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The Oort Cloud
The Oort cloud is an
immense spherical cloud
surrounding the planetary
system and extending
approximately 3 light years,
about 30 trillion kilometers
from the Sun. This vast
distance is considered the
edge of the Sun's orb of
physical, gravitational, or
dynamical influence.
The Oort Cloud
Within the cloud, comets are typically tens of
millions of kilometers apart. They are weakly
bound to the sun, and passing stars and other
forces can readily change their orbits, sending
them into the inner solar system or out to
interstellar space. This is especially true of
comets on the outer edges of the Oort cloud.
The structure of the cloud is believed to
consist of a relatively dense core that lies near
the ecliptic plane and gradually replenishes
the outer boundaries, creating a steady state.
One sixth of an estimated six trillion icy objects
or comets are in the outer region with the
remainder in the relatively dense core.
The Oort Cloud
In addition to stellar perturbations
where another star's Oort cloud passes
through or close to the Sun's Oort
cloud, are the influences of giant
molecular clouds and tidal forces. A
giant molecular-cloud is by far more
massive than the Sun. It is an
accumulation of cold hydrogen that is
the birthplace of stars and solar
systems. These are infrequently
encountered, about every 300-500
million years, but when they are
encountered, they can violently
redistribute comets within the Oort
cloud.
The Oort Cloud
Tidal forces affecting the Oort cloud
come from stars in the Milky Way's
galactic disk with some pull from the
galactic core. The tide results from the
sun and comets being different
distances from these massive
amounts of matter. The force on the
comets from these tides is greater
than the perturbations of passing
stars, and comets beyond 200,000 AU
are easily lost to interstellar space.
This pull contributes to the steady
state which replenishes the outer
comets that are randomly distributed
away from the ecliptic plane.
The Oort Cloud
The total mass of comets in the
Oort cloud is estimated to be
40 times that of Earth. This
matter is believed to have
originated at different distances
and therefore temperatures
from the sun, which explains
the compositional diversity
observed in comets.
The Oort Cloud
Typical noontime
temperatures are four
degrees Celsius above
absolute zero. As
temperatures move toward
absolute zero, the kinetic
energy of the molecules
approach a finite value.
Absolute zero should not be
considered a state of zero
energy without motion.
There still remains some
molecular energy, although
it is at a minimum, at
absolute zero.
The Oort Cloud
The Oort cloud is the source of long-period comets and possibly
higher-inclination intermediate comets that were pulled into
shorter period orbits by the planets, such as Halley and SwiftTuttle. Comets can also shift their orbits due to jets of gas and
dust that rocket from their icy surface as they approach the sun.
Although they get off course, comets do have initial orbits with
widely different ranges, from 200 years to once every million
years or more. Comets entering the planetary region for the first
time, come from an average distance of 44,000 astronomical
units.
The Oort Cloud
Long period comets can appear at any time and come
from any direction. Bright comets can usually be seen
every 5-10 years. Two recent Oort cloud comets were
Hyakutake and Hale-Bopp. Hyakutake was average in
size, but came to 0.10 AU (15,000,000 km) from Earth,
which made it appear especially spectacular. HaleBopp, on the other hand, was an unusually large and
dynamic comet, ten times that of Halley at comparable
distances from the sun, making it appear quite bright,
even though it did not approach closer than 1.32 AU
(197,000,000 km) to the Earth.
The Oort Cloud
Recognition of the Oort cloud gave explanation to the age old
questions: "What are comets, and where do they come from?" In
1950, Jan H. Oort inferred the existence of the Oort cloud from the
physical evidence of long-period comets entering the planetary
system. This Dutch astronomer, who determined the rotation of the
Milky Way galaxy in the 1920's, interpreted comet orbital distribution
with only 19 well-measured orbits to study and successfully
recognized where these comets came from. Additional gathered
data has since confirmed his studies, establishing and expanding
our knowledge of the Oort cloud.
