* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download The Oort Cloud
Outer space wikipedia , lookup
Tropical year wikipedia , lookup
Nebular hypothesis wikipedia , lookup
Rare Earth hypothesis wikipedia , lookup
Spitzer Space Telescope wikipedia , lookup
Aquarius (constellation) wikipedia , lookup
IAU definition of planet wikipedia , lookup
Planets beyond Neptune wikipedia , lookup
Astronomical unit wikipedia , lookup
Definition of planet wikipedia , lookup
Extraterrestrial life wikipedia , lookup
Planetary habitability wikipedia , lookup
High-velocity cloud wikipedia , lookup
Late Heavy Bombardment wikipedia , lookup
Astronomical naming conventions wikipedia , lookup
History of Solar System formation and evolution hypotheses wikipedia , lookup
Timeline of astronomy wikipedia , lookup
Formation and evolution of the Solar System wikipedia , lookup
Directed panspermia wikipedia , lookup
Solar System wikipedia , lookup
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