1. March 21, 2006 Astronomy 20101 Chapter 29: Life in the Universe Does life exist beyond Earth? In our own solar system? Does intelligent life exist? How can we search for life?

2. March 21, 2006Astronomy 20102 The Case for Life Beyond Earth There are billions of galaxies in the universe, and the typical galaxy contains billions of stars: 10 18 stars! Planets have been found around many nearby stars; planet formation seems to be a natural result of star formation. Even if life is rare, there is a strong possibility that it occurred more than once.

3. March 21, 2006Astronomy 20103 The Copernican Principle “There is nothing special about our place in the universe” Copernicus proposed a model for the solar system with planets orbiting the Sun. Later we discover that the Sun is not the center of the Milky Way, nor is the Milky Way the center of the universe. The atoms of which we’re made comprise just a fraction of the stuff of the universe.

4. March 21, 2006Astronomy 20104 Astrobiology “The study of the origin, distribution, and ultimate fate of life in the universe.” Combines astronomers, biologists, chemists, geologists, …

5. March 21, 2006Astronomy 20105 How to Search for Life A basic search for life should include the simplest form of single celled life. Can we find some requirements for life that will narrow a search? Water (can something take its place?) Energy source: light or chemical Chemistry: organic compounds or alternate

6. March 21, 2006Astronomy 20106 Organic Molecules Life on Earth is based on organic molecules: carbon-based molecules with hydrogen, nitrogen, oxygen, or a few other elements. Organic molecules have been found in clouds of interstellar gas and dust! Starting from these organic molecules, chemists formed amino acids in a simple apparatus that mimics the conditions of Earth 4 billion years ago.

7. March 21, 2006Astronomy 20107 Extremophiles Biologists have discovered that life can exist in rather extreme environments: Hot springs where the water temperature is close to 100ºC. Frozen for thousands of years in Antarctic ice, and able to reanimate when thawed. Living without light near deep sea vents called “black smokers”. Living in high acidity environments like the Rio Tinto.

8. March 21, 2006Astronomy 20108 Biomarkers To search for life around other stars, we should look for something distinctive of life: biomarkers. The simultaneous presence of oxygen and methane is a clear biomarker. Unfortunately the Earth didn’t have much free oxygen for much of the time that life existed. An indisputable signal is still being sought.

9. March 21, 2006Astronomy 20109 Habitable Zone The Habitable zone is the region around a star where a planet with liquid water could exist.

10. March 21, 2006Astronomy 201010 Could There Be Life on Mars? Or Europa? Or Encelades?

11. March 21, 2006Astronomy 201011 Intelligent Life Of course, electromagnetic signals from extra-terrestrial life would be a very clear signature. SETI: Search for Extra-Terrestrial Intelligence Scientists scan many radio frequencies searching for non-random signals. The latest idea is to search for brief signals in visible light.

12. March 21, 2006Astronomy 201012 Listening for Signals from Intelligent Life

13. March 21, 2006Astronomy 201013

14. March 21, 2006Astronomy 201014 The Drake Equation “Predicts” number of civilizations we could detect signals from in galaxy, N: R* = rate of star formation (about 10/year) f p = fraction of stars with planets n e = number of Earth-like planets / system f l = fraction of them where life develops f i = probability that intelligence evolves f c = prob. that advanced civilization evolves L = lifetime of advanced technology

15. March 21, 2006Astronomy 201015 The Drake Equation If we take: R* = 10/year f p = 1.0 n e = 1.0 f l = 0.1 f i = 1.0 f c = 1.0 then N = L, or the number of detectable intelligent civilizations equals the length of time that such civilizations survive. Our civilization has been at this stage since about 1900, or 100 years.

16. March 21, 2006Astronomy 201016 Sending Signals We have sent several space probes on voyages out of our solar system. These probes carry plaques with information about us, in case they are intercepted by other intelligent life. Kind of like a note in a bottle.

17. March 21, 2006Astronomy 201017

18. March 21, 2006Astronomy 201018 Discussion Question Should we actively try to send signals to any extra-terrestrial life that may be out there? Please consider what the consequences could be.