the oort cloud the oort cloud is an immense spherical cloud surrounding the planetary system and...

The Oort CloudThe Oort Cloud

The Oort cloud is an The Oort cloud is an immense spherical cloud immense spherical cloud surrounding the planetary surrounding the planetary system and extending system and extending approximately 3 light years, approximately 3 light years, about 30 trillion kilometers about 30 trillion kilometers from the Sun. This vast from the Sun. This vast distance is considered the distance is considered the edge of the Sun's orb of edge of the Sun's orb of physical, gravitational, or physical, gravitational, or dynamical influence. dynamical influence.


Within the cloud, comets are typically tens of Within the cloud, comets are typically tens of millions of kilometers apart. They are weakly millions of kilometers apart. They are weakly bound to the sun, and passing stars and other bound to the sun, and passing stars and other forces can readily change their orbits, sending forces can readily change their orbits, sending them into the inner solar system or out to them into the inner solar system or out to interstellar space. This is especially true of interstellar space. This is especially true of comets on the outer edges of the Oort cloud. comets on the outer edges of the Oort cloud. The structure of the cloud is believed to The structure of the cloud is believed to consist of a relatively dense core that lies near consist of a relatively dense core that lies near the ecliptic plane and gradually replenishes the the ecliptic plane and gradually replenishes the outer boundaries, creating a steady state. One outer boundaries, creating a steady state. One sixth of an estimated six trillion icy objects or sixth of an estimated six trillion icy objects or comets are in the outer region with the comets are in the outer region with the remainder in the relatively dense core. remainder in the relatively dense core.


In addition to stellar perturbations In addition to stellar perturbations where another star's Oort cloud passes where another star's Oort cloud passes through or close to the Sun's Oort through or close to the Sun's Oort cloud, are the influences of giant cloud, are the influences of giant molecular clouds and tidal forces. A molecular clouds and tidal forces. A giant molecular-cloud is by far more giant molecular-cloud is by far more massive than the Sun. It is an massive than the Sun. It is an accumulation of cold hydrogen that is accumulation of cold hydrogen that is the birthplace of stars and solar the birthplace of stars and solar systems. These are infrequently systems. These are infrequently encountered, about every 300-500 encountered, about every 300-500 million years, but when they are million years, but when they are encountered, they can violently encountered, they can violently redistribute comets within the Oort redistribute comets within the Oort cloud. cloud.


Tidal forces affecting the Oort cloud Tidal forces affecting the Oort cloud come from stars in the Milky Way's come from stars in the Milky Way's galactic disk with some pull from the galactic disk with some pull from the galactic core. The tide results from the galactic core. The tide results from the sun and comets being different sun and comets being different distances from these massive distances from these massive amounts of matter. The force on the amounts of matter. The force on the comets from these tides is greater comets from these tides is greater than the perturbations of passing than the perturbations of passing stars, and comets beyond 200,000 AU stars, and comets beyond 200,000 AU are easily lost to interstellar space. are easily lost to interstellar space. This pull contributes to the steady This pull contributes to the steady state which replenishes the outer state which replenishes the outer comets that are randomly distributed comets that are randomly distributed away from the ecliptic plane. away from the ecliptic plane.


The total mass of comets in the The total mass of comets in the Oort cloud is estimated to be Oort cloud is estimated to be 40 times that of Earth. This 40 times that of Earth. This matter is believed to have matter is believed to have originated at different distances originated at different distances and therefore temperatures and therefore temperatures from the sun, which explains from the sun, which explains the compositional diversity the compositional diversity observed in comets. observed in comets.


Typical noontime Typical noontime temperatures are four temperatures are four degrees Celsius above degrees Celsius above absolute zero. As absolute zero. As temperatures move toward temperatures move toward absolute zero, the kinetic absolute zero, the kinetic energy of the molecules energy of the molecules approach a finite value. approach a finite value. Absolute zero should not be Absolute zero should not be considered a state of zero considered a state of zero energy without motion. energy without motion. There still remains some There still remains some molecular energy, although molecular energy, although it is at a minimum, at it is at a minimum, at absolute zero. absolute zero.

