Size of the Earth is just right for the survival of human being. If the size of the Earth is either smaller or larger than the present one, we going to have serious problems.
Suppose the size of Earth is a half of the present one, mass of the Earth will reduce to 1/8 and surface gravity will decrease 1/2. If Earth has a small gravity, air molecules start to escape to outer space and will eventually lose most of the air we need to breathe. If this happened on Earth, we could not be here today. This dangerous situation has already happened in Mars and Moon. The size of the Mars is about 1/2 of the Earth and because of small gravity it lost most of air long times ago and retains only 1% of the air on Earth. The size of the Moon is 1/4 of the Earth and has lost all of the air long times ago.
If the size of Earth is twice than the present one, the surface gravity will also increase twice. The increased gravity will cause serious problems. Increased gravity prevents light gases (such as hydrogen and helium) and poisonous gases (ammonia and methane) from escaping to outer space. If the Earth accumulats these toxic gases, we could not have survived on Earth. The large gravity on Earth causes another hazardous situation. Large gravity attracts more asteroids and meteorites and the increased meteor impact rate will threaten life on Earth. We may need to wear helmet and armored vehicle if we want to go outside. Fortunately, current size of the Earth provides just right conditions and environments for the life on Earth.
Fig. 1.6 – Comparison of planet sizes. Small planets can’t hold air whereas large planets accumulate poisonous gases.