Saturday, April 2, 2016

Out of the Danger Zone: Why Habitable Zones Support the Rare Earth Hypothesis

Life here on Earth is miraculous; our Earth has numerous unique characteristics that create a habitable space for complex and intelligent life to form. These traits are the reason we exist today and point towards the rare Earth hypothesis, which demonstrates we are the only intelligent life in the universe. One of the most critical characteristics the Earth has is its location in the solar system. We are in the perfect spot; we are close enough to the sun so our climate and orientation can benefit, and we are far enough so we do not experience the harmful effects of the sun. This area in a solar system is known as the “habitable zone.”1 The habitable zone makes the Earth an ideal rocky planet for life to form and develop.

First hypothesized in the 1960s, the habitable zone in a solar system is “the region in a planetary system where habitable Earth clones might exist.”1 The Earth is close enough to the sun so that its oceans are not completely frozen (like Mars). However, if the Earth were closer to the sun, its oceans would be in danger of boiling away. Scientists estimate the oceans on Venus evaporated at least one billion years ago. The complex life on the Earth requires water, and without the sun warming the surface, the Earth would not be able to sustain this kind of life. Scientists do hypothesize, however, that certain forms of life may exist on places like Europa, and they may not need water to survive. These organisms could thrive using only the chemicals available to them.

On the flip side, Europa does not have a sun that can keep its climate mild. The sun warms the Earth’s surface and the atmosphere traps some of the rays. Although the temperature in Antarctica is drastically different from the temperature on the equator, the climate range we experience here on the Earth is minute and ideal for life relative to other rocky planets.

Another benefit of the habitable zone comes from the gravitational force of the sun. The suns gravity pulls on the Earth, and, in turn, the Earth’s gravity puts force on the sun. This exchange of forces puts the Earth into orbit around the sun. The specific position the Earth is in causes its orbit around the sun to be only slightly elliptical compared to other planets in our solar system. This allows the Earth to remain around the same distance from the sun throughout its entire orbit; it only varies by about five million kilometers.2 The almost circular orbit keeps the Earth in the habitable zone, and lets the sun continue to warm the surface most effectively.

The warm sun disappearing after the summer is not a result of the orbit, however. The seasons are caused by the Earth’s axial tilt in relation to the sun. The Earth is tilted at about 23.4 degrees. Throughout a year, the planet is tilted in the same direction relative to the background stars, meaning that when the Earth is on opposite ends of the orbit, different hemispheres are tilted towards the sun. For example, during the summer in the Northern Hemisphere, the top half of the Earth is tilted towards the sun, while the Southern Hemisphere tilts away and experiences winter. Seasons are key for intelligent life mainly because without them, humans would all stay close to the equator where the climate is most mild.

The habitable zone is a key part of our existence on the Earth; it allows the Earth to interact in a series of ways with the sun during its orbit, keeping the climate mild and the oceans liquid. The Earth’s atmosphere traps heat in, and the gravitational forces allow for a more circular orbit. Furthermore, the Earth’s axial tilt gives rise to seasons based on the hemisphere tilted towards the sun. All of these factors contribute to the planet’s suitability for complex and intelligent life, and when put together, these unique characteristics support the rare Earth hypothesis that we may actually be alone in the universe.


1Ward, Peter D., and Donald Brownlee. Rare Earth: Why Complex Life Is Uncommon in the Universe. New York: Copernicus, 2000.

2Williams, Matt. "Earth's Orbit Around the Sun." Universe Today. N.p., 21 Nov. 2014. Web. 1 Apr. 2016.

3Cain, Fraser. "Earth, Sun and Moon." Universe Today. N.p., 12 Mar. 2009. Web. 1 Apr. 2016.
- Sara Jahanian