The Oort CloudThe Oort Cloud The Oort cloud is the source of long-period comets and possibly The Oort cloud is the source of long-period comets and possibly

higher-inclination intermediate comets that were pulled into higher-inclination intermediate comets that were pulled into shorter period orbits by the planets, such as Halley and Swift-shorter period orbits by the planets, such as Halley and Swift-Tuttle. Comets can also shift their orbits due to jets of gas and Tuttle. Comets can also shift their orbits due to jets of gas and dust that rocket from their icy surface as they approach the sun. dust that rocket from their icy surface as they approach the sun. Although they get off course, comets do have initial orbits with Although they get off course, comets do have initial orbits with widely different ranges, from 200 years to once every million widely different ranges, from 200 years to once every million years or more. Comets entering the planetary region for the first years or more. Comets entering the planetary region for the first time, come from an average distance of 44,000 astronomical time, come from an average distance of 44,000 astronomical units. units.

The Oort CloudThe Oort Cloud Long period comets can appear at any time and come from Long period comets can appear at any time and come from

any direction. Bright comets can usually be seen every 5-10 any direction. Bright comets can usually be seen every 5-10 years. Two recent Oort cloud comets were Hyakutake and years. Two recent Oort cloud comets were Hyakutake and Hale-Bopp. Hyakutake was average in size, but came to Hale-Bopp. Hyakutake was average in size, but came to 0.10 AU (15,000,000 km) from Earth, which made it appear 0.10 AU (15,000,000 km) from Earth, which made it appear especially spectacular. Hale-Bopp, on the other hand, was especially spectacular. Hale-Bopp, on the other hand, was an unusually large and dynamic comet, ten times that of an unusually large and dynamic comet, ten times that of Halley at comparable distances from the sun, making it Halley at comparable distances from the sun, making it appear quite bright, even though it did not approach closer appear quite bright, even though it did not approach closer than 1.32 AU (197,000,000 km) to the Earth. than 1.32 AU (197,000,000 km) to the Earth.


Recognition of the Oort cloud gave explanation to the age old Recognition of the Oort cloud gave explanation to the age old questions: "What are comets, and where do they come from?" In questions: "What are comets, and where do they come from?" In 1950, Jan H. Oort inferred the existence of the Oort cloud from the 1950, Jan H. Oort inferred the existence of the Oort cloud from the physical evidence of long-period comets entering the planetary physical evidence of long-period comets entering the planetary system. This Dutch astronomer, who determined the rotation of the system. This Dutch astronomer, who determined the rotation of the Milky Way galaxy in the 1920's, interpreted comet orbital distribution Milky Way galaxy in the 1920's, interpreted comet orbital distribution with only 19 well-measured orbits to study and successfully with only 19 well-measured orbits to study and successfully recognized where these comets came from. Additional gathered recognized where these comets came from. Additional gathered data has since confirmed his studies, establishing and expanding data has since confirmed his studies, establishing and expanding our knowledge of the Oort cloud. our knowledge of the Oort cloud.


Comets also are cosmic debris, probably Comets also are cosmic debris, probably planetesimals that originally resided in the planetesimals that originally resided in the vicinity of the orbits of Uranus and Neptune vicinity of the orbits of Uranus and Neptune rather than in the warmer regions of the asteroid rather than in the warmer regions of the asteroid belt. Thus, the nuclei of comets are icy balls of belt. Thus, the nuclei of comets are icy balls of frozen water, methane, and ammonia, mixed frozen water, methane, and ammonia, mixed with small pieces of rock and dust, rather than with small pieces of rock and dust, rather than the largely volatile-free stones and irons that the largely volatile-free stones and irons that typify asteroids. In the most popular theory, icy typify asteroids. In the most popular theory, icy planetesimals in the primitive solar nebula that planetesimals in the primitive solar nebula that wandered close to Uranus or Neptune but not wandered close to Uranus or Neptune but not close enough to be captured by them were flung close enough to be captured by them were flung to great distances from the Sun, some to be lost to great distances from the Sun, some to be lost from the solar system while others populated from the solar system while others populated what was to become a great cloud of cometary what was to become a great cloud of cometary bodies, perhaps 10 trillion in number. Such a bodies, perhaps 10 trillion in number. Such a cloud was first hypothesized by the Dutch cloud was first hypothesized by the Dutch astronomer Jan Hendrik Oort. astronomer Jan Hendrik Oort.


