On the origin of planets

1. Context of the Planetary System

In this book we have discussed more than fifty worlds; some in considerable detail. These planets, satellites, asteroids and comets display an incredible diversity of composition and history. Yet they were all presumably formed at about the same time, condensing from the primordial solar nebular that also gave birth to the Sun. In spite of their individual diversity, these bodies carry many clues concerning their origins.

Lurking in the background is the question of the likelihood that there are other, perhaps similar, planetary systems around other stars. If we can understand the processes that formed the planets we know, we can then try to predict how probable it is that these same processes produced other planets, including some like ours. Similarly, if we could find planets orbiting other stars, their existence might help us to understand the origins and evolution of our own system.

Having stated what we would like to do, we must admit right away that it is not yet possible to do it. We are unable to work backward from the wealth of data on the present state of the solar system to derive a unique, detailed picture of how the system began. Neither can we work forward from a theory of star formation to the production of a solar system with all the properties we find today. Even the first and most important step, the formation of the Sun itself, is only poorly understood. Instead of a unique and all encompassing theory, we must work with a collection of reasonable explanations for those properties of the solar system that seem especially basic.

This somewhat unsatisfactory state of affairs may change dramatically within the next decade or two. Space-based telescopes launched before the end of this century should have the capability to survey hundreds of nearby stars for planets substantially smaller than Jupiter. At the same time, infrared and radio astronomy are constantly revealing more about the formation and early evolution of stars. The direct detection of other planetary systems, together with a deeper understanding of the star-formation process, may provide a much sharper perspective on these problems than is possible with our present limited information.

2. The ages of the Sun and Stars

At the beginning of the 20^th century, many astronomers thought that the planets were formed as the result of a remarkable accident, such as the near-collision of the Sun with another star. Since then, we have come to realize that stars naturally form in a cloud of dust and gas, what we have called the solar nebula in the case of our own system. Further, a variety of kinds of matter have been located orbiting nearby stars, and while none of these has yet turned out to be another planetary system like our own, their existence strengthens the idea that stars do not form alone. Finally, we now know that the ages of the Sun and of the planetary system are approximately the same. As we have seen, the Moon, the meteorites, and the Earth all formed 4.5 billion years ago. Astrophysicists who study stellar evolution give this same value for the age of the Sun. From all of these arguments, we conclude that the Sun and the planets probably formed together from a common source of material.

Of course, not all stars have the same age. The galaxy is at least 12 billion years old, and many of the stars have been present since its formation. Stars that formed early in the life of the galaxy contain much smaller quantities of the heavier elements, and for this reason they may be less likely to have formed planets. The very existence of the building blocks of our planetary system depends on the presence of heavy elements formed in previous generations of star and ejected back into interstellar space before our own system formed.

On a galactic time scale, the Sun is a relative newcomer. We are not among the latest arrivals, however. Stars that are more massive than the Sun have much shorter lifetimes. Their internal temperatures are hotter and their nuclear fires burn with greater intensity. The bright blue-white stars that dazzle us at night are all younger than the Sun. some of them have ages of only a few millions of years instead of billions. The most brilliant and massive of these stars are destined to explode as supernovas, generating a very special group of elements that can only be created in the unique conditions that briefly occur during these cosmic cataclysms.

3. Stellar Nurseries

Even younger stars exist, since they are being formed today. Star formation takes place in clouds of interstellar gas and dust such as those in the constellation of Orion. A typical interstellar cloud in which star formation is occurring has a mass hundreds of times greater than that of the Sun. It is composed primarily of hydrogen and helium, the predominant elements in the stars that it will spawn. The other elements that can be studied appear to be present in roughly the same proportions as they are found in the Sun and other young stars.