The Oort Cloud
Comets also are cosmic debris, probably
planetesimals that originally resided in the
vicinity of the orbits of Uranus and Neptune
rather than in the warmer regions of the asteroid
belt. Thus, the nuclei of comets are icy balls of
frozen water, methane, and ammonia, mixed
with small pieces of rock and dust, rather than
the largely volatile-free stones and irons that
typify asteroids. In the most popular theory, icy
planetesimals in the primitive solar nebula that
wandered close to Uranus or Neptune but not
close enough to be captured by them were flung
to great distances from the Sun, some to be lost
from the solar system while others populated
what was to become a great cloud of cometary
bodies, perhaps 10 trillion in number. Such a
cloud was first hypothesized by the Dutch
astronomer Jan Hendrik Oort.
The Oort Cloud
In the original version of the theory, the Oort cloud extended tens of
thousands of times farther from the Sun than the Earth, a significant
fraction of the way to the nearest stars. Random encounters with
passing stars would periodically throw some of the comets into new
orbits, plunging them back toward the heart of the solar system. As a
comet nears the Sun, the ices begin to evaporate, loosening the
trapped dust and forming a large coma that completely surrounds
the small nucleus, which is the ultimate source of all the material.
The solar wind blows back the evaporating gas into an ion tail, and
radiation pressure pushes back the small particulate solids into a
dust tail. Each solid particle is now an independently orbiting
satellite of the Sun, and the accumulation of countless such
passages by many comets contributes to the total quantity of dust
particles and micrometeoroids found in interplanetary space.
The Oort Cloud
The total mass contained in all the comets is highly uncertain.
Modern estimates range from 1 to 100 Earth masses. Part of the
uncertainty concerns the reality of a hypothesized massive "inner
Oort cloud" -- or "Kuiper belt" (if the distribution is flattened)--of
comets that would exist at distances from the Sun 40 to 10,000
times that of the orbit of the Earth. At such locations, the comets
would not be much perturbed by typical passing stars nor by the
gravity of the planets of the solar system, and the comets could
reside in the inner cloud or belt for long periods of time without
detection. It has been speculated, however, that a rare close
passage by another star (possibly an undetected companion of the
Sun) may send a shower of such comets streaming toward the inner
solar system. If enough large cometary nuclei in such showers
happen to strike the Earth, the clouds of dust and ash that they
would raise might be sufficient to trigger mass biological extinctions.
An event of this kind appears especially promising for explaining the
relatively sudden disappearance of the dinosaurs from the Earth.
The Oort Cloud
What’s in the Oort Cloud?
February 13, 2000: No one has ever seen
the Oort cloud, that spherical envelope of
comets and their residues that surrounds
our Solar System. No one has ever
measured its size and density or counted
the objects in it. Nor is anyone likely to do
so in the foreseeable future: the Oort cloud
is too distant, and the objects in it too
small and too dim to be detected by our
instruments.
The Oort Cloud
What’s in the Oort Cloud?
And yet, not only do scientists agree that
this ethereal cloud is out there, but they
are so confident that they actually argue
about its exact composition and
characteristics. A paper by Alan Stern of
the Southwestern Research Institute
and Paul Weissman of JPL in the
February 1 issue of Nature suggests
that the cloud is made up of smaller and
probably fewer objects than previously
thought. By using computer simulations
of conditions in the Solar System 4.5
billion years ago, the researchers
concluded that many of the objects that
could have ended up in the Oort cloud
were diverted from their course or
ground to dust before they never had a
chance to reach their cloudy destination.
The Oort Cloud
Origins
While the Oort cloud has probably been around for several billion
years, our knowledge of it is fairly recent. In 1950 the Dutch
astronomer Jan Hendrik Oort noted that the orbits of most observed
comets are shaped like extremely elongated ellipses. They
approach the sun at the very edge of their orbits, and then take off
again to distances as much as a hundred thousand times greater
than the distance of the Earth from the Sun. As a result these
comets spend most of their time far beyond the orbit of the
outermost planets, and, if they preserve their orbits, they will only
return to the inner Solar System once every several million years.