In the original version of the theory, the Oort cloud extended tens of 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 thousands of times farther from the Sun than the Earth, a significant fraction of the way to the nearest stars. Random encounters with fraction of the way to the nearest stars. Random encounters with passing stars would periodically throw some of the comets into new passing stars would periodically throw some of the comets into new orbits, plunging them back toward the heart of the solar system. As orbits, plunging them back toward the heart of the solar system. As a comet nears the Sun, the ices begin to evaporate, loosening the a comet nears the Sun, the ices begin to evaporate, loosening the trapped dust and forming a large coma that completely surrounds trapped dust and forming a large coma that completely surrounds the small nucleus, which is the ultimate source of all the material. 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 The solar wind blows back the evaporating gas into an ion tail, and radiation pressure pushes back the small particulate solids into a radiation pressure pushes back the small particulate solids into a dust tail. Each solid particle is now an independently orbiting dust tail. Each solid particle is now an independently orbiting satellite of the Sun, and the accumulation of countless such satellite of the Sun, and the accumulation of countless such passages by many comets contributes to the total quantity of dust passages by many comets contributes to the total quantity of dust particles and micrometeoroids found in interplanetary space. particles and micrometeoroids found in interplanetary space.


The total mass contained in all the comets is highly uncertain. The total mass contained in all the comets is highly uncertain. Modern estimates range from 1 to 100 Earth masses. Part of the Modern estimates range from 1 to 100 Earth masses. Part of the uncertainty concerns the reality of a hypothesized massive "inner uncertainty concerns the reality of a hypothesized massive "inner Oort cloud" -- or "Kuiper belt" (if the distribution is flattened)--of Oort cloud" -- or "Kuiper belt" (if the distribution is flattened)--of comets that would exist at distances from the Sun 40 to 10,000 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 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 would not be much perturbed by typical passing stars nor by the gravity of the planets of the solar system, and the comets could 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 reside in the inner cloud or belt for long periods of time without detection. It has been speculated, however, that a rare close detection. It has been speculated, however, that a rare close passage by another star (possibly an undetected companion of the passage by another star (possibly an undetected companion of the Sun) may send a shower of such comets streaming toward the inner Sun) may send a shower of such comets streaming toward the inner solar system. If enough large cometary nuclei in such showers solar system. If enough large cometary nuclei in such showers happen to strike the Earth, the clouds of dust and ash that they happen to strike the Earth, the clouds of dust and ash that they would raise might be sufficient to trigger mass biological extinctions. would raise might be sufficient to trigger mass biological extinctions. An event of this kind appears especially promising for explaining the An event of this kind appears especially promising for explaining the relatively sudden disappearance of the dinosaurs from the Earth. relatively sudden disappearance of the dinosaurs from the Earth.

The Oort CloudThe Oort CloudWhat’s in the Oort Cloud?What’s in the Oort Cloud?

February 13, 2000: No one has ever seen February 13, 2000: No one has ever seen the Oort cloud, that spherical envelope of the Oort cloud, that spherical envelope of comets and their residues that surrounds comets and their residues that surrounds our Solar System. No one has ever our Solar System. No one has ever measured its size and density or counted measured its size and density or counted the objects in it. Nor is anyone likely to do the objects in it. Nor is anyone likely to do so in the foreseeable future: the Oort cloud so in the foreseeable future: the Oort cloud is too distant, and the objects in it too is too distant, and the objects in it too small and too dim to be detected by our small and too dim to be detected by our instruments. instruments.

The Oort CloudThe Oort CloudWhat’s in the Oort Cloud?What’s in the Oort Cloud?

And yet, not only do scientists agree that And yet, not only do scientists agree that this ethereal cloud is out there, but they this ethereal cloud is out there, but they are so confident that they actually argue are so confident that they actually argue about its exact composition and about its exact composition and characteristics. A paper by Alan Stern of characteristics. A paper by Alan Stern of the Southwestern Research Institute the Southwestern Research Institute and Paul Weissman of JPL in the and Paul Weissman of JPL in the February 1 issue of Nature suggests February 1 issue of Nature suggests that the cloud is made up of smaller and that the cloud is made up of smaller and probably fewer objects than previously probably fewer objects than previously thought. By using computer simulations thought. By using computer simulations of conditions in the Solar System 4.5 of conditions in the Solar System 4.5 billion years ago, the researchers billion years ago, the researchers concluded that many of the objects that concluded that many of the objects that could have ended up in the Oort cloud could have ended up in the Oort cloud were diverted from their course or were diverted from their course or ground to dust before they never had a ground to dust before they never had a chance to reach their cloudy destination. chance to reach their cloudy destination.

