The Rare Earth hypothesis holds that one of the reasons scientists on Earth have not found other alien civilizations in our galaxy is that the combination of conditions of the Earth that make it suitable for complex life to ultimately form may be rare. Some factors on which the hypothesis is built include (but are not limited to) an orbit within a star system’s habitable zone, a large moon, plate tectonics, and an abundance of water. Each characteristic that contributes to the Earth’s ability to sustain life is then combined to form Peter Ward and Donald Brownlee’s Rare Earth Equation, which is an alternative version of the Drake Equation. The result is eleven total variables used to estimate the number (N) of planets in the Milky Way with complex life, or, as Ward and Brownlee put it, “the percentage of planets with worms.” Although the Rare Earth hypothesis is plausible, there are many parts that are impossible to test, making the theory questionable. In addition, there are multiple criticisms regarding certain parts of the theory, such as the idea that life only arises in the habitable zone of a star system, that Jupiter plays a protective role, and that rocky planets found within a habitable zone are rare.
Many of the criteria in the Rare Earth hypothesis cannot be tested. A few theories do exist that support that the Earth’s large moon, plate tectonics, and abundance of water are rare. However, current technology does not give scientists the ability to detect these features on planets in other star systems in the galaxy. Therefore, the possibility still stands that these characteristics exist on other planets similar to the Earth; we just may have not been able to detect their existence yet.
Technology also limits scientist’s ability to detect planets that are roughly the same size as the Earth, but the possibility that rocky planets form around stars also still stands. As a matter of fact, researchers have concluded that small rocky planets are likely to form around stars with “diverse elemental compositions,” and that the existence of these planets are not uncommon. Once these small planets are found, the next challenging step is classifying them. But scientists confirm that Kepler has found there are Earth-sized rocky planets out there in the galaxy, and that their compositions may also be chemically diverse.
In addition to discovering these planets, Kepler has also found Jupiter-like bodies relatively close to them. The Rare Earth hypothesis explains that Jupiter acts as a giant guardian from long period comets for the inner solar system planets; its gravitational pull slingshots these comets originating in the Oort cloud out of the solar system. The idea is that the presence of a body as large as Jupiter is beneficial to the development of life on the Earth and is also rare. There are two main problems with this theory, however. First, there is evidence that Jupiter sends short period comets from the asteroid belt flying towards the Earth and other inner solar system planets, which is ultimately dangerous for existing life. Second, the presence of a gas giant in a star system is not as uncommon as scientists may have thought, according to recent discoveries by Kepler and other research.
Although the Rare Earth hypothesis is a well-supported argument, there are still many criticisms. Parts of the Rare Earth hypothesis are used in the Rare Earth equation, which is used to estimate the number of planets in the galaxy with complex life. Unfortunately, many of the variables used are unable to be tested because of the limits of current technology. In addition, there is evidence that rocky planets within the habitable zone of a star system are not necessarily rare. The idea that Jupiter protects inner solar system planets has also been challenged due to recent Kepler discoveries. Overall, the Rare Earth hypothesis is convincing, but it still contains gaps like most other theories.
- Sara Jahanian