If you’ve ever looked at a model of the solar system, you’ve probably noticed that the sun, planets, moons, and asteroids lie roughly on the same plane. But why is this?
To answer this question, we have to travel to the very beginning of the solar system, around 4.5 billion years ago.
At the time, the solar system was just a huge cloud of spinning dust and gas, Nader Haghighipour, an astronomer at the University of Hawaii at Mānoa, told Live Science. This massive cloud measured 12,000 astronomical units (AU); 1 AU is the average distance between the Earth and the sun, or approximately 93 million miles (150 million kilometers). This cloud grew so big that even though it was only filled with molecules of dust and gas, the cloud itself began to collapse and shrink under its own mass, Haghighipour said.
Related: Why do galaxies have different shapes?
When the rotating cloud of dust and gas began to collapse, it also flattened out. Imagine a pizza chef tossing a spinning plate of dough into the air. As you turn, the dough expands but becomes increasingly thin and flat. This is what happened at the very beginning of the solar system.
Meanwhile, in the center of this ever-flattening cloud, all of these gas molecules have crowded together so tightly that they’ve warmed up, Haghighipour said. Under the immense heat and pressure, hydrogen and helium the atoms merged and set off a nuclear reaction that spanned billions of years in the form of a small star: the sun. Over the next 50 million years, the sun continued to grow, collecting gas and dust from its surroundings and emitting waves of intense heat and radiation. Slowly, the growing sun cleared a donut of the empty space around it.
As the sun grew larger, the cloud continued to collapse, forming “a disc around the star. [that] becomes more and more flat and expands and expands with the sun in the center, ”said Haghighipour.
Eventually, the cloud became a flat structure called a protoplanetary disk, orbiting the young star. The disc spanned hundreds of UAs and was only a tenth of that distance thick, Haghighipour said.
For tens of millions of years thereafter, the dust particles of the protoplanetary disk swirled gently, sometimes colliding with each other. Some even remained united. And over those millions of years, these particles became grains a millimeter long, and these grains became pebbles a centimeter long, and the pebbles kept bumping and sticking together. to each other.
Eventually, most of the material in the protoplanetary disk stuck together to form huge objects. Some of these objects grew so large that gravity shaped them into spherical planets, dwarf planets, and moons. Other objects have become irregular in shape, such as asteroids, comets, and a few small moons.
Despite the different sizes of these objects, they remained more or less on the same plane, from which their building materials came. This is why, even today, the eight planets and other celestial bodies in the solar system orbit at roughly the same level.
Originally posted on Live Science.