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

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

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

Earth Could it be the only one? Can there be others out there? Is it even worth looking?

Why look? Protection Scientific and cultural gain Curiosity

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

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

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

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

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

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

Protecting Life Gas giants needed – Reduces comet and asteroid chance – Radiation protection

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

Milky Way Four sections – Bulge – Thin Disk – Thick Disk – Halo

Milky Way Bulge – Wide range of metallicity – High levels of cosmic radiation

Milky Way Thin Disk – Where our solar system resides – Most likely to have life

Milky Way Halo and Thick Disk – Older stars – Generally metal poor regions

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

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

How Should We Look? Most bodies are too far away Many different kinds of telescopes – IR – Visible – UV

Kepler NASA space telescope Launched in 2009 Measures planetary transits in front of stars Analysts then estimate size accordingly

Kepler Imaging Planetary transit is only somewhat reliable

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

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

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

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

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