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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