1. Habitability in the Universe: Narrowing our Search for Life Bryan McMahon

2. Outline The Earth Why and Where Should We Look? Habitable Zones The Milky Way How Should We Look? Challenges Ahead

3. Earth Only known planet to harbor life Billions of organisms of varying complexity Life began 3.5 billion years ago Located in the Milky Way http://stacyjulian.com/wp-content/uploads/2009/10/earth-from-space-western.jpg

4. Earth Could it be the only one? Can there be others out there? Is it even worth looking? http://stacyjulian.com/wp-content/uploads/2009/10/earth-from-space-western.jpg

5. Why look? Protection Scientific and cultural gain Curiosity http://24.media.tumblr.com/tumblr_m68mbcqiux1rtbxcno1_500.jpg http://sesapzai.files.wordpress.com/2010/10/alien-abduction.png

6. Planets that Support Life Not all are inhabitable Special requirements for life – Organic molecules – Energy source – Liquid medium

7. What Makes Earth Special? Liquid water – Balancing effect of liquid water in quantities – Must base search on our own requirements – Look for planets that meet requirements

8. Galactic Habitable Zone Zone in a galaxy that would allow life – Hypothetical zone – Relative to galactic center

9. Circumstellar Habitable Zone Shaped much like GHZ Region where liquid water can exist – Minimum of a few billion years Earth fits inside our solar system’s CHZ

10. Metallicity Defined as an containing elements heavier than helium Probability of forming a terrestrial planet proportional to metallicity of parent star – Percent metallicity influences planet size – Larger planets can retain atmosphere – Sustained geologic activity

11. Metallicity Too much can be a problem as well – Strong gravity – Abundant rich volatile compounds – Covered in water The “bully” effect – Hurls planets into the sun or absorbs them

12. Protecting Life Gas giants needed – Reduces comet and asteroid chance – Radiation protection http://upload.wikimedia.org/wikipedia/commons/2/2e/Jupiter_Earth_Comparison.png

13. Milky Way Gradient of metallicity – From center of universe Need certain percentage for terrestrial planets – Defined as having land and sea Stars today – Much higher concentrations of metals – Leads to Mars or Venus like planets

14. Milky Way Four sections – Bulge – Thin Disk – Thick Disk – Halo http://www.news.wisc.edu/newsphotos/images/Milky_Way_galaxy_sun05.jpg http://rohlenscience.pbworks.com/f/1242137268/Side_milky_way.jpg

15. Milky Way Bulge – Wide range of metallicity – High levels of cosmic radiation http://rohlenscience.pbworks.com/f/1242137268/Side_milky_way.jpg

16. Milky Way Thin Disk – Where our solar system resides – Most likely to have life http://rohlenscience.pbworks.com/f/1242137268/Side_milky_way.jpg

17. Milky Way Halo and Thick Disk – Older stars – Generally metal poor regions http://rohlenscience.pbworks.com/f/1242137268/Side_milky_way.jpg

18. New Stars and Planets Generally higher in metallicity than our sun Less heat from potassium, thorium, and uranium Minimal plate tectonics Doesn’t allow for CO 2 regulation

19. Finding Life Looking within the Milky Way Must be able to search ring We probably have a head start – Due to our sun’s composition – Still may be the only ones – Probable we are the most advanced

20. How Should We Look? Most bodies are too far away Many different kinds of telescopes – IR – Visible – UV http://zebu.uoregon.edu/~imamura/122/images/electromagnetic-spectrum.jpg

21. Kepler NASA space telescope Launched in 2009 Measures planetary transits in front of stars Analysts then estimate size accordingly http://www.ballaerospace.com/gallery/kepler/img/08-0468-Kepler.jpg

22. Kepler Imaging Planetary transit is only somewhat reliable http://www.nasa.gov/images/content/511895main_Kepler-11_IntroShot_full.jpg

23. Telescope Needed Similar to the Hubble Operates in visible, UV, and near IR Lens around 8 m in diameter Can search atmospheres for signs of life http://3.bp.blogspot.com/-IDWpZdKa8eI/T7ivrxKzB7I/AAAAAAAAAFQ/4MxHaG8HPE8/s1600/hubble-telescope-photos.jpg

24. Why Don’t We Know Planets that support life are hard to find – Small – Cold (relatively) – Few False positives – Detection or content Mass determined by ratio to sun – Could be gaseous or terrestrial http://www.csmonitor.com/var/ezflow_site/storage/images/media/content/2012/1017/1016-planet/14056169-1-eng-US/1016-planet_full_600.jpg

25. Conclusion As far as we know, we are the only living beings in the universe We must logically narrow our search to even hope of finding life elsewhere Basing our search on what we know to be necessary is the most reasonable Investigating planets with telescopes with hopes of one day confirming life

26. Sources 1.G. Gonzalez, D. Brownlee, P.D. Ward. “Refuge for Life in a Hostile Universe” Scientific American, 2001: October; 62-67. 2.J.F. Kasting. “Habitable Planets: What Are We Learning from Kepler and Ground-Based Searches” Astrobiology, 2011: 11-4; 363-366. 3.J.L. Bertaux. “Solar Variability and Climate Impact on Terrestrial Planets” Space Science Reviews, 2006: 125; 435-444.

27. Questions Feel free to e-mail any questions to mcmahb@rpi.edu