The Oort CloudThe Oort CloudOriginsOrigins

While the Oort cloud has probably been around for several billion While the Oort cloud has probably been around for several billion years, our knowledge of it is fairly recent. In 1950 the Dutch years, our knowledge of it is fairly recent. In 1950 the Dutch astronomer Jan Hendrik Oort noted that the orbits of most observed astronomer Jan Hendrik Oort noted that the orbits of most observed comets are shaped like extremely elongated ellipses. They comets are shaped like extremely elongated ellipses. They approach the sun at the very edge of their orbits, and then take off 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 again to distances as much as a hundred thousand times greater than the distance of the Earth from the Sun. As a result these 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 comets spend most of their time far beyond the orbit of the outermost planets, and, if they preserve their orbits, they will only outermost planets, and, if they preserve their orbits, they will only return to the inner Solar System once every several million years. 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 Since there are very many of such long period comets (a new one is discovered every month or so), Oort concluded that the Solar discovered every month or so), Oort concluded that the Solar System must be surrounded by a cloud made of billion comets. System must be surrounded by a cloud made of billion comets. Spurred by a passing star, galactic tides, or molecule clouds, some Spurred by a passing star, galactic tides, or molecule clouds, some of these comets occasionally venture in among the planets. of these comets occasionally venture in among the planets.


The Oort cloud is essentially different from the The Oort cloud is essentially different from the other debris fields of our Solar system. Whereas other debris fields of our Solar system. Whereas the asteroid belt between Mars and Jupiter, and the asteroid belt between Mars and Jupiter, and the Kuiper belt beyond the orbit of Neptune, are the Kuiper belt beyond the orbit of Neptune, are shaped like flat rings, lying on the orbital plane shaped like flat rings, lying on the orbital plane of the planets, the Oort cloud is spherical, of the planets, the Oort cloud is spherical, enveloping the Solar System from all sides. We enveloping the Solar System from all sides. We know this because the long period comets know this because the long period comets (unlike the short period ones) come to us from (unlike the short period ones) come to us from all parts of the sky, and not just along the plane all parts of the sky, and not just along the plane of the ecliptic. of the ecliptic.


The different orbits of these comets have given The different orbits of these comets have given scientists clues as to their initial distance from scientists clues as to their initial distance from the Sun. The difference between the closest and the Sun. The difference between the closest and the furthest long-period comets provides a good the furthest long-period comets provides a good estimate of the "thickness," or depth, of the estimate of the "thickness," or depth, of the cloud. Astronomers now believe that it occupies cloud. Astronomers now believe that it occupies the space between 5000 and 100,000 the space between 5000 and 100,000 Astronomical Units from the Sun, where each Astronomical Units from the Sun, where each Astronomical Unit is equal to 93 million miles, Astronomical Unit is equal to 93 million miles, the average distance of the Earth from the Sun. the average distance of the Earth from the Sun. That makes for a very thick cloud, whose outer That makes for a very thick cloud, whose outer edges are almost half way to the nearest stars. edges are almost half way to the nearest stars.


Scientists have long assumed that the Oort cloud was formed during 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 infancy of our Solar System, about 4.5 billion years ago. When the giant planets, Jupiter, Saturn, Uranus, and Neptune, formed out the giant planets, Jupiter, Saturn, Uranus, and Neptune, formed out of the ancient protoplanetary disk, they did not use up all of the 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 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 left orbiting the sun along the planets' path and in between them. On occasion, when one of these objects, known as "planetesimals" occasion, when one of these objects, known as "planetesimals" would stray too close to one of its giant neighbors, it would be would stray too close to one of its giant neighbors, it would be catapulted into a planet-crossing path and ejected from the Solar catapulted into a planet-crossing path and ejected from the Solar System. This, as a matter of fact, was precisely the way the deep-System. This, as a matter of fact, was precisely the way the deep-space Pioneer and Voyager probes were sent on their way to the space Pioneer and Voyager probes were sent on their way to the stars. Most of the planetesimals were launched into interstellar stars. Most of the planetesimals were launched into interstellar space and lost to the Sun forever. Some, approximately 3%-10% of space and lost to the Sun forever. Some, approximately 3%-10% of them, ended up in the Oort cloud, orbiting the sun at fantastic them, ended up in the Oort cloud, orbiting the sun at fantastic distances with periods lasting millions of years. distances with periods lasting millions of years.