What may seem surprising, however, is the richness of the molecular chemistry that takes place in these clouds. Instead of just simple compounds like methane and ammonia, a large array of molecular species is being formed. A list of those known at the time of this writing would include more than sixty entries. Among the more interesting compounds, we call attention to ethyl alcohol, formaldehyde, used for preserving corpses, and hydrogen cyanide, a deadly poison. New molecules are constantly being discovered as astronomers use more sensitive radio telescopes and study new segments of the radio spectrum.

It is interesting to compare a list of interstellar molecules with the molecules found in comets. Some scientists think that the icy nuclei of comets contain unaltered interstellar material, trapped during the earliest stages of the formation of the solar system. Alternatively, it may be that the similarities simply reflect the universality of the processes that produce these compounds, whether they take place in the solar nebula or in the interstellar clouds.

4. A star is born

One of the first things to notice about stars is that most of them are members of multiple systems. Doubles and triples are more common than singles, but it seems unlikely on dynamical grounds that such multiple star systems will have many planets. Depending on the masses and distances of the stars, however, there may be regions around one or more of the stellar components where planetary orbits could be stable. Since we have neither the observations nor a theory for such systems, we will concentrate our attention on the much simpler case of single stars, like the Sun.

We start with a slowly rotating cloud of interstellar gas and dust that may itself be part of a much larger complex such as one of the giant clouds in Orion. At some point the cloud begins to collapse. Perhaps some gravitational instability has been created in its interior, by a random coming together of some of the material, or a nearby star has exploded as a supernova, perhaps seeding the cloud with short-lived radioactive elements and sending out shock waves that begin to compress the cloud. The collapse is possible as long as the energy of motion of the gas in the cloud is less than the gravitational energy represented by the mass of the cloud and the distance through which it collapses.

As the cloud becomes smaller, three things happen: 1) its rate of rotation increases, 2) it flattens into a disk, 3) it heats up. The heating is simply the conversion of gravitational energy to thermal energy. The increase in rotation rate results from conservation of angular momentum: as the mass of the cloud comes closer to the center, the angular velocity must increase to keep the momentum constant. The more rapid spin in turn causes the material to flatten into a disk.

In the disk, the gas and dust can radiate energy to space much more easily than in the center, where a spherical condensation develops. Here the temperature continues to rise until it finally reaches the point where nuclear fusion of hydrogen to helium can occur. At this stage, the Sun turns on and this spherical assemblage of matter begins its life as a star.

5. The mass and Dimensions of the Disk

There is still a great deal of dispute about the mass and the dimensions of the original solar nebula and of the disk itself. The central condensation must have had a mass nearly equal to that of the present Sun, but about the disk.

We can gain an idea of the minimum amount of mass that must have been present if we simply ask how much material of cosmic composition would be required to make all the present planets. The idea behind this calculation is that the solar nebula started with cosmic composition and then the individual planets formed from it with compositions reflecting the local temperature. If it had been cooler close to the Sun, massive planets like Jupiter and Saturn might have formed there.

To discover what these hypothetical planets would have been like, we can perform the thought experiment of adding hydrogen and helium to the existing planets until the ratio of these light elements to a key heavy element like silicon or iron is the same as it is in the Sun. The masses of the resulting planets are indeed similar to those of Jupiter and Saturn. Thus we might conclude that the initial disk must have had a mass roughly equal to at least ten times the present mass of Jupiter.

More likely, planet formation will not be efficient, and there was probably much more material available that has since been lost by the blowing away of the nebula or by gravitational ejection of larger bodies after the planets formed. Hence these days scientists generally adopt a value for the entire nebula of about 1.1 to 1.2 times the mass of the Sun. Recent observations of similar disks of matter around young stellar objects indicate masses of this magnitude. However, some theories hold out for much higher masses, on the order of twice the solar mass, which simply demonstrates how much more work to be done in this field to achieve some real certainty.

6. Dynamics of the Disk

We now have a newly born star at the center of a disk of gas and dust. This configuration can still be thought of as the primordial solar nebula, despite the fact that deep in the interior of the central condensation, nuclear reactions, are beginning to convert hydrogen to helium.

