Originally, scientists thought that intelligent life should be somewhat common in the universe. All one had to do was to estimate the number of planets in the habitable zones of their stars in our galaxy, and there were an unlimited number of candidates. The prebiotic molecules that help kick-start life can be found throughout the universe. So life made up of single cells is probably common on many planets.

However, a closer look at how life formed on Earth suggests that a number of very unlikely events had to occur for the planet to be habitable. First, the sun and its planet have to be favorable.

Our Sun, a very stable and bright star called a G-type main-sequence star, formed approximately 4.6 billion years ago. It will remain fairly stable for billions of years more, although its gradually increasing temperature will become too hot for life on Earth in one billion years. It is estimated to be brighter than about 85% of the stars in our Milky Way galaxy, most of which are red dwarfs. This stability, along with its brightness, has given life on Earth a generous amount of time to evolve, using abundant solar radiation as an energy source.

Obstacle 1 – No Early Source of Abundant Water

The planets in our solar system can be divided into two groups. The ones closest to the sun, inside the orbit of the asteroid belt, are rocky planets that formed without some volatile elements, including hydrogen and carbon. For those closest to it, the sun's heat kept volatile elements from condensing out of the dust and gas disk cloud which formed the planets. The planets further from the sun, from the center of the asteroid belt on, were able to accumulate volatile elements, including hydrogen, and form water.

In the very early solar system, there could have been over 20 proto-planets whose gravitational forces influenced each other as they grew larger, causing orbital instability. Some would have crashed into the sun or each other, especially Jupiter. The orbits of all present day planets have probably moved. Proto-Earth is thought to have developed as a rocky planet somewhere near its current orbit about 4.5 billion years ago.

According to the latest research, the hypothetical planet Theia developed as an ice world between the asteroid belt and the present orbit of Jupiter and contained carbon as well as abundant water ice. Thought to be Mars-sized, Theia was pulled out of its orbit until it eventually collided with Earth, about 9 million years after Earth's formation. The collision, very fortunate in its speed and angle, resulted in a merger giving Earth a metallic core larger than normal, a tilted axis, carbon, abundant water, and a moon much larger in relation to its planet's size than the moon of any other planet in our solar system.

The larger core gave Earth a stronger, longer-lasting magnetic field that shields it from solar wind particles that harm living organisms. The slightly tilted axis gave it seasons. The carbon not only made life possible, but allowed the Earth to have a solid inner core by combining with molten iron. As an ice world, Theia added abundant water early in Earth's history, allowing tectonic plates to form and giving life extra time to evolve. Ocean water today is 0.023% of Earth's mass and covers 71% of its surface. The large moon, tidally locked, provided rotational stability, although the need of its large size is open to debate.

Incidentally, a study of the number of foreign body impacts on the moon has concluded that proportionally, Earth could not have received enough asteroid and comet strikes to deliver the amount of water it has on its surface today. Asteroids and comets could only have delivered 5-10% of Earth's surface water. The water had to have come from collision with an icy planet.

The Earth's orbit lies within the habitable zone of the sun, where it is neither too hot for liquid water to exist, nor so cold that all water freezes. This habitable zone will continue for one billion years more.

Obstacle 2 – Too Little or Too Much Oxygen

Recent simulations show that in the earliest stage of planet formation, there has to be just the right amount of oxygen. As the interior of the forming planet becomes molten, heavy elements sink to the center and lighter elements rise. If oxygen levels are too low during this stage, phosphorous bonds with iron and sinks to the core, leaving too little phosphorous in the planet's outer layers where life develops. If there is too much oxygen, phosphorous stays in the upper layers, but nitrogen tends to drift into the atmosphere and is lost to the planet's crust. Earth's favorable conditions for life are caused in part by having the right amount of oxygen to balance its phosphorous and nitrogen after the merger of Earth and Theia.

Obstacle 3 – No Tectonic Plate Activity

Earth is the only planet in our solar system to have tectonic plate activity. One possibility is that these plates were caused by the weight of the large oceans which bore down on the Earth's crust and caused cracks, leading to upwelling at certain plate boundaries, and subduction at others. The introduction of a huge amount of water very early in Earth's history, may have made tectonic plates possible.