Introduction to Rare Earth

The Rare Earth hypothesis suggests that Earth-like planets containing complex life as we know it are likely quite rare in the Universe. This lesson will explore several parameters that have led the scientists Peter Ward and Donald Brownlee to put forth this hypothesis in their book, Rare Earth: Why Complex Life is Uncommon in the Universe. You may wish to refer to this book for a more detailed description of the Rare Earth hypothesis.

Scientists generally agree that the Earth formed about 4.5 billion years ago yet complex life has existed on the Earth for about the last 500 million years. It is still unclear exactly what chain of events led up to the emergence of complex life on this planet. One of the factors that scientists believe to be necessary is a long period of relatively stable climate resulting from a stable planetary orbit at just the right distance from an appropriate type of star.

Let's begin our search for an appropriate star by looking at the characteristics that make the Sun so appropriate for complex life to flourish on Earth. The Sun is a G-type star in the main-sequence phase of its life, which means that it is engaged in the stable burning of hydrogen by nuclear fusion to produce helium in its core, and radiating energy mostly in the form of visible light. The measure of a star's radiation energy is called luminosity. Although it has been brighter in the past, the Sun has been shining this way for about 5 billion years, making it about half-way through its main sequence lifetime of about 10 billion years. Let's explore the types of stars that are sufficiently Sun-like to allow complex life to evolve on their planets. Specifically, our first task is to determine what range of stellar masses meet the criteria for complex life, and to calculate the fraction of stars in the Universe that fall within that particular stellar mass range. In other words we want to know: What fraction of the stars in the sky are right for complex life?