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Is Earth Unique?
By the time the first astronauts went into space, science had long established that the Earth is round. Yet, to them, looking back on their home planet from outer space must have been a marvelous sight nonetheless: the blue planet suspended in empty space!
To some, Earth might simply be a pale blue dot in the vast cosmos (a term inspired by a picture the Voyager 1 probe took from a record distance), but it is undoubtedly a very special one. The precious blue marble is, of course, the only place in the universe so far confirmed to host life.
Of the over 4,000 exoplanets discovered so far (and that number will invariably increase while more advanced telescopes scan the sky), none seems suited for life as our home planet is. What is it that makes the Earth stand out?
This article covers the following 11 reasons that Earth alone among all known planets is uniquely equipped to support life:
- Earth is the right size
- Earth is the right distance from its star
- The sun is the right star for Earth
- Earth has liquid water
- Earth has an abundance of elements
- Earth's rotation
- Earth's tilted axis
- Earth has a large moon
- Earth has the right atmosphere
- Earth has a magnetosphere
- Earth has plate tectonics
1. The Right Size
Any planet to host life must have an atmosphere. If the Earth was smaller it couldn't hold its atmosphere and life couldn't be sustained. Conversely, if our home planet would be significantly larger it would hold on to too much atmosphere. The thick atmosphere in turn would trap too much heat and the planet would become uninhabitable.
Because of its thick atmosphere our neighboring planet Venus (about the size of the Earth), is the hottest planet in the solar system, despite Mercury being closer to the sun.
2. The Right Distance From Its Star
To host life a planet must be located within the so-called habitable zone: i.e. be placed at the right distance from its home star, where liquid water can exist on the orbiting planet's surface.
In our solar system, the habitable zone extends from about 0.9 to 1.5 astronomical units (1 AU being the average distance of the Earth from the Sun) with the blue planet perfectly placed to host life.
Some exoplanets (like Kepler-452b) are also thought to be within the habitable zone of their respective star.
3. The Right Star
By providing light and heat, solar energy is an indispensable ingredient for life. But for life to thrive, the planet-star relationship must be the perfect fit.
If the sun was hotter, then the Earth would be uninhabitable. On the other hand, with a less warm sun, there would be no liquid water on the Earth's surface, an indispensable prerequisite for life.
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As with any star, the sun's output energy varies over time; in its case typically over a period of 11 years (sun cycles). Yet at solar maximum, its brightness is only about 0.1 percent higher than at solar minimum. This relative stability over long periods of time has been crucial in sustaining life on Earth in the long run.
4. Liquid Water
For life, water in liquid form is essential. Due to its very unique properties, it is the indispensable solvent for all forms of life as we know them.
On a planetary scale, the right amount of water is also crucial: it has to be available in abundance, but not so much as to cover all the mountains and leave no land. Water is what lubricates plate tectonics, which in turn leads to continents and seafloors.
While other planets over time freeze or fry, Earth has kept its oceans, lakes, and rivers.
5. Abundance of Elements
Three chemical elements (oxygen, carbon, and hydrogen) make up about 93% of the human body mass. Yet overall there are at least 29 elements that are essential for the human body to function properly.
Though alien life need not necessarily be based on carbon molecules as are all forms of life on Earth (science fiction has long imagined alien worlds inhabited by silicon-based life), any planetary environment aspiring to host life would require a wide range of naturally occurring elements.
On Earth, virtually all elements of the periodic table occur naturally in some form, providing the fundamentals for higher forms of life.
The rotation of the Earth on its axis causes day and night cycles every 24 hours. If our planet did not rotate, one half would always be bright and warm, while the other would be dark and cold.
Instead, as the Earth rotates, each area of its surface gets a turn to face and be warmed by the sun, which affects the weather, agriculture, food, and our health.
The rotation periods of the planets vary widely with some cycles totally unsuitable for human life: Jupiter spins around itself in less than 10 hours, while a (solar) day on Venus lasts a never-ending 116 days!
7. Tilted Axis
Many people believe that in the summer it is hotter because the Earth is closer to the Sun. But that is not the case. Actually, the Earth is closest to the Sun (perihelion) in early January when in the Northern Hemisphere it is winter, and farthest from the Sun (aphelion) at the beginning of July.
The seasons instead are due to the Earth's tilted axis (23.5 degrees relative to its orbital plane). This way, throughout the year, different parts of Earth receive more energy from the Sun than other areas.
When the North Pole is tilted towards the Sun, the Northern Hemisphere experiences summer. Conversely, when the South Pole is tilted towards the Sun, it's summer in the Southern Hemisphere and winter for people leaving above the equator.
8. A Large Moon
Relatively to its size, the Earth has an exceptionally large natural satellite. This plays an important role in stabilizing our home planet's wobble on its rotation axis.
In absence of such a large moon, a wide range of the Earth's tilt would cause extreme climates: long daylight summers with searing heat and viciously cold perpetual winter months.
Our large Moon is therefore pivotal in mitigating the climate.
9. The Right Atmosphere
In order to survive, organisms need an adequate atmosphere: not too thin and neither too thick, and of the right composition.
Besides providing oxygen, the atmosphere protects life on earth by shielding it from incoming ultraviolet (UV) radiation that could otherwise damage our DNA. The Earth's atmosphere also helps regulate temperatures by preventing extremes between day and night.
Earth's atmosphere is about 300 mi (480 km) thick and is commonly divided into five major layers which are: the troposphere, stratosphere, mesosphere, thermosphere, and exosphere (from lowest to highest).
Not all planets have a magnetosphere (Venus and Mars do not), but to host life, a planet can hardly dispense with it: Earth's magnetic field provides an indispensable shield from cosmic radiation and high-energy particles launched from the sun and elsewhere in the cosmos.
The magnetic field is generated by electric currents due to the convective movements of liquid iron alloys within the Earth's core.
The famous Northern lights (aurora borealis) are formed, because of the peculiar structure of the Earth's magnetic field, when solar winds collide with the atmosphere near the pole.
11. Plate Tectonics
The Earth's outer crust is broken up into chunks called tectonic plates, floating on top of the magma interior.
Plate tectonics plays a life-essential role: it maintains dry land in the face of constant erosion. Without geological activity, over time erosion would eventually wear down the mountains and continents, creating an (almost) perfectly round planet entirely covered by water.
Plate tectonics also plays a pivotal role in the regulation of the carbon cycle and in regulating temperature.
Going from the fundamental elements of matter to even a bacterial cell already requires the existence and assembly of incredibly complex molecules. Yet the advancement from simple cells to mammals is still vastly more complex. And the enhancement from ordinary mammals to intelligent life is even more challenging.
Even if traces of primitive life forms were to be found on other planets (say Mars), that would not in any way confirm the existence of intelligent life elsewhere.
Giant radio dishes have been scanning distant stars for years listening for signals from space, yet the only place so far confirmed to host life is, of course, planet Earth.
- The Privileged Planet by Guillermo Gonzalez and Jay W. Richards, Regnery Publishing, Washington DC, 2004
- The Stargazer's Guide to the Night Sky, by Dr. Jason Lisle, Master Books, Green Forest AR, 2012
This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.
© 2021 Marco Pompili