The ages of the Sun and Stars

Part 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.

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.