As scientists are finding more planets orbiting other stars it is becoming more of a reality that we are not alone in the universe. We may never be able to contact or observe life on other planets, but no one can deny the possibility that life might take root these islands in space. Still, it is important to remember that life on Earth is due to special circumstances.
Ten Factors Required For Life On Earth
A planet orbiting a star does not necessarily result in the development of life. On our planet, we have at least ten factors that allowed life to develop.
1. Not Too Close to Other Stars (Location, location, location)
If our solar system was located near the center of the Milky Way Galaxy, life probably would not have been able to develop on Earth. Stars are dangerous. They do bad things including spitting out radiation that destroys the basic structures of complex life. A planet in a solar system with other stars in the neighborhood is asking for trouble.
2. Our Sun is Special
Life on any planet requires a star, but not just any star will do. Size matters in the development of life. So do the qualities of the star. Our Sun is bigger than most, but still it is a relatively small, stable star and it’s been that way for over four and a half billion years. It will be stable for about another 5 billion years. It also has a treasure of heavy elements that are necessary for planet formation. Without planets, there is no life.
Life around stars of different sizes are possible, but our Sun seems to be about the perfect size for the development of life. In addition, our Sun is 85% brighter than the rest of the stars in the Milky Way, which has been vital in ‘powering’ our planet.
3. A Star’s Habitable Zone
Planet hunters and the media have made a major issue out of the concept of the ‘Goldilocks Zone.’ This is the area around a Sun where a planet is not too close, nor too far away. It is an important aspect of the potential for development of life on a planet, but it is only one factor of many. For Earth, we are resting in the orbit that is just right.
It’s hard to overstate the importance of the Moon for the development of life on Earth. First, the Moon was likely formed in a collision when a small planet-sized object hit Earth and tilted our axis (more on this later.) In addition, the Moon has slowed the Earth’s rotation down (more on this later,) and helped enhance the tidal movement of the Earth’s oceans. The Moon has played an important role in human activity, but just as an important role for all our planet’s species.
5. Size of the Planet
Again, size matters. If a planet is too big and the gravity will inhibit the formation of larger, more complex molecular organic structures. Too small and there can be no atmosphere. Earth is in the zone.
6. Axial Tilt
If Earth’s axis was perpendicular to the plane of the solar system the Sun would heat up the equator creating a zone too hot for most life forms. The poles would have minimal solar heating and would be extremely cold. In between would be the combat zone between hot and cold. Constant violent storms and wind would batter the mid-latitudes.
The tilt of the Earth causes solar heating to warm one hemisphere and allows the other to cool down. Every six months the warm/cool cycle swaps hemispheres. This creates storm systems in both hemispheres, but they act to distribute the warmth more evenly. The tilt of Earth’s axis is almost perfect for nurturing life.
7. Length of Day (spin)
We take the 24 hour day for granted. We shouldn’t. Last year Takanori Sasaki, a planetary scientist with Kyoto University, pointed out that the Earth originally spun so fast that its ‘day’ was only four hours long. Multicellular life didn’t develop on Earth until the day was 23 hours long. It’s is not clear at what point a planet’s rotation makes it habitable, but it seems obvious that a planet’s spin is a factor in the possibility of life formation.
It may be obvious that an atmosphere is required for the development of life, but there are qualities to an atmosphere that are also required. The atmosphere cannot be too thick or too thin. It has to consist of an oxidizer, such as oxygen, to promote chemical reactions in cell structures. There is more to Earth’s air than just air.
9. Liquid Water
Water is necessary for all life that we are aware of, even though it is more important to some species than others. Liquid water is even more important to life than water vapor or ice. It is not an accident that the development of life happened on a planet where 71% of the surface is covered with liquid water.
10. Continent to Ocean Ratio
It’s not obvious, but life on Earth has been helped by the ratio of land to ocean. Land tends to have more temperature variance than the oceans between summer and winter. Land that is not covered in ice or vegetation absorbs much more heat in the summer. If most of our planet consisted of continents, the temperature change from summer to winter would be more dramatic, and less friendly to life.
Earth is Unique, Not Rare
Life on Earth was not an accident, nor is it divine. The conditions that led to the development of life here must exist on millions of planets, but there are an estimated 100 billion planets in the Milky Way Galaxy alone. We are unique, but we cannot be alone. Give life an opportunity and it will seize it.