Download Where do Comets come from?

Module 18:
Solar System Debris
Activity 1:
Where Do Comets Come From?
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.
Are comets bound to the Solar System?
Newton’s laws of motion
The Conic Sections
determine the orbit of an object
around the Sun and are used to
predict comet trajectories.
Parabola
Comets follow paths on one of
the family of curves known as
conic sections.
Ellipse
Circle
Conic sections include closed
circles and ellipses as well as
two open curves called
parabolas and hyperbolas.
The periodic comets we have
examined follow trajectories on
Hyperbola
closed elliptical curves.
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 aren’t 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.
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.
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...
Limit of our telescopes
…and by extrapolation
we can work out the full
trajectory of the comet
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
Limit of our telescopes
These comets take hundreds of thousands of years to
complete even one orbit - so we’ll never see them again.
Hence the name “Long Period Comets”.
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 arm’s length away. These
long period comet orbits would loop out to a kilometer
away and back. No wonder their periods are so long!
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
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
150,000 AU
Note: this picture is
not to scale. The
Solar System
would be invisibly
small if drawn to
scale
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 Pluto’s orbit, and sometimes not
even that far. These comets only take a hundred years or so
to complete an orbit.
Pluto’s 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.
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 don’t 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.
Pluto’s Orbit
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.
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 we’re 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 Pluto’s satellite Charon. There are probably
several million smaller ice worlds out there in the same
region, too small to see with current telescopes.
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 Sun’s 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.
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)
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 Neptune’s 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.
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.
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!).
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, they’d 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.
The Real Puzzle - the Origin of
the Oort Cloud
So far, we’ve 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 wasn’t the Oort cloud destroyed long ago by some
passing star or giant molecular cloud?
For a long time, astronomers thought that the Oort cloud was
disturbed by passing stars. Their gravity would stir up the iceworlds and cause some of them to fall in towards the Sun
(and others to fly out into space)
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.
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.
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...
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.
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, see
http://www.ras.org.uk/html/press/pn99-32.htm
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 Matese’s website at
http://www.ucs.louisiana.edu/~jjm9638/matese.html
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.
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.
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?
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...
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 isn’t
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, we’ll 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 Sun’s gravity
cannot hold it).
A star passes particularly close and scatters the Oort cloud.
Or the Sun passes through a giant molecular cloud and loses
the Oort cloud.
… so, we’ve 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?
An Inner Oort Cloud?
We don’t 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 we’ve 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.
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 wouldn’t 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?
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.
?
The Oort cloud:
source of long period
comets. Formed from
comets scattered out
from the inner Oort
cloud?
Regularly destroyed.
Nemesis and
sundry other planetsized things?
?
Inner Oort cloud. Unobserved and does not
supply comets at present. Supplies comets
to the Oort cloud? No idea how it formed.
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.
Part of Comet Halley’s ion tail
“breaking off”
Image Credits
Comet Halley detachment event - NASA
http://pds.jpl.nasa.gov/planets/gif/smb/haldet.gif
Candiate Kuiper Belt Object - A. Cochran (University of Texas) and NASA
http://oposite.stsci.edu/pubinfo/PR/95/26.html
Stellar Disks - D. Padgett (IPAC/Caltech), W. Brandner (IPAC),
K. Stapelfeldt (JPL) and NASA
http://oposite.stsci.edu/pubinfo/PR/1999/05/index.html
Triton - NASA
http://nssdc.gsfc.nasa.gov/image/planetary/neptune/triton_close.jpg
The Moon & Earth - Clementine Mission, NASA
http://nssdc.gsfc.nasa.gov/imgcat/html/object_page/clm_usgs_19.html
Gaseous Pillars in M16 (Eagle Nebula) - J. Hester and P. Scowen (Arizona State
University), and NASA
http://oposite.stsci.edu/pubinfo/pr/95/44.html
Now return to the Module 18 homepage and read
more about comets in the Textbook Readings.
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