Planetary habitability is the measure of a planet's or a natural satellite's potential to develop and sustain life. As the existence of life beyond Earth is currently uncertain, planetary habitability is largely an extrapolation of conditions on Earth and the characteristics of the Sun and solar system which appear favorable to life's flourishing—in particular those factors that have sustained complex, multicellular organisms and not just simpler, unicellular creatures. Research and theory in this regard is a component of planetary science and the emerging discipline of astrobiology.
An absolute requirement for life is an energy source, and the notion of planetary habitability implies that many other geophysical, geochemical, and astrophysical criteria must be met before an astronomical body can support life. In its astrobiology roadmap, NASA has defined the principal habitability criteria as "extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism."
The idea that planets beyond Earth might host life is an ancient one, though historically it was framed by philosophy as much as physical science. This article is an analysis of planetary habitability from the perspective of contemporary physical science.
The late twentieth century saw two breakthroughs in the field. The observation and robotic spacecraft exploration of other planets and moons within the solar system has provided critical information on defining habitability criteria and allowed for substantial geophysical comparisons between the Earth and other bodies. The discovery of extrasolar planets, beginning in the early 1990s and accelerating thereafter, has provided further information for the study of possible extraterrestrial life. Most importantly, it confirmed that the Sun is not unique among stars in hosting planets and expanded habitability research horizon beyond our own solar system.
From a Unification perspective, several points can be made:
- Planet Earth is our home. Without it, we would not be alive. Interestingly, it is the only place in the universe known to support living organisms of the type we are familiar with.
- From the perspective of the vast universe, the Earth is just a small speck. Some have likened it to a "pale blue dot," concluding that it is insignificant in importance as well as in size. Some have speculated that intelligent life must have evolved (by a series of chance events) on many planets (or moons of planets) in many other parts of the universe, and that our planet is just one example. The Unification view would agree that the Earth is indeed a small speck in the vast universe, but it takes the position that the Earth's small size does not diminish its uniqueness, importance, or value.
- Scientific research demonstrates that the Earth's position, structure, and composition make it ideally suited for living organisms to thrive. For instance, water and other chemicals in the Earth's surface play enormously significant roles in the development and sustenance of living organisms. In addition, our planet is located at just the right distance from the Sun to receive and trap the appropriate amounts of heat and light needed for such processes as photosynthesis and the water cycle. Moreover, the Earth's rotational and orbital movements allow for daily and seasonal cycles that maintain ecosystems. The Earth is orbited by a Moon that is sufficiently large to produce regular tides that are needed to maintain the health of the oceans and ocean-dwelling organisms. The Moon also stabilizes the planet's axial tilt. The Earth's atmosphere provides useful gases, transports water vapor, traps the Sun's heat, moderates temperature extremes, absorbs harmful ultraviolet radiation, and burns up most meteors before they strike the Earth's surface. The Earth's iron core helps generate a "magnetosphere" that protects the Earth's surface from bombardment by charged particles in the solar wind.
- From the Unification perspective, these and many other features needed for life are not the result of a series of chance events and blind evolutionary processes. Rather, they are the result of the creative work of a highly intelligent Creator. It would be more of a leap of faith to believe that it all happened by accidental events (and blind "natural selection") than to believe it to be the product of a higher intelligent Being.
- Although astronomers have detected planets orbiting stars far from the Solar System, the fact remains that despite years of efforts, scientists have been unable to detect signs of intelligent life (or any living organisms) in other parts of the universe. Some have concluded that intelligent life, if it does exist on faraway celestial bodies, is a rare occurrence in the cosmos.
- The significance of the Earth, its resident living organisms, and human beings in particular, is not based on the size and location of our planet. From the Unification perspective, the uniqueness of human life and other forms of life on our planet underscores the reasons to cherish and value life, and not take it for granted. We are compelled to think of Earth as a single "global village" that we all dwell in and need to share in harmony with one another and with the other animate and inanimate things of the world.
- Unification thought points out that God's own nature is reflected in His creation, and that human beings, endowed with a spiritual dimension as well as a physical dimension, have the capacity to reflect and substantiate that nature to the highest degree. The Earth happens to be the place where we spend our physical lives, during which time we need to exert our responsibility to learn and practice divine love for God, for one another, and for the rest of the created world. In Unification terminology, we need to reach spiritual maturity and fulfill the Three Great Blessings while we are alive on Earth. At that stage, we can enjoy an environment in which divine love is practiced habitually, leading to a world of true peace and happiness. Upon physical death, we are meant to carry this divine nature with us to the eternal spiritual realm, thereby infusing the spiritual world with divine love and happiness.
- From the Unification viewpoint, the Earth is an external expression of God's love for mankind and for the rest of creation. To reiterate, without a planet of this type, we and other creatures would not be alive. The Earth also indicates the unconditional nature of God's love, whether we "deserve" it or not. Our understanding of the Earth needs to remind us of that love, which we need to reciprocate and express in our own lives. In so doing, our spiritual dimension can grow, we can begin to resemble the higher Divinity who created us, and we can fulfill our ultimate goal of establishing a world of true love, peace, and joy.