We live on Earth’s hard, rocky surface, breathe the air that surrounds the planet, drink the water that falls from the sky, and eat the food that grows in the soil. But Earth did not always exist within this expansive universe, and it was not always a hospitable haven for life.
Billions of years ago, Earth, along with the rest of our solar system, was entirely unrecognizable, existing only as an enormous cloud of dust and gas. Eventually, a mysterious occurrence—one that even the world’s foremost scientists have yet been unable to determine—created a disturbance in that dust cloud, setting forth a string of events that would lead to the formation of life as we know it. One common belief among scientists is that a distant star collapsed, creating a supernova explosion, which disrupted the dust cloud and caused it to pull together. This formed a spinning disc of gas and dust, known as a solar nebula. The faster the cloud spun, the more the dust and gas became concentrated at the center, further fueling the speed of the
nebula
. Over time, the gravity at the center of the cloud became so intense that hydrogen atoms began to move more rapidly and violently. The
hydrogen
protons began fusing, forming helium and releasing massive amounts of energy. This led to the formation of the star that is the center point of our solar system—the sun—roughly 4.6 billion years ago.
Planet Formation
The formation of the sun consumed more than 99 percent of the matter in the
nebula
. The remaining material began to coalesce into various masses. The cloud was still spinning, and clumps of matter continued to collide with others. Eventually, some of those clusters of matter grew large enough to maintain their own gravitational pull, which shaped them into the
planets
and dwarf
planets
that make up our solar system today.
Earth is one of the four inner, terrestrial
planets
in our solar system. Just like the other inner
planets
—Mercury, Venus, and Mars—it is relatively small and rocky. Early in the history of the solar system, rocky material was the only substance that could exist so close to the Sun and withstand its heat.
In Earth's Beginning
At its beginning, Earth was unrecognizable from its modern form. At first, it was extremely hot, to the point that the
planet
likely consisted almost entirely of molten magma. Over the course of a few hundred million years, the
planet
began to cool and oceans of liquid water formed. Heavy elements began sinking past the oceans and
magma
toward the center of the
planet
. As this occurred, Earth became differentiated into layers, with the outermost layer being a solid covering of relatively lighter material while the denser, molten material sunk to the center.
Scientists believe that Earth, like the other inner
planets
, came to its current state in three different stages. The first stage, described above, is known as accretion, or the formation of a
planet
from the existing particles within the solar system as they collided with each other to form larger and larger bodies. Scientists believe the next stage involved the collision of a proto
planet
with a very young
planet
Earth. This is thought to have occurred more than 4.5 billion years ago and may have resulted in the formation of Earth’s moon. The final stage of development saw the bombardment of the
planet
with asteroids.
Earth’s early atmosphere was most likely composed of
hydrogen
and
helium
. As the
planet
changed, and the crust began to form, volcanic eruptions occurred frequently. These volcanoes pumped water vapor, ammonia, and carbon dioxide into the
atmosphere
around Earth. Slowly, the oceans began to take shape, and eventually, primitive life evolved in those oceans.
Contributions from Asteroids
Other events were occurring on our young
planet
at this time as well. It is believed that during the early formation of Earth,
asteroids
were continuously bombarding the
planet
, and could have been carrying with them an important source of water. Scientists believe the
asteroids
that slammed into Earth, the moon, and other inner
planets
contained a significant amount of water in their minerals, needed for the creation of life. It seems the
asteroids
, when they hit the surface of Earth at a great speed, shattered, leaving behind fragments of rock. Some suggest that nearly 30 percent of the water contained initially in the
asteroids
would have remained in the fragmented sections of rock on Earth, even after impact.
A few hundred million years after this process—around 2.2 billion to 2.7 billion years ago—photosynthesizing bacteria
evolved
. They released oxygen into the
atmosphere
via photosynthesis and, in a few hundred million years, were able to change the composition of the
atmosphere
into what we have today. Our modern
atmosphere
is comprised of 78 percent nitrogen and 21 percent oxygen, among other gases, which enables it to support the many lives residing within it.