Oort himself estimated that his cloud was composed of as many as Oort himself estimated that his cloud was composed of as many as 12 billion comets, and his estimate remains valid today. This, 12 billion comets, and his estimate remains valid today. This, however, does not tell us the actual mass, or the amount of matter, 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 that ended up in the Oort cloud. Since scientists have no way of measuring this directly, they have resorted to computer simulations measuring this directly, they have resorted to computer simulations that virtually "recreate" the cloud from scratch. The simulations that virtually "recreate" the cloud from scratch. The simulations reenact the effects of the gravitational pull of the planets and the 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 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 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, affected as a result? Would it end up crashing into the planet, launched into interstellar space, or orbiting in the Oort cloud? The launched into interstellar space, or orbiting in the Oort cloud? The simulations can give approximate answers to all these questions. simulations can give approximate answers to all these questions. Based on their results, scientists have concluded that the mass of Based on their results, scientists have concluded that the mass of the rocky cloud is probably no more than 40 Earth masses, but no the rocky cloud is probably no more than 40 Earth masses, but no less than 10. less than 10.


One crucial factor, however, One crucial factor, however, was left out, according to Stern was left out, according to Stern and Weissman: collisions. and Weissman: collisions. When billions of chunks of rock When billions of chunks of rock and ice move at high speed and ice move at high speed and in close proximity to each and in close proximity to each other, there are bound to be other, there are bound to be lots of scrapes, bumps, and lots of scrapes, bumps, and crashes. How would these crashes. How would these collisions affect the likelihood collisions affect the likelihood of the debris ending up in the of the debris ending up in the Oort cloud? That is the Oort cloud? That is the question that the two scientists question that the two scientists set out to answer in their set out to answer in their recent article in Nature. recent article in Nature.

The Oort CloudThe Oort CloudDiverted from Orbit…Diverted from Orbit…

According to Stern and Weissman, one effect the collisions would According to Stern and Weissman, one effect the collisions would have on the icy rocks spinning among the giant planets is to slow 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 them down and alter their orbits. This would reduce their chances of being launched beyond interplanetary space. In their simulation, being launched beyond interplanetary space. In their simulation, Stern and Weissman posited a common-sized planetesimal, about Stern and Weissman posited a common-sized planetesimal, about one kilometer in diameter, and with an average density. The course one kilometer in diameter, and with an average density. The course of such an object, they reasoned, would certainly be substantially of such an object, they reasoned, would certainly be substantially altered once it had collided with other debris, whose combined mass altered once it had collided with other debris, whose combined mass would be half its own. This does not necessarily imply a direct would be half its own. This does not necessarily imply a direct collision with another large object - it only means that over time the collision with another large object - it only means that over time the combined mass of small objects and dust, encountered in the rock's 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 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 would certainly be changed significantly, and the object's chances of escaping to the Oort cloud would be seriously diminished. escaping to the Oort cloud would be seriously diminished.


Naturally, the likelihood of this happening Naturally, the likelihood of this happening depends of the density of the ancient depends of the density of the ancient planetesimal belt around the giant planets. planetesimal belt around the giant planets. Since this is not known with certainty, the Since this is not known with certainty, the researchers posited 3 possible densities. researchers posited 3 possible densities. For each of these, they proceeded to For each of these, they proceeded to calculate how long it would take their calculate how long it would take their hypothetical rock to be diverted from its hypothetical rock to be diverted from its likely destiny in the Oort cloud. likely destiny in the Oort cloud.


