At the bottom of this entry, I give the Weekly Pondering 4 assignment, for those of you in ASTR 1/2. Any text that you need to read is on Blackboard or linked to here.

Often in science, we tackle difficult questions by first asking simpler ones. For instance, in thinking about whether there is life outside of Earth, we may wonder about our own star--the Sun. We may decide that there are many reasons to study our Sun, and not least of these is that, since our Sun is a fairly typical star and by far the closest star to us, it provides us with an incomparable opportunity to study a star up close. Therefore, it provides us with the ability to peer into the characteristics of distant stars that we would otherwise only be able to examine through telescopes. We are able to send craft proximate to the Sun, so that we may not only observe its surface but measure the waves which propagate through it. This is the field of helioseismology, which is analogous to seismology, the study of the propagation of waves through Earth. We are thus able to construct a picture of the interior of the Sun, and use this information to study much more distant stars.

As we further consider the question of whether there is life in the cosmos outside of Earth, we may seek to also answer many different questions—such as, for instance: what kinds of stars are there; how do they differ from each other; how common are Sun-like planets; and, how many planetary systems are there which are similar to our Solar System? Answering these may give us insight into whether there are features of our planet, Sun, and perhaps Solar System in general, that are unique. For example, throughout the twentieth century we developed a much better understanding of how to categorize stars in distant parts of the universe. This is primarily accomplished via their spectra, and we discuss this in Lecture 6: Measuring the Stars.

We may wonder if we can directly search for life outside of our planet. During Spring 2021, the NASA rover Perseverance landed on Mars. This rover will search for past biosignatures on the Martian surface, in the hope that, if life developed on Mars, we might be able to detect evidence of this life and begin to understand it. This would provide us not only with a dramatic discovery, but the opportunity to study extraterrestrial life and therefore perhaps to better understand life in the cosmos.

Please watch the interview with Professor Sara Seager linked above, from 15:27 to 40:43. She is asked about the Drake Equation, which is a framework for thinking about the search for habitable worlds. Also, please read Reflections on the Equation (located here), by Frank Drake (2013). Then, open the Drake Equation Calculator. Play around with some of the numbers. Bear in mind that this equation cannot be used to compute the precise number of advanced life forms in the galaxy; this number is unknown. The Drake Equation gives us a quantitative way of thinking about this problem, to aid us in our search for habitable planets. In a paragraph or two, discuss your thoughts on the possible prevalence of extraterrestrial life in the galaxy (or universe, if you wish to be so bold!), given the linked interview, essay, and PBS site.