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,
Module 18: Solar System Debris
Where Do Comets Come From?
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.
The Conic Sections
Are comets bound to the Solar System?
Newton’s 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.
Conic sections include closed circles and ellipses as well as two open curves called parabolas andhyperbolas.
The periodic comets we have examined follow trajectories on closed elliptical curves.
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...
…and by extrapolation we can work out the full trajectory of the comet
Limit of our telescopes
Limit of our telescopes
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 we’ll never see them again. Hence the name “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.
Note: this picture is not to scale. The Solar System would be invisibly small if drawn to scale
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.
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.
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.
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 ice-worlds 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.
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...
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, seehttp://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
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.
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.
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...
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.
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?
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.
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?
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.
Part of Comet Halley’s 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.
Comet Halley detachment event - NASA
Candiate Kuiper Belt Object - A. Cochran (University of Texas) and NASA
Stellar Disks - D. Padgett (IPAC/Caltech), W. Brandner (IPAC), K. Stapelfeldt (JPL) and NASA
Triton - NASA
The Moon & Earth - Clementine Mission, NASAhttp://nssdc.gsfc.nasa.gov/imgcat/html/object_page/clm_usgs_19.htmlGaseous Pillars in M16 (Eagle Nebula) - J. Hester and P. Scowen (Arizona State University), and NASA
Now return to the Module 18 homepage and read more about comets in the Textbook Readings.
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