The results they obtained were significant indeed. When The results they obtained were significant indeed. When they assumed the highest density for this planetesimal they assumed the highest density for this planetesimal field (which is also the closest to current estimates of its field (which is also the closest to current estimates of its true density), they found that a given planetesimal would true density), they found that a given planetesimal would almost certainly be diverted from its course long before it almost certainly be diverted from its course long before it would be launched beyond the planets. The medium would be launched beyond the planets. The medium density gave the rock a somewhat better than even density gave the rock a somewhat better than even chance to reach its destination in the cloud. Only the chance to reach its destination in the cloud. Only the very lowest (and least likely) density promised the rock a very lowest (and least likely) density promised the rock a good chance of staying its course long enough to good chance of staying its course long enough to encounter one of the giant planets and be launched encounter one of the giant planets and be launched beyond the inner Solar System. beyond the inner Solar System.

The Oort CloudThe Oort Cloud… Or Ground to Dust… Or Ground to Dust

The other potential effect of the collisions is even more The other potential effect of the collisions is even more serious: each scrape and crash erodes the rocks to a serious: each scrape and crash erodes the rocks to a certain degree, and may ultimately reduce them to dust. certain degree, and may ultimately reduce them to dust. Fine dust particles are unlikely to end up in the Oort Fine dust particles are unlikely to end up in the Oort cloud, since they are much more likely to be affected by cloud, since they are much more likely to be affected by Solar radiation pressure and end up being blown out to Solar radiation pressure and end up being blown out to interstellar space or sucked into the Sun. The question interstellar space or sucked into the Sun. The question before the researchers was what would happen first: before the researchers was what would happen first: would a rocky planetesimals encounter a giant planet would a rocky planetesimals encounter a giant planet and be launched into interstellar space or the Oort cloud, and be launched into interstellar space or the Oort cloud, or was it more likely to be ground to nothing before such or was it more likely to be ground to nothing before such an encounter ever took place? an encounter ever took place?


Again, Stern and Weissman turned to computer Again, Stern and Weissman turned to computer simulations. This time they assumed a fixed simulations. This time they assumed a fixed density for the asteroid belt, and studied the density for the asteroid belt, and studied the effects collisions would have on icy rocks, effects collisions would have on icy rocks, depending on their orbits and internal strength. A depending on their orbits and internal strength. A highly eccentric orbit would result in a faster rate highly eccentric orbit would result in a faster rate of erosion, because the objects tend to travel at of erosion, because the objects tend to travel at higher speeds at the time of collision. Similarly, higher speeds at the time of collision. Similarly, an internally weak object would naturally erode an internally weak object would naturally erode much faster than an internally strong one. much faster than an internally strong one.


In all cases, however, the verdict of the In all cases, however, the verdict of the simulation was clear: rocky planetesimals simulation was clear: rocky planetesimals were far more likely to be reduced to dust, were far more likely to be reduced to dust, than to survive long enough to be than to survive long enough to be launched towards the Oort cloud. As a launched towards the Oort cloud. As a result, the researchers estimate, the total result, the researchers estimate, the total mass that ended up in the Oort cloud is mass that ended up in the Oort cloud is somewhere between 0.6 and 2 Earth somewhere between 0.6 and 2 Earth masses - far less than the common masses - far less than the common estimates of 10 to 40 Earth masses. estimates of 10 to 40 Earth masses.

The Oort CloudThe Oort CloudSo What?So What?

Estimating the precise mass of an unseen, Estimating the precise mass of an unseen, unmeasurable, and unreachable cloud unmeasurable, and unreachable cloud may not seem like a very promising pursuit may not seem like a very promising pursuit at first glance. But in the surprising ways at first glance. But in the surprising ways of science, such a seemingly quaint study of science, such a seemingly quaint study may provide us with clues about some of may provide us with clues about some of the most intriguing mysteries of our world. the most intriguing mysteries of our world.


A smaller, less massive Oort cloud, for example, A smaller, less massive Oort cloud, for example, has interesting implications for the history and has interesting implications for the history and formation of our Solar System. Whenever a formation of our Solar System. Whenever a planet like Jupiter or Neptune ejects an object planet like Jupiter or Neptune ejects an object into interplanetary space, it is also, at the same into interplanetary space, it is also, at the same time, pushed slightly inwards, towards the Sun. time, pushed slightly inwards, towards the Sun. This is the inevitable consequence of Newtonian This is the inevitable consequence of Newtonian dynamics. If the giant planets had in fact sent 10 dynamics. If the giant planets had in fact sent 10 to 40 Earth masses into the Oort cloud, and to 40 Earth masses into the Oort cloud, and many times that mass into interplanetary space, many times that mass into interplanetary space, we would expect them to have moved we would expect them to have moved significantly towards the Sun. significantly towards the Sun.


