Cosmologic philosophy: rare Earth hypothesis (Introduction)

by David Turell @, Friday, August 05, 2022, 17:08 (5 days ago) @ David Turell

A list of reqasons:

"The Rare Earth hypothesis focuses on numerous aspects of Earth and its environment that played a role in allowing complex life to develop. Some of the key factors Ward and Brownlee felt were critical to the formation of complex life included:

"A planet that exists in a favorable part of the right kind of galaxy, where significant amounts of heavy elements are available and sterilizing radiation sources are located far away.

"An orbit around a star that has a long lifetime (billions of years) but does not give off too much ultraviolet radiation.

"An orbital distance that allows liquid water to exist at or near the planet’s surface.

"An orbital distance that is far enough away to prevent the planet from becoming tidally locked to its host star.

"An orbit that is stable around its host star over cosmic timescales.

"A planetary tilt that allows for seasonal atmospheric changes to be mild, not severe.

"A solar system that includes gas giants capable of preventing debris from polluting the inner solar system, reducing the odds of major cosmic impacts and subsequent mass extinctions.

"A planetary mass large enough to both retain an atmosphere and allow for liquid oceans.

"A moon large enough to help stabilize the tilt of the planet’s axis.

"A molten planetary core that generates a significant global magnetic field, largely protecting the surface from solar radiation.

"The presence of oxygen, and the right amount of oxygen, at the right time for complex life to utilize it.

"The presence of plate tectonics, which build up land masses, create diverse ecosystems, cycle carbon into and out of the atmosphere, prevent a runaway greenhouse effect, and help stabilize the surface temperature worldwide.


"Ward and Brownlee challenged many widely held notions that supported the idea that complex life is out there waiting to be found. For example, while astronomer Carl Sagan often opined that our Sun is an unremarkable star, in reality, about 80 to 95 percent of stars are significantly different from our own in terms of size, mass, luminosity, lifespan, and many other factors.

"Furthermore, prior researchers who had attempted to answer the question of why life on Earth was so plentiful yet so rare in the universe had not included plate tectonics in their thinking at all. Indeed, an entire chapter in Rare Earth is devoted to the topic, going to great lengths to explain the role of plate tectonics in shaping Earth into a good place for life. Earth is, to the best of our knowledge, the only body in the solar system with active plate tectonics. And there are many other features of our life-friendly planet that we haven't seen replicated anywhere else in the universe, too.

"It’s important to remember that the Rare Earth hypothesis only applies to the emergence of complex life. Ward and Brownlee believe that simple life, such as bacteria, is widespread in the universe — after all, even the harshest habitats on Earth harbor microbes. However, the pair feel that complex life, metazoans like animals and us, are exceptionally rare.

“"If you find life elsewhere, it's likely to be microbial,” said Brownlee. “You know, Earth will have a lifetime of about 12 billion years, but [compared to bacteria], metazoans have a much more restricted range of environmental criteria that they can survive in.” That means that a planet’s environment is conducive to simple life for much longer than it is conducive to complex life."

Comment: a discussion of fine tuning that points out simple life might be everywhere but complex life requires very exact circumstances.

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