Editor’s Note: As a series at ENV, we are pleased to present "Exoplanets." Daniel Bakken is an engineer who teaches astronomy at the college level, and an entrepreneur in compound semiconductor crystal growth. In a series of articles he critically examines recent claims about exoplanets beyond our solar system, asking whether our own planet Earth is a rarity, or common, in the cosmos.
It is often speculated that life could arise on moons, dwarf planets, or other space bodies as well as planets. Yet these bodies are very limited in their ability to offer the right kind of habitat for intelligent life, the kind that could build rockets and radios. If we just want to look for a body that can sustain some kind of life, they may provide a broader field. However, in answering the question "Is there anybody out there?" we likely won’t be satisfied with microbes barely surviving on a moon. We are looking for much complex life, with a brain capacity similar to our own, and the ability to modify its surroundings into complex technology. This means the factors that can allow complex multicellular technological life.
A classification system has been proposed that puts habitable planets into four classes. Class I are those that maintain liquid water on their surface by being exposed to energy from their host star, and would include the Earth.
Class II bodies start out their cosmic history able to support liquid water on their surface, yet lose that ability within a few billion years. They most likely cannot produce technological life, and are presently lifeless. Mars and Venus are probably in this class.
A Class III body has a liquid water environment under its crust of ice, as Jupiter’s moon Europa does. This water has contact with a silicate core, providing a supply of materials that can interact with the water. Bill Nye, among others, has speculated that "We May Discover Life on Europa."
Class IV is similar, yet has its ocean bounded above by an ice crust and below by ice separating this ocean from the silicates below. Ganymede, Callisto, and many of the Saturnian satellites likely fall into this class.1 This classification system roughly follows the most habitable bodies with liquid water to the least, but also takes into account cosmic history. As we can see, it can be argued that liquid water by itself is necessary, but may not be sufficient for life.
There can be too much water on a planet. It is difficult to imagine technological civilizations taking hold on a "waterworld," since smelting metals using fire would most likely be impossible. The conditions that can allow for the continued support of life may also be significantly different from the conditions that are required for life’s origin. For an example, many origin-of-life pathways that are suggested include wet-dry cycles that would be impossible on a waterworld.2 For this reason just "habitable" seems too optimistic, since we are also constrained to a habitable planet that at least had in its past the conditions to allow life’s origin. This means that there are likely planets where life could be sustained, yet are lifeless.
Considering these four classifications, it is hard to imagine higher life forms populating anything other than Class I. It is straightforward then to limit our search for alien technological civilizations to Class I habitable worlds.
Next up: What’s a Circumstellar Habitable Zone?
(1) H. Lammer et al, "What Makes a Planet Habitable?" The Astronomy and Astrophysics Review 17 (2009):218-219, accessed March 29, 2014, DOI 10.1007/s00159-009-0019-z.
(2) Guillermo Gonzalez, "Setting the Stage for Habitable Planets," Life 4, No. 1 (February 21, 2014): 36, accessed May 23, 2014, doi:10.3390/life4010035.