Since there are very many of such long period comets (a new one is
discovered every month or so), Oort concluded that the Solar
System must be surrounded by a cloud made of billion comets.
Spurred by a passing star, galactic tides, or molecule clouds, some
of these comets occasionally venture in among the planets.
The Oort Cloud
Origins
The Oort cloud is essentially different from the
other debris fields of our Solar system. Whereas
the asteroid belt between Mars and Jupiter, and
the Kuiper belt beyond the orbit of Neptune, are
shaped like flat rings, lying on the orbital plane
of the planets, the Oort cloud is spherical,
enveloping the Solar System from all sides. We
know this because the long period comets
(unlike the short period ones) come to us from
all parts of the sky, and not just along the plane
of the ecliptic.
The Oort Cloud
Origins
The different orbits of these comets have given
scientists clues as to their initial distance from
the Sun. The difference between the closest and
the furthest long-period comets provides a good
estimate of the "thickness," or depth, of the
cloud. Astronomers now believe that it occupies
the space between 5000 and 100,000
Astronomical Units from the Sun, where each
Astronomical Unit is equal to 93 million miles,
the average distance of the Earth from the Sun.
That makes for a very thick cloud, whose outer
edges are almost half way to the nearest stars.
The Oort Cloud
Origins
Scientists have long assumed that the Oort cloud was formed during
the infancy of our Solar System, about 4.5 billion years ago. When
the giant planets, Jupiter, Saturn, Uranus, and Neptune, formed out
of the ancient protoplanetary disk, they did not use up all of the
material in their vicinity. A large amount of icy and rocky debris was
left orbiting the sun along the planets' path and in between them. On
occasion, when one of these objects, known as "planetesimals"
would stray too close to one of its giant neighbors, it would be
catapulted into a planet-crossing path and ejected from the Solar
System. This, as a matter of fact, was precisely the way the deepspace Pioneer and Voyager probes were sent on their way to the
stars. Most of the planetesimals were launched into interstellar
space and lost to the Sun forever. Some, approximately 3%-10% of
them, ended up in the Oort cloud, orbiting the sun at fantastic
distances with periods lasting millions of years.
The Oort Cloud
Origins
Oort himself estimated that his cloud was composed of as many as
12 billion comets, and his estimate remains valid today. This,
however, does not tell us the actual mass, or the amount of matter,
that ended up in the Oort cloud. Since scientists have no way of
measuring this directly, they have resorted to computer simulations
that virtually "recreate" the cloud from scratch. The simulations
reenact the effects of the gravitational pull of the planets and the
Sun on an average-sized piece of debris: How long would it take
before it is likely to encounter a giant planet? How would its orbit be
affected as a result? Would it end up crashing into the planet,
launched into interstellar space, or orbiting in the Oort cloud? The
simulations can give approximate answers to all these questions.
Based on their results, scientists have concluded that the mass of
the rocky cloud is probably no more than 40 Earth masses, but no
less than 10.
The Oort Cloud
Origins
One crucial factor, however,
was left out, according to Stern
and Weissman: collisions.
When billions of chunks of
rock and ice move at high
speed and in close proximity to
each other, there are bound to
be lots of scrapes, bumps, and
crashes. How would these
collisions affect the likelihood
of the debris ending up in the
Oort cloud? That is the
question that the two scientists
set out to answer in their
recent article in Nature.
The Oort Cloud
Diverted from Orbit…
According to Stern and Weissman, one effect the collisions would
have on the icy rocks spinning among the giant planets is to slow
them down and alter their orbits. This would reduce their chances of
being launched beyond interplanetary space. In their simulation,
Stern and Weissman posited a common-sized planetesimal, about
one kilometer in diameter, and with an average density. The course
of such an object, they reasoned, would certainly be substantially
altered once it had collided with other debris, whose combined mass
would be half its own. This does not necessarily imply a direct
collision with another large object - it only means that over time the
combined mass of small objects and dust, encountered in the rock's
orbit, would add up to one half its own mass. By that point its orbit
would certainly be changed significantly, and the object's chances of
escaping to the Oort cloud would be seriously diminished.