This, scientists are convinced, never happened. If the This, scientists are convinced, never happened. If the planets had traveled in such a manner they would have planets had traveled in such a manner they would have been pushed closer together, with the result that their been pushed closer together, with the result that their orbits would have been locked into synchronized orbits, orbits would have been locked into synchronized orbits, their periods bearing a simple and fixed ratio to each their periods bearing a simple and fixed ratio to each other like 2:1 or 3:2. Since the orbital periods of the giant other like 2:1 or 3:2. Since the orbital periods of the giant planets do not correspond to such simple ratios, planets do not correspond to such simple ratios, astronomers conclude that they are probably orbiting astronomers conclude that they are probably orbiting close to where they first formed. If the Oort cloud is as close to where they first formed. If the Oort cloud is as massive as previously thought, this is very puzzling; with massive as previously thought, this is very puzzling; with Stern and Weissman's new estimate, we may have the Stern and Weissman's new estimate, we may have the beginnings of a solution to the mystery. beginnings of a solution to the mystery.


These clues about the formation of our own Solar System, in turn These clues about the formation of our own Solar System, in turn opens up interesting possibilities regarding the planets discovered opens up interesting possibilities regarding the planets discovered orbiting faraway stars. Many of these are true giants, several times orbiting faraway stars. Many of these are true giants, several times the size of Jupiter but orbiting at dazzling speeds, very close to their the size of Jupiter but orbiting at dazzling speeds, very close to their home star. According to current models, these planets could not home star. According to current models, these planets could not have formed where they are now: the tremendous heat emanating have formed where they are now: the tremendous heat emanating from the star in its early life would have burnt out the gasses from the star in its early life would have burnt out the gasses necessary for the formation of giant planets. They must therefore necessary for the formation of giant planets. They must therefore have migrated from their more distant orbits before ending up in have migrated from their more distant orbits before ending up in their current position near the home star. This migration may well their current position near the home star. This migration may well have occurred as a result of the planets launching planetesimal have occurred as a result of the planets launching planetesimal debris into higher orbit - the exact same mechanism that formed our debris into higher orbit - the exact same mechanism that formed our own Oort cloud. Is it possible, then, that giant planets orbiting close own Oort cloud. Is it possible, then, that giant planets orbiting close to their stars are surrounded by particularly massive cometary to their stars are surrounded by particularly massive cometary clouds? It is too early to say. Nevertheless, the new study does hint clouds? It is too early to say. Nevertheless, the new study does hint at a possible relationship between cometary clouds and the orbits of at a possible relationship between cometary clouds and the orbits of extrasolar planets. extrasolar planets.


Stern and Weissman's study also has interesting Stern and Weissman's study also has interesting implications regarding the nature and composition of implications regarding the nature and composition of comets, and possibly about the origins of life. Since long-comets, and possibly about the origins of life. Since long-term comets originate in the Oort cloud, then a less term comets originate in the Oort cloud, then a less massive Oort cloud implies that there are either fewer of massive Oort cloud implies that there are either fewer of them than astronomers think, or, more likely, they are them than astronomers think, or, more likely, they are less massive than the current estimates suggest. Some less massive than the current estimates suggest. Some scientists believe that many of the components scientists believe that many of the components necessary for life were brought to Earth initially through a necessary for life were brought to Earth initially through a cometary bombardment. New insights into the mass and cometary bombardment. New insights into the mass and composition of comets may well affect our views on the composition of comets may well affect our views on the emergence of life on our planet. emergence of life on our planet.


The Oort cloud is The Oort cloud is distant, undetectable, distant, undetectable, and invisible. And yet, and invisible. And yet, we find, even the we find, even the study of so ethereal study of so ethereal an object can have an object can have far-reaching far-reaching implications for our implications for our knowledge of the knowledge of the universe.universe.


