object - A. Cochran
(University of Texas) and NASA http//oposite.stsc
i.edu/pubinfo/PR/95/26.html Stellar Disks - D.
Padgett (IPAC/Caltech), W. Brandner (IPAC), K.
Stapelfeldt (JPL) and NASA http//oposite.stsci.e
du/pubinfo/PR/1999/05/index.html Triton -
NASA http//nssdc.gsfc.nasa.gov/image/planetary/ne
ptune/triton_close.jpg The Moon Earth -
Clementine Mission, NASAhttp//nssdc.gsfc.nasa.go
v/imgcat/html/object_page/clm_usgs_19.htmlGaseous
Pillars in M16 (Eagle Nebula) - J. Hester and P.
Scowen (Arizona State University), and
NASA http//oposite.stsci.edu/pubinfo/pr/95/44.htm
l
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• Now return to the Module 18 homepage and read
  more about comets in the Textbook Readings.

Hit the Esc key (escape) to return to the Module
18 Home Page
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