The Earth: Just Right

In our march through the terrestrial planets, the next logical stop would be Earth. We have mentioned some of the unique aspects of our home planet in earlier chapters, and will mention more in the course of the book. In particular, we will look at the earth as a home to life when we discuss the search for life elsewhere in the Milky Way. But let’s take a brief moment to think of the earth as just another one of the terrestrial planets. The earth is almost the same size as Venus, and has a rotational period and inclination on its axis almost identical to Mars. How is it, then, that the earth is apparently the only one of these three planets to support life?
As in real estate, it comes down to three things: location, location, and location. The earth is far enough from the sun that it has not experienced the runaway greenhouse effect of Venus. It is close enough to the sun to maintain a surface temperature that allows for liquid water, and massive enough to hold onto its atmosphere. The molten rock in the mantle layer above its core keeps the crust of the earth in motion (called plate tectonics), and the rotation of this charged material has generated a magnetic field that absorbs and holds on to charged particles that escape from the sun in the solar wind.
These conditions have created an environment in which life has gotten a foothold and flourished. And life has acquired enough diversity that the occasional setback (like the asteroid that struck the earth some 65 million years ago) may change the course of evolution of life on the planet, but has not yet wiped it out. Our home planet is truly remarkable, and remarkably balanced. The more we learn about our terrestrial neighbors, the more we should appreciate the delicate balance that supports life on Earth.