Lesson 1: Ecology of the System 1.0 Introduction and Overview Figure 1.0.1 Artist's rendition of early Earth We know that the solar system is a good place to look for Life, because the third planet from the Sun is our home, and we live on it together with a myriad of other organisms. What we would like to know is what it is about the solar system that makes it a good habitat for life. Is there anything special about the Sun as a star that favors the development of life, once we have an acceptable planet to host life? Is there something special about the dust cloud from which the solar system formed? Is there something special about the Earth - its composition and its size - without which there could be no life? If there were no other planets but Earth, would this still be a good place for life to develop and persist? Or is it not possible to have Earth without having at least some of the other planets? What we know is that all the conditions were just right (otherwise we would not be here). But we do not know precisely which of the conditions we see are of vital importance (literally) and which are incidental, that is, which ones we could have done without. It is unlikely that the solar system is exactly duplicated elsewhere, because it has grown historically from conditions containing elements of chance. If we knew which of the conditions are of vital importance, we could make better estimates of the likelihood of life elsewhere, than if we just look for twins of the solar system. The dilemma we have described is a special case of the so-called "anthropic principle", propounded by the British physicist Brandon Carter (born 1942). Carter pointed out (what seems intuitively obvious) that any observation by an intelligent observer (such as a human) implies a privileged place in the universe, where such an observer could evolve. Thus, our solar system is definitely privileged. The question is, can we find out what are the ingredients of privilege and what are the accessories. Figure 1.0.2 The Solar System To deal with this rather difficult problem (it is by no means solved) we must look at the various properties of the solar system, and their historic emergence, beginning with the Sun, and moving outward through the planets. At each step, we must ask ourselves whether a property and the way it evolved is of vital importance, or whether we could envision a different path with a different outcome, and still have a place where Life could find a home. Since we do not know how Life arose, all such musings are necessarily speculative. However, they are not useless. They force us to come to grips with the most fundamental questions that science can ask - how unique is Life? Is it something that arises spontaneously from lifeless matter? Once or many times? Here only or in many places? Clearly, the single most interesting thing about the solar system is that it provides an abode for living organisms. It is the reason why it is worth knowing how the Sun produces sunlight - without the Sun's long-lasting energy production Life would not have enough time to evolve. It is the reason why it is worth knowing how the elements are made that we consist of. It is the reason why we would like to know how planets are made. And it is the reason to study how the planet Earth evolved along with the organisms it sustains. Without our evolving planet, we would be nowhere.