The Earth is different because it has life on it…obviously. And the factors that make it so, is why it is regarded as a fragile planet.
Explanation:
For example, one of the reasons Earth supports life is its location in the solar system. The venerable Habitable Zone. If the earth was, say in Venus’ orbit for example, runaway greenhouse effect would create so much heat, it would not be able to hold surface water.
So we’re lucky to be in the correct distance from the Sun, however, distance is not the only factor here. If the planet was covered in ice for example, its reflectivity (also called albedo) would cause the surface temperature to be either cool, if albedo is high and vice versa.
So, again, we’re lucky to have just the right amount of surface reflectivity. How about our atmosphere? The atmosphere provides us with life sustaining weather, temperature regulation and shields us from deadly radiation. The composition of our atmosphere is crucial in how atmosphere interacts with solar radiation. Some gases, such as Carbon Dioxide and Water vapor, allow visible light to pass through which is good for those important things like…seeing. Unfortunately, these same gases prevent radiation from being reflected back out into space in what is referred to as The Greenhouse Effect. Every planet with an atmosphere has this at different degrees and is what heats up the planets. The dissipation of the greenhouse gases also determines for how long the planet will retain its heat, for example, water vapor may take a week while Carbon dioxide takes about 100 years. Again, luckily for us, we only have a trace amount of Carbon dioxide, less than 0.1% of it.
How about the one ingredient that we can’t do without? Oxygen? It turns out that we don’t have that much in the atmosphere. Only about 21% of our atmosphere contains oxygen. If we had less, we would have weak hearts, prone to diseases, slow and probably still be in the dark ages. If there was too much of it and someone farted, the atmosphere would simply combust and murder everyone since oxygen is highly reactive. The highest oxygen content that could be safely stored in our atmosphere is about 35%. Luckily we have giant recycling plants called…well, plants. We also have oxygen reserves in our oceans, crust and beneath ice caps.
The odds for all these factors that have occurred on our little blue marble, albeit over millions of years, to be replicated on another planet and support life as we know on earth is literally a 1 in a not-happening.
All these things are maintained by a very delicate natural balance and can unfortunately be undone by both natural and man-made effects. Hence, our planet is very fragile.
