Water is an indispensable element for all living creatures and Earth is the only planet containing vast amount of water on the surface in a liquid form. If the Earth is placed closer to the Sun, all water on Earth will be boiling and will eventually be escaping to outer space. If the Earth is placed further away from the Sun, all water will be frozen and Earth will no longer be a comfortable place to live.
Fig. 1.1 – Earth seen from the space
In solar system, there exists a special zone where water remains not in boiling or frozen state but in liquid state. This zone is called a Habitable Zone (or Goldilocks zone). If we calculate range of the Habitable Zone in solar system, it becomes 0.95 – 1.15 AU (Astronomical Unit: 1 AU = distance from Earth to Sun). Thus, if the location of the Earth was 5% closer to the Sun, all of the water on Earth would have boiled and if it was 15% further away from the Sun, all water would have frozen.
The green area in Fig. 1.2 shows the Habitable Zone and occupies only 0.05% of the area of orbital plane swept out by all 8 planets. Amazingly, our Earth is located right in the center of the Habitable Zone.
Fig. 1.2 – Habitable Zone (blue color) in solar system. The circles outside of the Habitable Zone are orbits of Mars, Jupiter, Saturn, Uranus, and Neptune, respectively.
Another important factor for the Earth to remain within the Habitable Zone is that Earth has to rotate around the Sun in a circular orbit. If the Earth has elliptical orbit with an eccentricity larger than 0.1, all of the water will be frozen at A, C points and boiling at B, D points in Fig. 1.3. Fortunately, the eccentricity of the Earth is very small (0.017) and the Earth rotates around the Sun in almost circular orbit.
Fig. 1.3 – If the Earth (green point) orbits around Sun (yellow point) with eccentricity larger than 0.1 (dotted line), water on Earth will be frozen at A, C points and will be boiling at B, D points.