The Oort Cloud
Diverted from Orbit…
Naturally, the likelihood of this happening
depends of the density of the ancient
planetesimal belt around the giant planets.
Since this is not known with certainty, the
researchers posited 3 possible densities.
For each of these, they proceeded to
calculate how long it would take their
hypothetical rock to be diverted from its
likely destiny in the Oort cloud.
The Oort Cloud
Diverted from Orbit…
The results they obtained were significant indeed. When
they assumed the highest density for this planetesimal
field (which is also the closest to current estimates of its
true density), they found that a given planetesimal would
almost certainly be diverted from its course long before it
would be launched beyond the planets. The medium
density gave the rock a somewhat better than even
chance to reach its destination in the cloud. Only the
very lowest (and least likely) density promised the rock a
good chance of staying its course long enough to
encounter one of the giant planets and be launched
beyond the inner Solar System.
The Oort Cloud
… Or Ground to Dust
The other potential effect of the collisions is even more
serious: each scrape and crash erodes the rocks to a
certain degree, and may ultimately reduce them to dust.
Fine dust particles are unlikely to end up in the Oort
cloud, since they are much more likely to be affected by
Solar radiation pressure and end up being blown out to
interstellar space or sucked into the Sun. The question
before the researchers was what would happen first:
would a rocky planetesimals encounter a giant planet
and be launched into interstellar space or the Oort cloud,
or was it more likely to be ground to nothing before such
an encounter ever took place?
The Oort Cloud
… Or Ground to Dust
Again, Stern and Weissman turned to computer
simulations. This time they assumed a fixed
density for the asteroid belt, and studied the
effects collisions would have on icy rocks,
depending on their orbits and internal strength. A
highly eccentric orbit would result in a faster rate
of erosion, because the objects tend to travel at
higher speeds at the time of collision. Similarly,
an internally weak object would naturally erode
much faster than an internally strong one.
The Oort Cloud
… Or Ground to Dust
In all cases, however, the verdict of the
simulation was clear: rocky planetesimals
were far more likely to be reduced to dust,
than to survive long enough to be
launched towards the Oort cloud. As a
result, the researchers estimate, the total
mass that ended up in the Oort cloud is
somewhere between 0.6 and 2 Earth
masses - far less than the common
estimates of 10 to 40 Earth masses.
The Oort Cloud
So What?
Estimating the precise mass of an unseen,
unmeasurable, and unreachable cloud
may not seem like a very promising pursuit
at first glance. But in the surprising ways
of science, such a seemingly quaint study
may provide us with clues about some of
the most intriguing mysteries of our world.
The Oort Cloud
So What?
A smaller, less massive Oort cloud, for example,
has interesting implications for the history and
formation of our Solar System. Whenever a
planet like Jupiter or Neptune ejects an object
into interplanetary space, it is also, at the same
time, pushed slightly inwards, towards the Sun.
This is the inevitable consequence of Newtonian
dynamics. If the giant planets had in fact sent 10
to 40 Earth masses into the Oort cloud, and
many times that mass into interplanetary space,
we would expect them to have moved
significantly towards the Sun.
The Oort Cloud
So What?
This, scientists are convinced, never happened. If the
planets had traveled in such a manner they would have
been pushed closer together, with the result that their
orbits would have been locked into synchronized orbits,
their periods bearing a simple and fixed ratio to each
other like 2:1 or 3:2. Since the orbital periods of the giant
planets do not correspond to such simple ratios,
astronomers conclude that they are probably orbiting
close to where they first formed. If the Oort cloud is as
massive as previously thought, this is very puzzling; with
Stern and Weissman's new estimate, we may have the
beginnings of a solution to the mystery.
The Oort Cloud
So What?