The disk is revolving around this central condensation in the same sense that the condensation itself is rotating on its axis. Immanuel Kant and Pierre Simon Laplace proposed nebular theories for the origin of the solar system as early as the 18^th century. The main reason these theories were challenged was the attention given to the fact how the Sun could slow down by transferring angular momentum to the disk and hence the planets.

It’s enlightening to put this problem in context. It turns out that all stars with masses 15% or more greater than that of the Sun rotate much more rapidly than our star. If one calculates the rotation rate the Sun should have, given conservation of angular momentum as the original cloud collapsed, this rate turns out to be similar to that of the more massive stars. It is also the rate that would result if the present angular momentum of the planets were put back into the Sun.

7. Magnetic Braking

One solution to this problem invokes magnetic braking. The material of the disk is following orbits defined by Kepler’s laws. Thus it is orbiting the Sun more slowly than the Sun is rotating. The situation is analogous to the interaction of the material in the Io plasma torus with the rapidly spinning magnetic field of Jupiter. The material in the inner part of the disk is ionized. The Sun’s magnetic field encounters resistance from this plasma as the Sun rotates, slowing down the spin.

This idea has several problems. It doesn’t explain why only stars with low masses exhibit slow rotation, since it is well known that some stars with large masses have strong magnetic fields. A solution to this dilemma would be to postulate that only low-mass stars form with disks, but we now know that disks are not uncommon about young stars with masses greater than the Sun’s. Finally, magnetic braking would transfer angular momentum to the disk and hence to the planets. Yet as we have seen, the problem is not that the planets have too much momentum, but only that the Sun has too little.

8. The Solar Wind: Blowing the Problem Away

A second solution involves the solar wind. Recall that the gases in the outer fringes of the solar atmosphere have enough energy to escape into space, flowing steadily outward through the solar system at speeds of about 400 km/s. The amount of matter lost in this way is a tiny fraction of the Sun’s total mass. Observations of very young stars with small masses like the Sun’s indicate that they generate intense stellar winds shortly after they form. While it is difficult to make an accurate estimate of this effect, it appears sufficient to account for the present slow rotation of the Sun.

An especially appealing aspect of this theory of solar wind braking is its natural explanation of the mass-dependence of stellar rotation. It turns out that only stars with masses comparable to or less than that of the Sun have the proper atmospheric structures to produce steady stellar winds. Hence the same strong wind that ultimately clears residual gas and dust from the disk can slow down the rapidly rotating star that generates it. More massive stars will not produce such winds, so they will continue to exhibit rapid rotation.

Tasks

Make sure you know the words and word combinations

1. In considerable detail 2. To display a diversity 3. Composition 4. Presumably 5. The wealth of data 6. Propagation 7. To be poorly understood 8. All encompassing theory 9. State of affairs 10. To reveal 11. To make up 12. The planetary system 13. The solar system 14. To constrain a theory 15. To emerge 16. Chemical composition 17. The solar nebula 18. To strengthen an idea 19. Stellar evolution 20. A relative newcomer 21. Internal/external 22. To dazzle 23. To be destined 24. To occur 25. Constellation 26. To absorb 27. To spawn 28. Sensitive telescope 29. Unaltered material 30. Universality of processes

31. Stellar component 32. Rate of rotation 33. To heat up/to cool down 34. Conversion 35. To result from 36. Angular momentum 37. Nuclear fusion 38. Assemblage of matter 39. Ejection 40. To achieve certainty 41. To convert smth into smth 42. To rotate on the axis 43. To revolve around 44. As early as the Xth century 45. To challenge a theory 46. Large-scale 47. It’s enlightening 48. To invoke 49. To encounter resistance 50. To postulate 51. An accurate estimate 52. To account for 53. Phenomenon – phenomena 54. Satellites 55. Asteroids 56. Comets 57. To condense 58. To survey 59. A perspective on smth 60. Array of species

Дата добавления: 2015-10-21