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Despite the extraordinary diversity of life on Earth, our planet is far from hospitable. According to one estimate, 99.5 percent of the Earth’s habitable space is completely inaccessible to humans.

For a planet to be habitable, it must meet four criteria:

First, it has to be just the right distance from a star. A planet that is too close to a star will be far too hot to sustain life – but too far away, and it will be too cold for life to thrive.

In fact, if the Earth were just 5 percent closer to the sun, or just 15 percent further away, life would not have developed.

Second, the planet must have an atmosphere that shields life from cosmic radiation. On Earth, we can thank our planet’s molten core for providing us with a protective atmosphere.

Third, we need a perfectly sized moon. Without our rocky, dimpled companion, the Earth would spin much faster. Its dizzying spin would cause the climate and weather to go haywire.

Fourth, timing is everything. The complex sequence of events that led to our existence had to play out in a particular manner at particular times to produce life. For example, our moon was formed after a planet the size of Mars crashed into Earth around 4.4 billion years ago. We can thank this collision for giving us our perfectly sized moon. We can also thank the fact that it happened billions of years ago, before the development of life. Had it happened later, it might have snuffed out life on Earth altogether.