10. Galactic Habitable Zone

Just as habitable zone in solar system, there exists a special region in the Galaxy where the life can thrive safely. This region is called a Galactic habitable zone (GHZ). The radius of our Galaxy is about 45,000 light years and Solar system is located about 26,000 light years from the center. If we are too close to the Galactic center, Earth’s rotation axis will be disturbed by nearby crowding stars. In addition, we will have frequent supernova explosions around us. Supernova explosion generates high energy gamma-rays and x-rays. These high energy cosmic rays destroy Earth's ozone layer and causing the unfiltered ultraviolet light to damage human cells and DNAs.

The phytoplankton is a bottom food chain of the food pyramid in marine ecosystem. Strong ultraviolet light can destroy phytoplankton and as a result all of the marine ecosystem can be wiped out. Phytoplankton also plays an important role for removing carbon dioxide in the atmosphere. The amount of carbon dioxide removed by phytoplankton is about the same as that removed by all trees on land. If we don’t have enough phytoplankton, Earth will become extremely hot due to increased green house effect and will no longer be a cozy habitable planet.

Fig. 1.19 – Phytoplankton that constitutes base food chain in marine food pyramid. It also plays an important role for removing carbon dioxide in the atmosphere.

The distribution of heavy elements in planets depends on the radius from the Galaxy. The amount of heavy elements decreases as we move outside of the Galaxy. If the Earth was too far away from the Galactic center, Earth would not have enough heavy elements to form inner metal core. If the Earth doesn’t have inner metal core, it can’t generate magnetic field. If there's no magnetic field, we will be directly exposed to high energy cosmic rays and UV rays.

Heat generated from radioactive activity inside the Earth contributes global heat budget on Earth. If we were too far away from the center of the Galaxy, we would not have enough radioactive elements to heat up the Earth. In contrast, if we were too close to the center of the Galaxy, Earth would have too much radioactive elements and will generate too much heat to be a habitable planet.

The estimated Galactic habitable zone ranges between 23,000 to 29,000 light years from the Galactic center (green region in Fig. 1.20). Interestingly, our Earth lies exactly in the center of the Galactic habitable zone.

The rotation speed of stars in Galaxy is a function of radius. Stars in the inner part of the Galaxy rotate fast and outer part of the Galaxy rotate slowly. Because of this difference, spiral galaxies can have curved arms. There exists a region where the rotation speed of stars is the same as rotation speed of the Galaxy. This region is called a co-rotation radius and is the safest place in Galaxy. Amazingly, our solar system is located exactly in the center of the co-rotation radius.

Fig. 1.20 – Galactic Habitable Zone (green region).

So far, we have seen many amazing facts about the Earth that we have never thought of before. If one of these ten conditions we discussed was not satisfied, we may not have existed here on Earth today. How come the Earth has such fine-tuned conditions to provide safe and comfortable environments for all living creatures?

Is it a coincidence or God’s design?

"For this is what the LORD says-- he who created the heavens, he is God; he who fashioned and made the earth, he founded it; he did not create it to be empty, but formed it to be inhabited-- he says: "I am the LORD, and there is no other." Isaiah 45:18