The Earth is located at an optimal distance from the Sun that allows for the presence of liquid water, moderate temperatures, a source of energy for life to thrive, and a protective magnetic field.
There are planets that have been described as “Earth-like” because they have similar gravitational forces, an equivalent distance from their host star that keeps them warm but not too hot, and the potential for liquid to exist. However, in reality, Earth possesses a range of other characteristics that make it particularly suitable for life.
First Factor
“First, a type of liquid is needed, an environment where molecules can react,” says Sara Seager, a planetary scientist at MIT, to OurAmazingPlanet. A liquid provides a space for fundamental components, such as DNA and proteins, to swim and interact with each other, facilitating the chemical reactions that lead to life.
And the most promising liquid, which is also the origin of life on Earth, is water. Water is an excellent solvent due to its ability to dissolve many substances. It also floats when frozen, unlike many other liquids, preventing the layer beneath from freezing further.
Liquid water is the first factor that facilitates life on Earth. (Photo: Reuters).
If water sank when frozen, a layer of water on the surface would continue to freeze and sink, eventually leading to all the water freezing and halting the chemical reactions necessary for life.
Astronomers searching for extraterrestrial life often focus on planets located in the “habitable zone” around their host stars. In this zone, planets are neither too hot nor too cold, allowing liquid water to exist on their surfaces.
Earth is a prime example of a “goldilocks” position, situated in the habitable zone around the Sun. If Earth’s orbit were further away or closer, it would be a frigid desert like Mars or a fiery furnace like Venus, and life would not emerge.
Of course, extraterrestrial life might follow different rules than those on Earth. An increasing number of astrobiologists argue for a broader view beyond typical “goldilocks” locations. For instance, liquid water currently does not exist on the surface of Mars or Venus, but it may have once existed. Life could have evolved at that time and then retreated to safer places, such as underground, or gradually adapted to harsher environments, much like some extremophiles found on Earth.
Earth is located in the ideal position relative to its host star to maintain temperatures that are neither too hot nor too cold. (Photo: Reuters).
Moreover, other solvents might harbor life. “Titan, Saturn’s moon, has methane and liquid ethane,” Seager states.
Additional Features of Earth
Second, life requires energy. The most apparent source of energy is the host star. For Earth, sunlight drives the photosynthesis process in plants, which in turn creates nutrients that serve as energy for most life on Earth.
However, many organisms on Earth also exist thanks to alternative energy sources, such as chemicals from deep-sea hydrothermal vents, so energy is not as scarce as solvents.
With all the components in place, life also needs time. Planets require host stars that can live long enough to provide heat and energy for at least several billion years, long enough for life to evolve, as was the case with Earth.
Some stars only live for a few million years before they die. “However, life could begin very quickly, so the age of the star is not that crucial,” says Jim Kasting, an astrobiologist at Pennsylvania State University. For instance, Earth is about 4.6 billion years old. The oldest known organisms first appeared on Earth around 3.5 billion years ago, meaning life evolved over 1.1 billion years or less.
Tardigrades, or water bears, are creatures that demonstrate how life can form in harsh and extreme conditions. (Photo: Shutterstock).
More complex life forms still require more time. It wasn’t until about 600 million years ago that the first multicellular animals emerged on Earth. Because both the planet and the Sun have existed long enough, higher life forms, including humans, had time to evolve.
There are also other details that contributed to the emergence of life on Earth, such as the stability of the Sun’s radiation compared to other host stars and the magnetic field protecting the planet from charged particle storms from the Sun. Intense radiation bursts from the host star could wipe out fragile life forms in their early stages.
“Nonetheless, the research community continues to rethink these factors and their actual importance. We are trying to be more open-minded and discover the gray areas where life might arise,” Seager says.
Earth remains the only known planet with life due to the unique combination of many factors. However, one day humans may discover other planets with similar attributes or find other ways in which life could arise in the universe.
