The sun, moon and vast array of stars littering the night sky were all essential in generating life on Earth.
“Earth really is a perfect planet for life, but also complex life,” Pennsylvania State University Astronomy Researcher Paul Robertson said.
No other known planet is quite like Earth. It is covered in liquid water, sits within a habitable proximity to the warm glow of a sun and still retains an atmosphere capable of sustaining life. However, it could have turned out much differently, leaving the planet to float aimlessly, desolate and lifeless, across the cold expanse of space for all time.
The stars of the night sky have astonished humanity dating back to the earliest civilizations, even dictating religious beliefs, societal norms and architecture. These heavenly bodies have also inspired philosophers, provided direction to weary travelers navigating long journeys and deposited the very fabric of our existence.
Nearly 14 billion years ago, all matter, energy, space and time were unified as one, exploding outward in a violent, chaotic expansion at a rate that challenges comprehension.
In its infancy, the cooling expansion of the hot, dense universe would allow for the formation of atomic nuclei to create molecules of hydrogen and helium, eventually giving rise to nebulae of gas that spawned stars, incandescent orbs of superheated gas.
All of the heavier elements found on Earth, including those that compose the human body, were created in the cores of these brilliant, glowing stars. Without stars, the ingredients for planets and life would have never formed.
No other star is more important to life on Earth than our Sun, the star that warms the planet and holds the Solar System together.
Large clouds of gas and dust were instrumental in the creation of the Sun. Once formed, the churning, gravitational force generated by the star, which comprises 99.8 percent of all mass in the Solar System, forced the planets to form around it.
Nearly 4.6 billion years ago, the Earth began to form in a spinning disk of accumulating matter driven by the most pervasive force in the Universe: gravity.
However, the early Earth was far from the habitable home it is today. The planet was once a world comprised of molten oceans and a blazing atmosphere of volcanic gas with nearly zero oxygen.
“Very early on, it was molten,” Robertson said, adding that as the Earth cooled, violent impacts from comets likely helped create the atmosphere.
“The formation of the planets was a violent process,” according to NASA. “Huge, rocky planetary embryos smashed into one another, forming oceans of magma surrounded by white-hot silicate atmospheres. Between the huge impacts, Earth's magma oceans probably cooled rapidly, perhaps in only a few years or decades.”
According to NASA, astronomers theorize that it is likely that the Moon was formed after a great impact to Earth.
“The impact blasted a lot of rock into orbit, but it was in small pieces, like rubble,” Robertson said. “Over time, the smaller bits coalesced into the moon.”
Additional impacts would create the Earth’s atmosphere and oceans over time.
“Comets are mostly water or ice,” Robertson said. "These would crash into the early Earth and would be incorporated into volcanic eruptions.”
As volcanic eruptions blasted out gases, the Earth’s density provided enough gravitational force to retain the water vapor released, which would eventually form an atmosphere and oceans, according to Robertson.
Eventually, these processes created the carbon and water cycles that govern Earth’s climate, he added.
Even though the molten Earth eventually cooled and was able to retain a sustainable atmosphere and liquid water, the formation of complex life was also dependent on other factors, including the Earth’s density and its location from the Sun, nearly 93 million miles away.
"This is something we refer to as the 'habitable zone' in astronomy," Robertson said. "We can use Venus and Mars as comparison cases that are right on the edges of the 'habitable zone.'"
Venus, which orbits 25 million miles away from Earth, is similar in size and composition to Earth, Robertson said.
But unlike the blue planet, Venus is a scorching world, stifled by a thick atmosphere of heavy greenhouse gases. Venus is approximately 80 percent as massive as Earth, Robertson said, which allowed the planet enough gravitational pull to form an atmosphere also.
"We think that Venus used to have oceans," he said, adding that somewhere in the past, Venus' carbon cycle failed to regulate the temperature, causing the planet to lose its former habitability.
The carbon cycle on Earth is essential in creating a sustainable environment for life through the release and recycling of carbon, which includes interactions between the geosphere, biosphere, hydrosphere and atmosphere.
One reason for this loss of habitability could be a lack of volcanic activity, which is instrumental in regulating the carbon dioxide content in the atmosphere and also impacts climate, he said. Venus is thought to have a solid core similar to Earth's, but it is not spinning in a molten shell.
"When we look at Mars, it is sort of the opposite," Robertson said. "The mass of Mars is much lower than Earth."
Mars, about 10 percent as massive as Earth, at one point also had liquid water based on evidence obtained by NASA’s exploration of the planet, he said.
However, because of a lack of mass, the gravitational force of Mars is much weaker than Earth’s, which is not suitable to retaining a thicker atmosphere.
Mars, located 35 million miles from Earth and farther from the Sun than Venus, is now a cold, red planet with a thin atmosphere. The water is now trapped in the form of ice, primarily in the Martian polar ice caps. "It would be interesting if we were able to switch Venus and Mars," Robertson said, adding that a thicker atmosphere on Mars could have done a better job of creating a more habitable climate.
In addition to the Earth's proximity to the Sun and its mass, the blue planet also has a moon that offers unique protection from dramatic shifts in climate.
“The Moon is quite massive relative to the size of the planet,” Robertson said. “What that does, it stabilizes the tilt of the Earth.”
The Earth rotates at 1,040 mph as it completes its revolution around the Sun at nearly 67,000 mph.
The planets travels on an elliptical orbit at a 23.5 degree tilt, or obliquity, from the Sun which is responsible for Earth’s change in seasons.
As the Earth spins, it wobbles like a top, and every 26,000 years, it completes one full cycle of precession.
"The moon dampens that wobble," Robertson said.
The Moon influences the Earth’s tides, but according to Robertson, it also protects the Earth from more drastic temperature fluctuations by stabilizing the wobble of Earth’s spin.
"You would see these short-term swings in the longer-term climate," he said.
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