Rare Earth Hypothesis
The Rare Earth Hypothesis is actually the summation of two hypotheses. The first is that simple life (like bacteria, for instance) is very widespread in the universe. The second hypothesis states that complex life (and therefore intelligent life) is extremely rare. In defending the two hypotheses, the Rare Earth examines a set of variables that contribute to the likelihood that complex life develops elsewhere in the universe. Their logic follows directly from the variables outlined in the Drake equation. When attempting to assign realistic values to each of these variables, one must consider a wide variety of situations and constraints. It is a subset of these that the Rare Earth Hypothesis focuses on.
If we imagine searching for life forms that are governed by similar rules as life on Earth, then we might expect that many of the events and circumstances under which complex life developed here might be similar to those on another world harboring complex life. For instance, in the Rare Earth Hypothesis there is a discussion about something called the Galactic Habitable Zone. This is a region within a spiral galaxy, like the Milky Way, in which the cosmic conditions are favorable for life to develop on a planet within that region. The Rare Earth Hypothesis purports that this region is fairly small. Within our own Milky Way Galaxy, it is surmised that the density of stars is so great toward the center that the amount of radiation emitted would prevent life from forming on a planet. On the flip side, the density of gas and dust toward the outer regions of the galaxy are so dispersed that there might not be enough “metals” to form stars accompanied by a terrestrial planet like Earth. Therefore, there is a restricted region in which life could form. The size of this region is highly debated amongst astrobiologists and is bolstered by results from astronomy research.
Along with the Galactic Habitable Zone, the Rare Earth Hypothesis asks one to consider the habitable zone around individual stars. Habitable zones are loosely defined as the region around a star where water on the surface of a planet can remain liquid. Since liquid water is the one necessity for all life as we know it, it seems fair to define a habitable zone for life as one which includes water. As main sequence stars grow older, they also grow brighter and emit more radiation. As a result, the size of the habitable zone around a star changes over time. The Rare Earth Hypothesis suggests that a transient habitable zone would be detrimental to the formation and sustainability of complex life on an orbiting planet. However, this neglects other planetary factors (such as mass and atmosphere) that would influence the ability for water to remain liquid despite changes in stellar luminosity.
The Rare Earth Hypothesis also takes into account the characteristics of stars that may influence the development of complex life. For instance, it is important that a star be of significant mass so that its lifetime is sufficiently long for complex life to develop. However, a star cannot be too massive, otherwise it will emit large amounts of harmful radiation and will likely burn out long before life has a chance to become complex. In addition, its energy output should be rather constant, large fluctuations might be hard to accommodate on a young planet with budding life. Of the billions of stars in our galaxy, this limits the types of stars around which we might look for life to F, G, and K spectral types. The Rare Earth Hypothesis argues that this decreases the number of possible stars significantly.
In addition to examining the host star, the characteristics of the planet itself are important to the likelihood that complex life will be able to develop. A planet should be large enough to maintain a healthy atmosphere that will be able to regulate planetary climate and protect life from harmful radiation. Planets that are much larger than Earth tend to be gaseous, like Jupiter and Saturn in our own solar system. These planets are unlikely to develop complex life because they do not have a stable surface and planetary temperatures and pressures are much too great. The Rare Earth Hypothesis proposes that the actual number of Earth-sized planets is extremely low. This guess is based on the current number of planets that have been discovered beyond our solar system. So far, the majority of extrasolar planets has been Jupiter-sized or larger and these planets orbit very close to their host stars. However, it is important to keep in mind that the current technology used to detect extrasolar planets favors the discovery of high mass planets. Current missions like Kepler and Terrestrial Planet Finder (TPF) will be able to locate more of the types of planets, like Earth, that would be likely to develop and foster life.
There are many other factors that contribute to the Rare Earth Hypothesis. These include the idea that Earth has a unique situation due to the fact that Earth has a large moon to stabilize its orbit, or that Earth has a nearby Jupiter to protect it from frequent impacts from comets and other space debris, or that Earth is covered with oceans that stabilize the climate of Earth. All of these factors are proposed to be very unique to the situation of complex life on Earth, and it is argued that it is unlikely that all of these factors would come into play simultaneously at another time and place in the universe. An interesting thought, to say the least. Is Earth unique? Are we truly alone in the universe? The answers to those questions will likely remain unknown for a good many years. But the implications of an answer, whatever the conclusion, will be staggering.
The Drake Equation
Evolution and the Cosmic Environment
The Search for Extraterrestrial Intelligence
Where Are They?
Solar System Formation and Evolution