Peter D. Ward
Professor: Biology, Earth & Space Sciences, U. Washington
Author: Rare Earth: Why Complex Life is Uncommon in the Universe 
(w/D. Brownlee, 2003, Springer)
Author: The Life and Death of Planet Earth 
(2003, Times Books)

How will humanity first discover ET Life?
Best guess:  biosignatures (Solar System) 
Microbial Life
May be right next door, on Mars, or further out in the ice moons of Sol’s children
Likely first found: under the surface of Mars  
Complex Life
Likely never found (including technosignatures) 
Big problem of current life and planet hunters: Calling so many exoplanets “Earth-Like”
Bigger Problem: Are tidally locked planets “habitable” for complex life?

A) Planetary systems may frequently have microbial life
	Robust organisms -  modest environmental requirements
	Easy to form - hard to extinct
	Microbes were on Earth soon after late heavy bombardment
B) Animal-like organisms are likely to be rare
	Need special environments
	Hard to form - easy to extinct
	Took ~4 billion years to evolve on Earth

Which planet is Earth-like?

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