Searching for Life on Other Worlds: Drake Equation and the Possibility of Intelligent Civilizations

1. Phys 1810: Lecture 35 Life on Other Worlds continued. No more Star Fleet Academy Monday is last office hour. Coming up: Can we communicate with aliens? Hunt for them? Review for exam. Example of Extremophiles: Tardograde DNA could survive rentry from space! http://www.scientificamerican.com/article/dna-can-survive-reentry-from-space/?WT.mc_id=SA_Facebook

2. 28.3 Intelligent Life in the Galaxy The Drake equation, illustrated here, is a series of estimates of factors that must be present for a long-lasting technological civilization to arise. Estimates the number of civilizations we could attempt to communicate with in the Milky Way Galaxy at any representative time (such as the present). Make your own estimate!

3. Life as we know it – searching for Mr. Spock. Term 1  More accurately determined. Term 2  Term 3  Term 4  Term 5  Difficult to estimate. • N = the number of civilizations now = # of stars in the Milky Way * fraction of appropriate stars * fraction of those stars with planetary systems * # of planets suitable for life in each exoplanet system *fraction of suitable planets upon which intelligent life appears * fraction of planets that produce a civilization with interstellar communication * lifetime of that civilization / time that appropriate stars have existed. Term 6  Term 7  Term 8 

4. Drake’s Equation: Term 4 • The number of planets suitable for life in each exoplanet system: How many rocky planets reside in the Habitable Zone (HZ)? Using our solar system as an example, almost 3 rocky planets are in the HZ. • 1 planet is too hot • 1 planet has too little mass to retain its solvent as a liquid. • adopt the value of 1 appropriate planet in the HZ • 1 * 1.5 * 10**9 stars When you make your own calculation, adjust this up to “3 times” if you like.

5. Drake’s Equation: Term 5 • What fraction of suitable planets produce life? • e.g. given the ingredients (C, N, H and H20) and assuming life spontaneously arises. • 50-50 chance • ½ * 1.5 * 10**9 stars = roughly 1 * 10**9 stars with a planet with life on it. Textbook uses 1 – optimistically = 1.5 billion stars. Dimitar Sasselov TED talk 2010: 100 million habitable planets. Sara Seager TEDX talk 2013: ~2 dozen Earth-like planets discovered.

6. Drake’s Equation: Term 6 • What fraction of planets with life produce intelligent civilizations that develop a technology that releases detectable signs of their existence into space? • e.g. of Issues - Mass extinctions: • one needed to wipe out the dinosaurs. • alternatively it could wipe out life either altogether or to the microbial stage. • perhaps less than 50% of the exoplanet systems are lucky to have this happen and survive???  1/3 * 10**9 stars The least constrained term is term 7, life time of the technological civilization. Let’s leave this for the moment.

7. What is true about this image? • It is the famous Ring Nebula. • Our sun will look like this as it dies. • Carbon is an element in these objects. • It is a Planetary Nebula. • All of the above.

8. Drake’s Equation: Term 8 • How long have appropriate stars existed? • i.e. How long has carbon existed? • study elements in Planetary Nebulae and those that are younger than ~6 * 10**9 years old have enough carbon.  the civilizations we seek have occurred within the last 10 billion years (c.f. Universe’s age 13.5 billion years)

9. Life as we know it – searching for Mr. Spock. What is your estimate for N? N = your_fraction * lifetime of that civilization • N = the number of civilizations now N = 1/3 * 10**9 stars * lifetime of that civilization / 10 * 10**9 yr N = lifetime of that civilization / 30

10. Life as we know it – searching for Mr. Spock. • Other Estimates: • Most Favourable Case: • N = 120 * lifetime of that civilization • Least Favourable Case: • N = lifetime of that civilization / 10 * 10**9 • Textbook Estimate: • N = the number of civilizations now N = 1/3 * 10**9 stars * lifetime of that civilization / 10 * 10**9 yr N = lifetime of that civilization / 30

11. Life as we know it – searching for Mr. Spock. Discuss with your neighbour how long civilizations last. Consider the Egyptians right through to how long radio broadcasts have existed. Does nuclear war or climate change play a role in your estimate of a technological civilization’s lifetime? • N = lifetime of that civilization / 30 • N = 120 * lifetime of that civilization • N = lifetime of that civilization / 10 * 10**9 • N = lifetime of that civilization

12. What are our survival chances? Astrobiology meets sustainability science “Using the Drake Equation as a vehicle to explore the gamut of astrobiology, we focus on its most import factor for sustainability: the mean lifetime L¯ of an ensemble of Species with Energy-Intensive Technology (SWEIT).” Maybe other technological civilizations have been through this before us. Is L 200 years, 500 years or 50,000 years? Fig. 3 Solution space for SWEIT evolution. Shown is a schematic of possible stability domains in a single N (population), e c (energy harvest rate per capita) plane. As shown schematically in the figure, region III is most likely to drive unstable incre... Adam Frank , Woodruff Sullivan Sustainability and the astrobiological perspective: Framing human futures in a planetary context Anthropocene, Volume 5, 2014, 32 - 41 http://dx.doi.org/10.1016/j.ancene.2014.08.002

13. What are our survival chances? Astrobiology meets sustainability science Our current climate change  • Thus the evidence is indeed strong that during our present Anthropocene epoch the coupled Earth systems are being altered on an extremely rapid time scale. Although such rapid changes are not a new phenomenon, the present instance is the first (we know of) where the primary agent of causation is knowingly watching it all happen and pondering options for its own future. In this paper we have argued that it is unlikely that this is the first time such an event as occurred in cosmic or even galactic history. The point is to see that our current situation may, in some sense, be natural or at least a natural and generic consequence of certain evolutionary pathways. Given that fact it may be possible to use the data and perspectives of astrobiology to tell us something about optimal pathways forward. One point is clear, both astrobiology and sustainability science tell us that the Earth will be fine in the long run. The prospects are, however, less clear for Homo sapiens. • http://www.washington.edu/news/blog/what-are-our-survival-chances-astrobiology-meets-sustainability-science/

14. Life as we know it – searching for Mr. Spock. Discuss with your neighbour how long civilizations last. Consider the Egyptians right through to how long radio broadcasts have existed. Does nuclear war or climate change play a role in your estimate of a technological civilization’s lifetime? • N = lifetime of that civilization / 30 • N = 120 * lifetime of that civilization • N = lifetime of that civilization / 10 * 10**9 • N = lifetime of that civilization Calculate how many civilizations that we might be able to communicate with could exist in the Milky Way.

15. TED talks Dimitar Sasselov http://www.ted.com/talks/dimitar_sasselov_how_we_found_hundreds_of_potential_earth_like_planets?language=en#t-1013911 Sara Seager http://www.youtube.com/watch?v=NnM4SaGc8R0