Debate Notes

Ward, Brownlee, and Gonzalez’s 2001 paper estimates 0.1% of stars as in the habitable interstellar zone. Too close to center= gamma ray bursts. Too far away= high risk of being struck by space debris.

The shifting of the galactic habitable zone- only Earth is in the zone, and it shifts relative to the age of the solar system

The planet has to have suitable geological activity- not having geological activity prevents complex life. Plate tectonics also allows for wide biodiversity and prevents extinction. Plate tectonics may allow biogeochemical cycle water cycle.

Stellar type is highly important. A star that is too large will burn out before complex life appears. A star that is too small will prevent photosynthesis. Red dwarfs overheat one side of the planet.

If a planet is too small, it will not have enough liquid water. It also won’t have enough of an atmosphere. A planet that is overly large will have too much atmosphere and be like Venus- therefore uninhabitable.

The solar system has to have planets that attract stellar debris, otherwise heavy bombardment. Large gas giants have to be on the exterior, with smaller, rocky planets on the inside.

Responding to Marty’s Case:

The large scale of the universe doesn’t prove anything- according to an article in Science, 90% of galaxies are unsuitable for complex life due to gamma ray bursts.

The Drake equation can give a massive range of values, from very small (8x10-20) to extremely large (36.4 million). Many of the terms are highly speculative, including fl = the fraction of planets that could support life that actually develop life at some point

fi = the fraction of planets with life that actually go on to develop intelligent life (civilizations)fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into spaceL = the length of time for which such civilizations release detectable signals into space[8]

Early arising of life proves absolutely nothing. It took 2 billion years for mitochondria to evolve. Additionally, Ayala calculates that the probability of intelligent life is 1 in 10100000.