These clues about the formation of our own Solar System, in turn
opens up interesting possibilities regarding the planets discovered
orbiting faraway stars. Many of these are true giants, several times
the size of Jupiter but orbiting at dazzling speeds, very close to their
home star. According to current models, these planets could not
have formed where they are now: the tremendous heat emanating
from the star in its early life would have burnt out the gasses
necessary for the formation of giant planets. They must therefore
have migrated from their more distant orbits before ending up in
their current position near the home star. This migration may well
have occurred as a result of the planets launching planetesimal
debris into higher orbit - the exact same mechanism that formed our
own Oort cloud. Is it possible, then, that giant planets orbiting close
to their stars are surrounded by particularly massive cometary
clouds? It is too early to say. Nevertheless, the new study does hint
at a possible relationship between cometary clouds and the orbits of
extrasolar planets.
The Oort Cloud
So What?
Stern and Weissman's study also has interesting
implications regarding the nature and composition of
comets, and possibly about the origins of life. Since longterm comets originate in the Oort cloud, then a less
massive Oort cloud implies that there are either fewer of
them than astronomers think, or, more likely, they are
less massive than the current estimates suggest. Some
scientists believe that many of the components
necessary for life were brought to Earth initially through a
cometary bombardment. New insights into the mass and
composition of comets may well affect our views on the
emergence of life on our planet.
The Oort Cloud
So What?
The Oort cloud is
distant, undetectable,
and invisible. And yet,
we find, even the
study of so ethereal
an object can have
far-reaching
implications for our
knowledge of the
universe.
The Oort Cloud
In 1950 Jan Oort noticed that no comet had been observed coming
from interstellar space. Instead, all long period comets, normally
have orbits that lie at a great distance and have no preferential
direction. For these reasons he deduced the existence of a vast
cloud of comets, named the Oort cloud, with the shape of a diffuse
spherical shell at about 50,000 AU from the Sun (which is about 1/5
of the distance to Alpha Centauri, the nearest star).
The statistics imply that this cloud, which surrounds the entire solar
system, could contain perhaps up to objects. In this hypothesis, the
Oort Cloud may account for a significant fraction of the mass of the
solar system (perhaps even more than Jupiter itself). Unfortunately,
since the individual comets are so small and at such large distances,
scientists have only very little evidence about the Oort Cloud.
Image courtesy of The Electronic Universe Project
The Oort Cloud
Why Do Comets Leave the Oort
Cloud?
Some of the comets that inhabit the Oort cloud,
can, from time to time, fall into the inner solar
system and come under the gravitational control
of the planets, becoming long period comets.
But which are the physical mechanisms that can
actually make comets leave the Oort cloud
pulling them inside the inner solar system, or
ejecting them to interstellar space?
The Oort Cloud
Why Do Comets Leave the Oort
Cloud?
First of all, comets of the Oort Cloud can be
perturbed by the gravity of passing stars. In fact,
all the stars in the disk of the Milky Way share a
common motion around the center of the galaxy
but also move relative to each other. Stars
approach from random directions, so the velocity
changes are sometimes positive, sometimes
negative. The combined effect is a chaotic
perturbation of the velocity (with more precision,
a "random walk") such that, after 10,000 stars
have passed by, the original orbit of the comet
has been drastically altered.
The Oort Cloud
Why Do Comets Leave the Oort
Cloud?
How long does this take? The answer depends
on where in the Oort Cloud the comet comes
from. In fact, most of the comets are thought to
come from the outer edge of the cloud, where
the attraction to the sun is weak and passing
stars have bigger effects. Closer to the sun, the
comets are tightly held and may never be
dislodged by the gravity of passing stars.
The Oort Cloud
Why Do Comets Leave the Oort
Cloud?
Other Perturbations might occur, such as the gravity of
the Milky Way disk itself that can disturb the orbits of
comets in the Oort Cloud, with an effect comparable in
size to that of passing stars.
Also the sun may disturb the Oort Cloud's objects (on
very rare occasions, when passing through a Giant
Molecular Cloud) causing a shower of comets to rain on
the inner planetary system. Another possible mechanism
to make comets leave the Oort cloud can be the shock
wave from an explosive event such as a supernova.
The Oort Cloud
The End