Why Earth is a Nice Place to Live
Science fiction writers may invent planets other than our own as wonderful places to live but as far as we know there is nothing like Earth in this solar system or in any other. Our Sun is a long-lived stable star with a reliable energy output over billions of years, and our planet is just the right distance away to avoid getting overheated like Venus or being out in the cold like Mars. Earth is big enough (that is, its gravity is strong enough) to retain an atmosphere (which would escape on a small planet like the Moon). Also, the atmosphere is dense enough and has the right composition to form a pleasant greenhouse blanket. Seventy percent of Earth is covered by water, which ensures there is plenty of water vapor available for the atmosphere, and this, in turn, ensures that the greenhouse effect is strong enough for producing an average balmy 60°F on the surface of Earth.
The Earth rotates about its axis once every 24 hours, which results in alternating heating and cooling in a daily cycle, so that extremes stay close to the mean. The ocean and the presence of freshwater also aid in temperature control because of the large heat capacity of water. In fact, much of the heat generated by the human-induced excess greenhouse effect appears to enter the ocean. Likewise, water is important in carrying heat from where it is hot to where it is cold, both in the form of "latent heat" as the water vapor in the winds, and as warm water in the ocean currents.
In addition to having the right distance from the sun, an ocean, an atmosphere, and a 24-hour rotation, the Earth also has a rotational axis that is at a convenient angle to the plane of the orbit. The inclination of the axis (23.5° relative to the vertical to the orbital plane) results in considerable annual variation in the sunlight received in middle and high latitudes, which makes for the seasons, summer and winter! If the Earth's axis were vertical, the polar regions would have much more ice than now, because the summer sun would be too weak to remove winter snow poleward of 50 degrees latitude or so.
Earth as a Habitable Planet
Earth is the only planet we know where water exists widely in its three phases as a solid (as snow and ice), a fluid (in the ocean and in lakes, rivers and groundwater) and as a gas (in the air, as a vapor). Why does Earth have an ocean, while the Moon, Mars, and Venus do not? A satisfying answer to this deceptively simple question can only come from a careful consideration of the evolution of each planet. For now, a crude explanation is that the moon is too small, Mars is too cold, and Venus is too hot.
To illustrate this let us make a simple thought experiment. First, we reduce gravity on Earth to that of the moon. In this situation, the Earth would have a problem holding on to its atmosphere. The air would start to get thinner as the gas molecules of the atmosphere leave the planet. The ocean would release its own gas content, mainly carbon dioxide, and then it too would slowly evaporate in an attempt to replenish the atmosphere. These atmospheric molecules would escape into space until the ocean would be used up.
In another mental experiment, we keep Earth's gravity as it is but move Earth into the orbit of Mars. We are now a good deal further from the sun (1.5 times to be exact), and much less energy would arrive from the Sun, with the solar constant being reduced by a factor of 1.5 squared. Earth would immediately enter an ice age worse than anything seen in geologic history. The oceans would completely freeze over, down to great depth. Volcanic ashes and wind-blown dust would by and by cover this ocean of ice, and it would disappear from view. Because of the low temperature, water content in the atmosphere would be very small. The composition of the atmosphere would change drastically, since its composition is currently maintained by life’s activities. The end result might not be much different from what we see on Mars.
Finally, let us move Earth into the orbit of Venus. Immediately, it would become unpleasantly hot, since we are now much closer to the sun (about 1.4 times closer). The entire atmosphere would rapidly turn into a steam bath. Thick clouds would form in the upper atmosphere where the vapor would condense. The Earth would turn white and thus reflect much of the incoming sunlight. However, the steamy atmosphere would also act like a thick thermal blanket. The sunlight that does reach the surface would keep heating it, evaporating ever more water and making the atmosphere ever less transparent to outgoing heat radiation. The temperature would rise until rocks called limestones, made of calcium carbonate (CaCO3), would disintegrate and give off carbon dioxide. Large amounts of this greenhouse gas would now be added to the atmosphere. The result would be something like a wet Venus, a state perhaps not unlike a former state of that planet.
The Gaia Hypothesis
These thought experiments, however fictional, illustrate that Earth's benign climate is rather delicately balanced. One of the truly astonishing facts about the history of this Earth is that it has had an ocean for a very long time. Life needs water, and life has existed for more than 3 billion years. Hence, free water must have been present uninterruptedly for most of the age of Earth (4.6 billion years). If the sun had been much colder or hotter in the past, the hydrosphere would either have frozen or boiled away into the atmosphere.
Astronomers tell us that the Sun was less bright early in Earth history. It sent considerably less energy three billion years ago than today, yet the Earth would have had to stay warm enough to harbor living organisms. How the sun could have been less bright while the Earth maintained a constant temperature is called the “Faint Young Sun Paradox.” One answer to this problem is that the atmosphere may have been much richer in greenhouse gas than today, with carbon dioxide content presumably many times greater.
Life processes are intimately involved in helping to determine environmental conditions on Earth; in fact the present composition of Earth's atmosphere is entirely the product of bacterial and photosynthetic processes. The intimate association of biological and geochemical processes is studied in the field of “biogeochemistry,” whose results have given rise to the “Gaia hypothesis,” a suggestion that life processes control Earth's environment for the benefit of living organisms. (See the Glossary to read more about biogeochemistry and the Gaia Hypothesis.)