1. Search for extraterrestrial intelligence (SETI) The Drake equation (Frank Drake, 1961) is a back-of-the- envelope calculation estimating how many other planets with civilizations might be in our galaxy to communicate with. Make your own estimate by starting with 400 billion stars in our galaxy and gradually reducing this number: (Number of stars in our galaxy)  (Fraction of stars that are single stars)  (Fraction of single stars that are Sun-like)  (Fraction of Sun-like stars that have Earth-like planets)  (Fraction of Earth-like planets with life)  (Fraction of those with intelligent life)  (Fraction of those with technology, such as radio) = Number of civilizations in our galaxy

2. This exercise reveals that the most uncertain assumptions are at the bottom of the list, such as guessing the probability for developing life on an Earth-like planet, the chance that intelligent life arises, and how long it lasts.

3. How many planets are out there ? In recent years, a planet hunt has started in our cosmic neighborhood. The Kepler satellite was launched 2009 with this purpose, and lots of ground-based telescopes are searching as well. Thousands of planets have been found. Most are much bigger than Earth, because large planets are easier to detect. Recently, an Earth-sized planet was found at one of our closest stars (  -Centauri, only 4 light-years away). But it is way too hot (glowing orange at 1200 o C). Bottom line: Planets appear to be plentiful, but those suitable for life are at our detection limit.

4. How quickly did life develop ? The Earth formed 4½ billion years ago, but was under heavy meteorite bombardment until 3.9 billion years. Primitive life developed rather quickly thereafter, with the first chemical evidence appearing after 0.1 billion years, and the first fossils appearing after 0.5 billion years. It took 3.5 billion years to form complex, multicellular organisms, such as humans.

5. What about intelligent life ? Early humans began using tools 2-3 million years ago, developed agriculture 10 000 years ago, and invented radio 100 years ago. This period for communicating with intelligent life is rather short compared to the 3.5 billion years that it took for humans to develop. How long will the rapid pace of technology continue ? Humans may self-destruct in another 100 years, and the Earth will be ruled by cockroaches. These hardy critters have survived extinctions for 300 million years.

6. Where is everybody ? In 1950 Enrico Fermi wondered over lunch why we have not seen any evidence for intelligent life, with so many planets likely to exist in our galaxy. A few possibilities : 1) Interstellar travel is impractical (Lect. 17, Slides 4,5). 2) Radio communications take a long time, because our galaxy has a diameter of 100 000 light-years. 3) Intelligent life self-destructs too quickly via nuclear war, overpopulation, exhaustion of natural resources.

7. A somber possibility Suppose that intelligent life with radio develops quickly on a geological time scale (say every million years some- where in our galaxy). Also assume that such a civilization lasts only briefly (say a hundred years). Then the civiliza- tion preceding us on some other planet would have self- destructed long before we developed radio. The next ci- vilization would arrive in a million years. There would be a window of a hundred years to connect. But it takes 100 000 years for a round trip radio signal half way across the galaxy. By then, we and the other civiliza- tion would have self-destructed. Communication would be one way (listening or sending).