In 1950 Jan Oort noticed that no comet had been observed coming In 1950 Jan Oort noticed that no comet had been observed coming from interstellar space. Instead, all long period comets, normally from interstellar space. Instead, all long period comets, normally have orbits that lie at a great distance and have no preferential have orbits that lie at a great distance and have no preferential direction. For these reasons he deduced the existence of a vast direction. For these reasons he deduced the existence of a vast cloud of comets, named the Oort cloud, with the shape of a diffuse 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 spherical shell at about 50,000 AU from the Sun (which is about 1/5 of the distance to Alpha Centauri, the nearest star). of the distance to Alpha Centauri, the nearest star). The statistics imply that this cloud, which surrounds the entire solar The statistics imply that this cloud, which surrounds the entire solar system, could contain perhaps up to objects. In this hypothesis, the system, could contain perhaps up to objects. In this hypothesis, the Oort Cloud may account for a significant fraction of the mass of the Oort Cloud may account for a significant fraction of the mass of the solar system (perhaps even more than Jupiter itself). Unfortunately, solar system (perhaps even more than Jupiter itself). Unfortunately, since the individual comets are so small and at such large since the individual comets are so small and at such large distances, scientists have only very little evidence about the Oort distances, scientists have only very little evidence about the Oort Cloud. Cloud. Image courtesy of The Electronic Universe Project Image courtesy of The Electronic Universe Project

The Oort CloudThe Oort CloudWhy Do Comets Leave the Oort Why Do Comets Leave the Oort

Cloud?Cloud?

Some of the comets that inhabit the Oort cloud, Some of the comets that inhabit the Oort cloud, can, from time to time, fall into the inner solar can, from time to time, fall into the inner solar system and come under the gravitational control system and come under the gravitational control of the planets, becoming long period comets. of the planets, becoming long period comets. But which are the physical mechanisms that can But which are the physical mechanisms that can actually make comets leave the Oort cloud actually make comets leave the Oort cloud pulling them inside the inner solar system, or pulling them inside the inner solar system, or ejecting them to interstellar space? ejecting them to interstellar space?


Cloud?Cloud? First of all, comets of the Oort Cloud can be First of all, comets of the Oort Cloud can be

perturbed by the gravity of passing stars. In fact, perturbed by the gravity of passing stars. In fact, all the stars in the disk of the Milky Way share a all the stars in the disk of the Milky Way share a common motion around the center of the galaxy common motion around the center of the galaxy but also move relative to each other. Stars but also move relative to each other. Stars approach from random directions, so the velocity approach from random directions, so the velocity changes are sometimes positive, sometimes changes are sometimes positive, sometimes negative. The combined effect is a chaotic negative. The combined effect is a chaotic perturbation of the velocity (with more precision, perturbation of the velocity (with more precision, a "random walk") such that, after 10,000 stars a "random walk") such that, after 10,000 stars have passed by, the original orbit of the comet have passed by, the original orbit of the comet has been drastically altered. has been drastically altered.


Cloud?Cloud?

How long does this take? The answer depends How long does this take? The answer depends on where in the Oort Cloud the comet comes on where in the Oort Cloud the comet comes from. In fact, most of the comets are thought to from. In fact, most of the comets are thought to come from the outer edge of the cloud, where come from the outer edge of the cloud, where the attraction to the sun is weak and passing the attraction to the sun is weak and passing stars have bigger effects. Closer to the sun, the stars have bigger effects. Closer to the sun, the comets are tightly held and may never be comets are tightly held and may never be dislodged by the gravity of passing stars. dislodged by the gravity of passing stars.


Cloud?Cloud?

Other Perturbations might occur, such as the gravity of Other Perturbations might occur, such as the gravity of the Milky Way disk itself that can disturb the orbits of the Milky Way disk itself that can disturb the orbits of comets in the Oort Cloud, with an effect comparable in comets in the Oort Cloud, with an effect comparable in size to that of passing stars. size to that of passing stars. Also the sun may disturb the Oort Cloud's objects (on Also the sun may disturb the Oort Cloud's objects (on very rare occasions, when passing through a Giant very rare occasions, when passing through a Giant Molecular Cloud) causing a shower of comets to rain on Molecular Cloud) causing a shower of comets to rain on the inner planetary system. Another possible mechanism the inner planetary system. Another possible mechanism to make comets leave the Oort cloud can be the shock to make comets leave the Oort cloud can be the shock wave from an explosive event such as a supernova. wave from an explosive event such as a supernova.


The EndThe End

the oort cloud the oort cloud is an immense spherical cloud surrounding the planetary system and...

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