1. Lecture 35: Are We Alone? Astronomy 1143 Spring 2014

2. Key Ideas Finding planets in habitable zone key to finding life Liquid water critical to life on Earth – always true? Spectra of planets can reveal the presence of life The Drake equation Estimate of the number of intelligent civilizations It only takes 1 civilization and a lot of time to colonize the Galaxy Where are they already? Possible solutions to the Fermi paradox

3. Basic Requirements for Life Stable, Long-lived Source of Energy Energy to fuel chemical reactions Warmth to permit liquid water Complex Chemistry Elements heavier than H and He Carbon, liquid water, inorganics Benign Environmental Conditions Stable, well-regulated climate Protection from harmful UV radiation Location for life to emerge Oceans, land masses (place to swim/stand)

4. The Importance of Water Liquid water may be a requirement for life to exist on any world Certainly true on Earth Life needs to extract energy by chemical reactions (forming new molecules) Many, many substances dissolve in water, so individual chemical reactions can take place Water is liquid at temperatures where chemical reactions happen rapidly

5. The Habitable Zone is the region around a star where liquid water is stable on a planet’s surface Planet too close: Runaway greenhouse effect superheats the atmosphere and vaporizes all the water. Planet too far: Water freezes out and won’t be liquid on the surface.

6. 101 0.1 AU Habitable Zones for Stars 8.5 – 12.5AU 1.5 – 2.2AU 0.08–0.12AU 0.38 – 0.56AU 0.95 – 1.4 AU

7. Are we imaginative enough?

8. The big challenge: Earths are extremely faint compared to the light of their parent stars. The spectrum of the Earth has two components: Combined, the Earth is about 2 billion times fainter than the Sun. Reflection Thermal Reflected Sunlight Thermal Infrared Emission

9. Signs of Life Spectra from exoplanets can show if they have an atmosphere have oxygen in their atmosphere have water in their atmosphere show other features of life such as a “red edge” Discovery of these “biomarkers” would be extremely interesting Don’t have to travel to the planet to look for life, but do need exquisite instruments

10. from Beichmann et al. – NASA Origins website

11. Earth plants are strongly reflective in the infrared, giving their spectrum a distinctive “Red Edge” The way plants keep cool in full sunlight.

12. How many civilizations are out there? Life, of whatever kind, on other planets is fascinating. Intelligent life that we could communicate with would also be fascinating. The Drake equation provides a useful way to think about what we know, what we don’t know, and the probability of intelligent civilizations

13. One reason we think intelligent life must have arisen elsewhere is the sheer number of stars ~200 billion galaxies in the visible Universe ~100 billion stars per galaxy Total of ~2x10 22 (20 billion trillion) stars Even a chance of 1 in 10 12 would yield more than 20 billion possible sites for life.

14. The Drake Equation estimates the number of advanced, communicating civilizations in our Galaxy R  = rate of star formation f p = fraction of stars with planets n e = number of Earth-like planets per system f l = fraction with life. f i = fraction with intelligence f C = fraction with communication technology L = lifetime of an advanced civilization Frank Drake

15. R , the rate of star formation per year, is known from extensive observations. A reasonable estimate: N   100 Billion Stars Age  13 Billon Years R   7 stars/year

16. f p, the fraction of stars with planets, is becoming known from exoplanet searches. Present observed fraction is Optimistic Estimate: f p  0.5 f p  0.15 But, this is mostly giant planets – rocky planets may be more common.

17. n e, the number of planetary systems with Earth-like planets is still unknown, but soon measurable. Should become known in the next decade or so. Depends on the detailed distribution of rocky planets around stars. Right now, we don’t know. Optimistic Guess: n e  1 Allows us establish an upper limit…

18. f l, the fraction of Earth-like planets with life, is currently unknown and conjectural. Some guidance from the history of Earth Life arose within ~100 Myr of the end of the epoch of Heavy bombardment. Optimistic Guess: f l  1

19. f i, the fraction of life that is intelligent, is even harder to guess. Clues from Earth’s history: First life arose ~3.8 Gya Multi-cellular life ~1.2 Gya Cambrian explosion ~545 Mya Land colonization ~475 Mya Homo sapiens emerged ~100,000 years ago Took 100 Myr for life to emerge, but ~40x longer for an intelligent species (us) to appear. Wild Guess: f i  0.1 (is it rarer, or just take longer?)

