Goal: To find life in our universe. Objectives: To understand what to look for To learn about What type of stars and planets to look for if we want to find life

Looking for life outside our solar system: To find life we first need to be able to image planets directly. We need a spectrum

Which molecule, if found in some abundance, would indicate that there was some form of life on the planet? A) Carbon Dioxide B) Nitrogen C) Water D) Ozone

What determines the makeup of the atmosphere? There are 3 processes: 1) geological – volcanoes mostly. Volcanoes spew water, Carbon Dioxide, Nitrogen, and Sulfur Dioxide into the atmosphere

Interactions with the sun Two ways here: 1) UV rays can break apart molecules. This will form some oxygen in an atmosphere for example, but only trace amounts. This can break apart water. This is how Carbon 14 is formed from Nitrogen 14.

Solar wind If a planet has no sizable magnetic field certain gasses (such as water vapor) will be removed from the atmosphere.

Biological This is the one we want to search for. If there are molecules that are a result of biological processes, are short lived, and do not occur much naturally, if we find them, we have found life! Note this will be life in general, like bacterial and plant life, not intelligent life. So, what do we look for?

Smoking guns for life? Nitrogen can be useful. However, it is difficult to detect, and many atmospheres have it naturally (Venus + Mars have 3%, and Titan has mostly Nitrogen). How about molecular Oxygen (O2)? Well, it is even more difficult to observe. Very trace amounts are produced naturally, so you would have to show a lot of it (like our 26%) to be able to say it was life induced, but we still can’t detect it…

Guns? Methane and Nitrous Oxide Methane does not survive long in an atmosphere as it gets destroyed by UV rays. NO tends to react with Oxygen or goes to molecular Nitrogen. Either way both are too trace to be seen with the instruments coming out.

The true gun However OZONE is the key! To have significant amounts of Ozone you need a lot of free Oxygen, To have a lot of free Oxygen you need life! Also, Ozone is fairly easy to detect!

Star type Red dwarf – lack of magnetic field for tidally locked planets. Min mass 0.4 solar. Big stars – die too quickly. Lifetime is: 10 billion / Mass squared So, max mass for life in general is about 4 solar masses. 40 billion stars like this

Metallicity Stars with too little metals will have smaller planets and will lack gas giants to prevent massive asteroid impacts. Stars with too much metals will be more likely to have larger gas giants and hot Jupiters and less terrestrial planets. So the range is probably 1/3rd solar to 3X solar.

Galactic Goldilocks Since there is a slight increase in metallicity as you get to the center of the galaxy you expect life within about 10 kpc of the center. This would increase with time. Develop inner boundary as metals get high.

Planet sizes Too small – loose mag field, loose atmosphere (like Mars). Min radius maybe 2/3s Earth (about ¼ Earth mass). Too big – too much atmosphere, runaway greenhouse. Max, tough to tell, maybe 5 Earth masses.

So that leaves About 20% of stars like our sun are estimated to have an earthlike planet in the habitable zone. So that means we could have 8 billion planets suitable for life.

Wildcard: A large moon. Produced by large impact during solar system formation. No idea how common. Maybe 1 in 10??? Still 800 mil planets with life potential.

How far? Volume of galaxy is ~16 trillion cubic light years. One planet per 2000 cubic light years. One per 13 light years (27 if need moon). Stars that close to us: Tau Seti Alpha Centauri system Epsilon Eridani

Conclusion We have found what a planet needs to be capable of supporting life. We have found what to look for to determine if a planet has life.