Conditions to sustain life
The Earth is known to be an only planet with life. Venus, which is located inside of the orbital plane of the Earth and Mars which is located outside of the orbital plane of the Earth have no sign of presence of life.
Suitable temperature, presence of oxygen, presence of carbon dioxide, and presence of liquid water are considered as indispensable conditions to sustain life. The area that satisfies these conditions are called habitable zone. Our body does not work well without liquid water. Scientists consider that basic structure of creatures, if any outside of the Earth, are similar, because structures of our body are so well designed.
In the solar system, the area which allows liquid water and oxygen is a zone between Venus and Mars. Only planet existing in this area is the Earth.
Is life present only on the Earth?
Recent research revealed that there is a habitable zone outside of the Earth. It is found in Europa, a satellite of Jupiter. Heat inside of Europa is not given by the Sun. Heat is due to tidal force generated when Europa rotates around Jupiter. Heat is produced by internal friction produced by tidal motion.
Searching habitable zone outside of the Earth is a hot topic of science of today.
Europa is one of 4 satellites, so called Galileo satellites, discovered by Galileo Galilee. The surface is covered by thick ice with cracks.
A possibility of life is expected under the ice, because ice seems to melt in the interior of the satellite.
The tidal force between the Earth and moon is not constant. The force at the surface of the Earth near Moon is stronger than the surface of the opposite side. Tidal force shows variation by anomaly of density of the matter inside of the Earth. These variations of force induce high tide and low tide.
These twisting force due to tide is very strong in Europa, so that friction takes place inside of the satellite and heat is generated by friction heat.