20. Do we qualify as “Intelligent Life”? You have to wonder sometimes: Only had radio communications technology for ~100 years. Only had limited (short-duration) manned spaceflight for ~50 years. Only sent robotic spacecraft to the edges of our Solar System in the last decade. May or may not yet have sufficiently sensitive radio reception technology.

21. f c, the fraction of intelligent life that is capable of (or interested in) communication, is purely conjectural. The rise of science and technology is very recent cultural development. It entails the ability (and willingness) to make sense of the world in terms of logic and physical principles. Shameless and baseless optimism: f c  1

22. L, the lifetime of an advanced, communicating civilization, is difficult (and uncomfortable) to guess Lower Bound: We’ve only had radio technology for ~100 years Upper Bounds: Next year? 100 years? When habitable zone moves past Earth in ~3 Gyr? When the Sun runs out of core Hydrogen (5 Gyr) Only example is us…

23. A shamelessly optimistic guess… N  = 100 Billion stars f p = 0.5 n e = 1 f l =1 f i =0.1 f c =1 L =100 years (we made it this far … so far …) Age = 10 billion years With 50 civilizations, average distance is  7000 light years

24. The Drake Equation is open to numerous criticisms, and is not without its detractors. Relies on many unknown quantities, and so it is heavy on conjecture. Doesn’t account for population dynamics if interstellar colonization is possible. So where are they already? A single civilization could colonize a whole Galaxy.

25. Interstellar Colonization is a rapid, exponential process. The time to colonize the entire Galaxy is ~500 Myr even with very modest assumptions

26. The Fermi Paradox If any civilizations have been around in our Galaxy for at least ~10–100 Myr they should have colonized the entire Galaxy by now. We should have been visited many times over. But, we have no evidence of extraterrestrial visitation, nor have we found any alien artifacts on Earth. A number of explanations have been proposed … 1950 New Yorker Cartoon by Alan Dunn

27. UFOs Are Real? Extraterrestrial Visitations? No. No reliable, repeatable, or reputable evidence has been offered: Fuzzy photographs Anecdotal accounts of visits and abductions Claims of government conspiracies There are unexplained sightings, but failure to explain them does not justify leaps to truly wild explanations.

28. Civilizations have colonized the Galaxy, but… They aren’t telling us… “Zoo Hypothesis” “Sentinel Hypothesis” A “Prime Directive” (ethics) We are too “primitive” to tell They are too advanced or too different from us. Many examples of “fatal impacts” between peoples in human history, even when well-intentioned.

29. Civilizations colonized us long ago In this theory, we are actually part alien! But this flies in the face of the genetic and archaeological record However, building blocks of life could have been brought on comets….

30. Civilizations have not colonized the Galaxy… …because it is too expensive …because they don’t want to Use slow autonomous robotic probes instead of expensive starships Self-replicating Von Neumann machines Bracewell “Messenger Probes” But not a lot of precedent in human history

31. … because civilizations self-destruct before they can colonize the Galaxy Human populations tend to grow faster than sustainability Tendency toward aggression Poor long-range planning Irresponsibility with technology “Doomsday Hypothesis” Nuclear war Biological warfare Accidental contamination Nanotechnology catastrophe Environmental catastrophe

32. …because Nature can be cruel, which limits the lifetimes of even peaceful civilizations. Possible exterminating events: Asteroid or Comet Impact Supernova or Gamma-Ray Burst sterilizes planet. Ample precedent for this in Earth’s geological history.

33. The Rare Earth Hypothesis posits that complex intelligent life like on Earth is extremely rare We are a product of an extremely unlikely combination of geological & astronomical circumstances. We’re in the right part of our Galaxy We are around the right kind of star We have a relatively benign impact environment Earth has a large moon that stabilizes its rotation Complex life arose late, and intelligence even later Concludes that we are the only intelligent species.

34. My Best Guess Life exists on many other planets, but space travel is prohibitively hard and random communication unlikely Earth is already getting “darker” in its broadcasts We have sent very few deliberate messages into space toward very few locations Nowhere close to long range space flight Very interesting if I’m wrong