What Makes the Earth Just Right for Life?

Over at the OUP blog is a great piece by Karel and Iris Schrijver on What Makes Earth ‘Just Right’ for Life?

There is nothing religious or theological about it, however, it looks like a great piece of evidence for intelligent design. The earth just happens to be the one place in the galaxy we know of where it is possible for life to exist.

It appears that what was essential to life on Earth from the outset was the abundance of liquid water. And that has to do with the irradiation of the Earth by sunlight; closer to the Sun would be too warm, further too cold. A somewhat heavier Sun would not have provided stable irradiation for almost five billion years. A lighter Sun would have its ‘habitable zone’ so close that magnetically-driven ‘weather in space’ and gravitational tides would likely pose severe hazards to life. The abundance of water in the Earth’s oceans and atmosphere caused ‘geochemical weathering’ of rocks and soil, a chemical process that removes carbon dioxide from the atmosphere and stores it in sediments. This process removed much of the carbon dioxide in Earth’s early atmosphere, but continued sequestration would ultimately remove so much that plants couldn’t survive. As plants ‘breathe in’ carbon dioxide to grow, this would mean no food, and ultimately no oxygen for mammals, including humans. Carbon dioxide is, however, brought back into the atmosphere by volcanic eruptions (with fossil fuel burning as a modern-day additional source). Volcanism works because of the high temperature inside the Earth, which drives plate tectonics and its associated volcanic eruptions. It also happens to drive the magnetic dynamo that shields life on Earth from hazardous cosmic rays from the Sun and throughout the Galaxy.