In two previous essays, I discussed the Rare Earth Hypothesis of Peter Ward and Donald Brownlee. Earth is a lucky planet, first because the Sun is a stable star; and second because Jupiter and the Moon help to stabilize conditions on Earth. In this third essay, I explain how Earth itself is a mighty lucky planet.
Earth happened to form in the “habitable zone” of the new Solar System, in which temperatures were in the right range to allow water to exist in liquid form. No medium other than water is known in which lifelike processes can occur. Some scientists speculate that the liquid methane on Saturn’s moon Titan may be a medium for life. Even if this is the case, molecules move very slowly in liquid methane, and any metabolism of life forms on Titan would be very slow and the resulting life forms would be very simple. And Earth has plenty of water. The water may have been delivered to the young Earth by comets hitting it before 3.9 billion years ago. The ice in the comets melted and vaporized, creating a haze of steam, much of which was lost into space as new comets continued to rain from the sky. When the collisions became less frequent, and Earth cooled down, the steam became oceans, and water vapor saturated a hot, dense atmosphere of carbon dioxide and nitrogen gases. Mars, Earth’s little brother, also had oceans when it was a young planet.
The Earth is also just the right size. If the Earth were too large, its gravity would be so great that complex organisms (not to mention mountains or even continents) would not be able to stand up. If the Earth were too small, however, it would be unable to hold onto its atmosphere, partly due to a lack of sufficient gravity, and also because particles streaming from the Sun would have stripped the gases away. Mars, which is about half the size of Earth, has an atmosphere—just barely. Its atmosphere is about one percent as thick as ours. At first this seems strange, given that its gravity is at least one-quarter as strong as that of the Earth. But Mars is small enough that its core has cooled off and solidified. On Earth, the currents of molten lava produce a magnetic field that deflects much of the dangerous solar particle stream. Mars has no such protection. The solar particles have scoured away most of its atmosphere, as well as its surface water. When Earth and Mars first formed, they were both wet planets with carbon dioxide atmospheres. Earth kept its atmosphere, and evolved; Mars lost its atmosphere, and (apparently) died. The surface of Mars is without life; and if there is life on Mars, it is deep in the rocks and therefore microbial in size.
The fact that the Earth is not too large and not too small, and is just the right distance from the Sun, has been compared by some scientists to the story of Goldilocks, the Aryan girl who thought that she had the right to barge into somebody else’s house. She found that Baby Bear’s food and bed were “just right.” And if Earth were not “just right” in size and chemical composition, there would be no life on Earth more complex than microbes.
Our planet is also fortunate to have radioactive elements in its core. Without radioactivity, the core of the Earth might have cooled down and solidified, causing Earth to have a fate similar to that of Mars, though it would not have happened quite as quickly.
Our planet also happens to have plenty of carbon, which may be the only element from which life can be built and sustained. As any aficionado of Star Trek knows, silicon-based life forms such as the Horta are conceivable, since silicon atoms have several chemical similarities to carbon. However, silicon is probably too heavy to allow a silicon-based life form to participate in a silicon-based ecosystem. Silicon is a mineral; it is always a mineral. Carbon atoms can form carbon dioxide gas, which circulates through the atmosphere and is turned into complex molecules by photosynthesis. But silicon dioxide is quartz and just stays in the crust of any planet on which it is found.
The most obvious way in which Earth is lucky is that it has water—lots of it. Earth is not quite unique in this respect—Jupiter’s moon Europa is covered with oceans that are capped with ice. But on Earth, the water exists in all three states: ice, liquid water, and water vapor. Not only do life processes, as we know or imagine them, occur in liquid water, but water moves from oceans to continents and back in gaseous form. Europa has liquid water (kept from freezing not by any warmth from the Sun but by the pull of Jupiter’s gravitational field) but has no water cycle as does the Earth. Thus when you consider carbon and water, Earth is lucky not just in what it has but in what it can do with it: carbon and water can circulate around and around on the planet.
Ward and Brownlee conclude that simple microbial life may be common in the universe. But for advanced life to evolve, it is necessary that planetary conditions remain within certain limits for a long time. Such long term stability, and the complex life and advanced civilizations that would require such stability, appear to be vanishingly rare in the universe.
This essay is adapted from chapter 1 of my forthcoming book, Life of Earth: Portrait of a Beautiful, Middle-aged, Stressed-out World, to be released soon by Prometheus Books.