Origin of the Universe: There are a lot of theories about how the universe began, but most scientists believe the Big Bang Theory is correct.
Big Bang Theory: Our universe began around 13.8 billion years ago from a single point smaller than an atom. It was extremely hot and dense and started expanding outward (the ‘bang’). The new matter was incredibly hot at the beginning but eventually cooled to form different objects like the planets, moons, and stars. The universe is STILL expanding.
Fun Fact: When we see stars in the night sky, we are observing light that left them years ago. For example, the closest star to us besides the sun is called Proxima Centauri. It is about 4.2 light-years away from us. That means that if you look at Proxima Centauri tonight, you’re actually seeing it as it was 4.2 years ago. And 4.2 years from now, you’d see it as it actually looks today. Basically, we can look ‘back in time’ to the early days of the universe by looking at stars and galaxies that are really far away because the further away they are, the further back into their past you can look.
Origin of the Solar System: Our solar system began around 4.6 billion years ago as a nebula. A shockwave, probably from a nearby exploding star, caused the nebula to start rotating and condensing. It eventually flattened into a disk shape and gravity pulled the bits of gas, dust, and ice into larger clump of matter. Eventually the temperature and pressure of all the matter at the center became so hot and high that fusion started and a star, our sun, was born.
The rest of the gas, ice, and dust of the nebula pulled together into larger and larger clumps, forming the planets, moons, and other space objects. Because the energy from the sun was so intense, lighter elements were swept away from the inner solar system.
As a result, the inner planets close to the sun are composed mostly of heavy elements, while the outer planets are composed mostly of lighter elements and gases.
Solar System Models: Today, we know that our solar system has eight planets and smaller objects that revolve around the sun. In the past, however, people thought the solar system was set up differently.
Earth-Centered Model: Greek scientists, such as Aristotle and Ptolemy, believed Earth was at the center of the solar system. They thought that the sun, moon, and the five planets they knew of at the time all revolved around Earth.
Sun-Centered Model: In 1543, Nicolaus Copernicus published a paper saying that Earth and all other planets revolved around the sun. Only the moon, he said, revolved around Earth. Using his observation of the planet Venus, among other observations, Galileo Galilei also theorized that the sun was at the center of the solar system. These models were not perfect, but they set us on the right track to figure out how our solar system works.