With the discovery of exoplanets in the 1990's, enthusiasm was mounting for the search for life on other worlds. In 2000, a popular book "Rare Earth" was published by Ward and Brownlee suggesting that primitive life is very common but technological life is rare. Without a single other example of life, the book spurred a lot of discussion among scientists and the interested public.
The premise of the book was that too many random, chance conditions were required for homosapiens to evolve. In addition, once our technological species appeared, there were new threats to the survival of life (nuclear war, climate change, over-population). Some of the essential conditions and circumstances identified by Brownlee and Ward are listed in the Table below.
|Right mass planet||Too low mass and the planet could not hang on to its atmosphere; too high mass and the high surface pressure would impede evolution of animals.|
|Right mass star||Too high mass and the star would have a very short lifetime and emit most of its energy at damaging ultraviolet wavelengths; too low mass and strong magnetic activity from the star would strip away the planet atmosphere.|
|Habitable zone orbit||Too close and the planet would experience runaway greenhouse warming, like Venus; too far and liquid water would be frozen and biochemical reaction rates would be sluggish.|
|Atmosphere||An atmosphere is needed to protect life from ultraviolet radiation.|
|Oxygen||The invention of photosynthesis was required to produce atmospheric oxygen for efficient aerobic metabolisms. Microbes could be successful organisms in anaerobic conditions, but not complex eukaryotes.|
|Liquid water||Some surface water, but not so much as to cover all of the land (hard for technology to develop under water).|
|Plate tectonics||To participate in a negative feedback loop and stabilize the climate.|
|Global magnetic field||Without a global magnetic field, the wind of charged particles from the surface of the host star will strip away the atmosphere.|
|Large moon||To stabilize the tilt of the planet.|
|Mars-like world||So prebiotic chemistry and life could get a head start.|
|Gas Giant outer planet||To act as a sink for incoming comets.|
|Right location in the galaxy||Too close to the galactic center and high energy radiation from massive stars would be a threat; too far out in the galactic disk and there would be less chemical enrichment.|
|Mass extinctions||Some mass extinctions to allow for biodiversity, but not too many.|
The counter point made by many scientists was that argument was too anthropomorphic. Some characteristics make sense (right mass planet, atmosphere, water) but others seem less relevant, for a few different reasons:
Not a bottleneck: The circumstances that allowed life on our planet might not be a requirement for life elsewhere. Some characteristics do not present a bottle neck. There are hundreds of billions of "right mass stars," and very few OB stars with short lives and peak energy emission of damaging ultraviolet radiation. It now appears that there are also billions of planets similar in mass to the Earth and orbiting in the habitable zone. Many of these stars and planets reside in intermediate regions in the galactic disk.
Redundant: Some of the characteristics are interrelated. In the same way that there is redundancy in the requirement: "you have to have a tree and it has to have leaves and it has to have a trunk and branches" some of the characteristics follow "for free" from others. A large subset of Earth like planets in the habitable zone will have liquid water, plate tectonics and global magnetic fields. They will naturally outgas or accrete atmospheres.
Uncertain impact: Other factors in Ward & Brownlee's list have an uncertain impact. Did we really need Mars to jump-start life on Earth? If we didn't have Jupiter, would there be fewer objects trapped in the asteroid belt? Would a few more impacts have been a good thing for life on Earth? Does it matter if the obliquity of the Earth precesses at a faster or slower rate?
Mass extinctions fall on a spectrum of natural selection, which must be fundamental for life. It does seem likely that simple microbial life will be far more common that complex organisms. There is likely a pyramid scheme for life, with single cell organisms at the bottom, and complex organisms like whales, velociraptors, and humans near the top. This is exactly what astrobiologists are trying to learn.