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Rare Earth
Right distance from starRight mass of starStable planetary orbitsRight planetary massJupiter-like neighbor Plate tectonicsOcean and continentsLarge moonRight tilt
Rare Earth
Right number/size of giant impactsRight amount of carbon and ironRight atmospheric propertiesRight kind of galaxyRight position in galaxyRight orbit in galaxyRight history (Snowball Earth/ice age episodes)
Example: Photosynthesis
In order for photosynthesis to work, you need the right temperature blackbody curve and the right atmosphere.
Evolution
Photosynthesis evolved on Earth to take advantage of the abundant energy supplied by the Sun. We only have one kind because that is the most efficient use of our Sun’s energy.
The Sun didn’t just happen to have the perfect conditions to match photosynthesis
Convergent Evolution
A number of different species evolve independently toward the same solution to optimize survival.
Does Earth have the perfect conditions and history for the formation and development of life, or did life though the process of evolution adapt to perfectly fit Earth’s conditions and history?
SETI
Tesla – 1901 picked up signals “from Mars” on a large Tesla coil.
Marconi – encouraged everyone to listen to Martian broadcast during the 1924 opposition with Mars.
Low energy radio waves
Long wavelength radio waves get reflected from Earth’s ionosphere
Early detections of extraterrestrial signals likely due to distant lightning strikes
Drake Equation
N = 1/year × 1 × 1/10 × 1 × 1 × 1 × 100 years
N = 10 civilizations in our galaxy that are currently able to communicate with us.
Distance between civilizations
If N = 10, the average distance between communicating civilizations is about 9000 light years.
Signals from other civilizations
1. Radio/TV broadcasts
2. Interplanetary communications
3. Signal beacons
Interstellar de-coherence
Charged particles in interstellar space tend to spread out or smear coherent radio signals. Interstellar radio messages may not be decipherable.
Discussion
Despite repeated attempts a signal from this region of sky has never been detected again. Should we consider this evidence of extraterrestrial life?