Chapter 19: Life in the Universe

Closeup front view of one antenna of the Allan Telescope Array, a radio telescope for combined radio astronomy and SETI (Search for Extraterrestrial Intelligence) research being built by the University of California at Berkeley, outside San Francisco.

Hollywood has a long history of creating movies that speculate about the existence of complex life beyond Earth. Unfortunately, we currently have no data to suggest that any life exists elsewhere in the universe, let alone complex life. Like many people, astrobiologists have ideas and theories about the prevalence of complex, and intelligent, life in the universe. But just like everyone else, astrobiologists' ideas and theories may differ at times. For instance, scientists working at the SETI (Search for Extraterrestrial Intelligence) Institute in Mountain View, CA do not ask "if" there is intelligent life in the universe, they ask "when" we will discover its existence. In stark contrast, some scientists believe that, although simple life forms may be prevalent in the universe, the occurrence of complex life (and therefore intelligent life) is a very rare event indeed. In 2000, Pete Ward and Donald Brownlee, two scientists at the University of Washington, published a book called The Rare Earth Hypothesis: Why Complex Life is Uncommon in the Universe. It was here that they outlined all of the cosmic, geologic, and biologic happenings that led them to the conclusion that Earth may be the exception, rather than the rule, concerning life in the universe. Widely debated amongst scientists and lay audiences alike, the Rare Earth Hypothesis provides an opportunity to examine the conditions that have resulted in the only example of intelligent life discovered in the universe: ourselves.

This is the galactic habitable zone of the Milky Way, as predicted by Lineweaver et al (2004).

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 contends 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 hotly debated amongst astrobiologists.

A diagram that shows the range of the habitable zone for different sizes of stars.

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 of water to remain liquid despite changes in stellar luminosity.

A Hertzsprung-Russell diagram showing color and size of stars.

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 is of significant mass so that its lifetime is sufficiently long for complex life to develop. However, a star cannot be too massive, or 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.

 

The exoplanets discovered by Kepler that are in the habitable zone of their star systems.

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 was based on the current number of planets that had been discovered beyond our Solar System. In this case, the hypothesis has been overtaken by events — Kepler has discovered several thousand extrasolar planets in the past 5-6 years, and the overall total exceeds 5000. Several hundred are close to Earth's size. Many of these planets, like Earth, are likely to be able to develop and foster life.