Universe and Solar System

2.3 The Solar System

Geocentric Model

Humans’ view of the solar system has evolved as technology and scientific knowledge has increased. The ancient Greeks identified five of the planets, and they were the only planets known for many centuries. Since then, scientists have discovered two more planets, many other solar-system objects, and even planets found outside our solar system. (Introduction to the Solar System | Earth Science, n.d.)

The ancient Greeks believed that Earth was at the center of the universe. This view is called the geocentric “Earth-centered” model. In the geocentric model, the sky, or heavens, are a set of spheres layered on top of one another. Each object in the sky is attached to a sphere and moves around Earth as that sphere rotates. From Earth outward, these spheres contain the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. An outer sphere holds all the stars. Since the planets appear to move much faster than the stars, the Greeks placed them closer to Earth.

The geocentric model worked well by explaining why all the stars appear to rotate around Earth once per day. The model also explained why the planets move differently from the stars and each other. One problem with the geocentric model is that some planets seem to move backward, in retrograde, instead of in their usual forward motion around Earth.

Around 150 A.D., the astronomer Ptolemy resolved this problem by using a system of circles to describe the motion of planets. In Ptolemy’s system, a planet moves in a small circle, called an epicycle. This circle moves around Earth in a larger circle, called a deferent. Ptolemy’s version of the geocentric model worked so well that it remained the accepted model of the universe for more than a thousand years.

Heliocentric Model

Ptolemy’s geocentric model worked, but it was not only complicated, it occasionally made errors in predicting the movement of planets. At the beginning of the 16th century A.D., Nicolaus Copernicus proposed that Earth and all the other planets orbit the Sun. With the Sun at the center, this model is called the heliocentric or “sun-centered” model of the universe. Copernicus’ model explained the motion of the planets, as well as Ptolemy’s model, did, but it did not require complicated additions like epicycles and deferents.

Although Copernicus’ model worked more simply than Ptolemy’s, it still did not entirely describe the motion of the planets because, like Ptolemy, Copernicus thought planets moved in perfect circles. Not long after Copernicus, Johannes Kepler refined the heliocentric model so that the planets moved around the Sun in ellipses (ovals), not circles. Kepler’s model matched observations perfectly.

Because people were so used to thinking of Earth at the center of the universe, the heliocentric model was not widely accepted at first. However, when Galileo Galilei first turned a telescope to the heavens in 1610, he made several striking discoveries. Galileo discovered that the planet Jupiter has moons orbiting around it. This provided the first evidence that objects could orbit something besides Earth. Galileo also discovered that Venus has phases like the Moon, which provides direct proof that Venus orbits the Sun.

Galileo’s discoveries caused many more people to accept the heliocentric model of the universe, although Galileo himself was found guilty of heresy for his ideas. The shift from an Earth-centered view to a Sun-centered view of the universe is referred to as the Copernican Revolution.

Watch this animation of the Ptolemaic and Copernican models of the solar system. Ptolemy made the best model he could with the assumption that Earth was the center of the universe, but by letting that assumption go, Copernicus came up with a much simpler model. Before people would accept that Copernicus was right, they needed to accept that the Sun was the center of the solar system.

Today we know that just as Earth orbits the Sun, the Sun and solar system orbit the center of the Milky Way galaxy. The center of the Milky way may likely be a massive black hole. One orbit of the solar system takes about 225 to 250 million years. It is believed that the solar system has orbited 20 to 25 times since it formed 4.6 billion years ago.

Modern Solar System

Today, we know that our solar system is just one tiny part of the universe. Neither Earth nor the Sun is at the center of the universe. However, the heliocentric model accurately describes the solar system. In our modern view of the solar system, the Sun is at the center, with the planets moving in elliptical orbits around the Sun. The planets do not emit their light, but instead, reflect light from the Sun.

Esri has created an excellent story map called the Solar System Atlas. NASA has created a great website called Solar System Exploration, and National Geographic has created a great resource called Solar System. Both websites are splendid sources to introduce yourself to our solar system.


Since the early 1990s, astronomers have discovered other solar systems, with planets orbiting stars other than our own Sun, called extrasolar planets or simply, exoplanets. Some extrasolar planets have been directly observed, but indirect methods have discovered most. One technique involves detecting a star’s very slight motion periodically moving toward and away from us along our line-of-sight, known as a star’s radial velocity. This periodic motion can be attributed to the gravitational pull of a planet or, sometimes, another star orbiting the star.

A planet may also be identified by measuring a star’s brightness over time. A temporary, periodic decrease in light emitted from a star can occur when a planet crosses in front of the star it is orbiting, called a transit, momentarily blocking out some of the starlight. More than 3,600 extrasolar planets have been identified, and the rate of discovery is increasing rapidly.

Planets and Their Motions

Since the time of Copernicus, Kepler, and Galileo, we have learned a lot more about our solar system. Astronomers have discovered two more planets (Uranus and Neptune), four dwarf planets (Ceres, Pluto, Makemake, Haumea, and Eris), more than 150 moons, and many, many asteroids and other small objects.

Although the Sun is just an average star compared to other stars, it is by far the most massive object in the solar system. The Sun is more than 500 times the mass of everything else in the solar system combined. The table below gives data on the sizes of the Sun and planets relative to Earth. (Introduction to the Solar System | Earth Science, n.d.)

Object Mass (Relative to Earth Diameter of Planet (Relative to Earth)
Sun 333,000 Earth’s mass 109.2 Earth’s diameter
Mercury 0.06 Earth’s mass 0.39 Earth’s diameter
Venus 0.82 Earth’s mass 0.95 Earth’s diameter
Earth 1.00 Earth’s mass 1.00 Earth’s diameter
Mars 0.11 Earth’s mass 0.53 Earth’s diameter
Jupiter 317.8 Earth’s mass 11.21 Earth’s diameter
Saturn 95.2 Earth’s mass 9.41 Earth’s diameter
Uranus 4.6 Earth’s mass 3.98 Earth’s diameter
Neptune 7.2 Earth’s mass .81 Earth’s diameter

Size and Shape of Planetary Orbits

The figure below shows the relative sizes of the orbits of the major planets within our solar system. In general, the farther away from the Sun, the higher the distance from one planet’s orbit to the next. The orbits of the planets are not circular but slightly elliptical with the Sun located at one of the foci.

While studying the solar system, Johannes Kepler discovered the relationship between the time it takes a planet to make one complete orbit around the Sun, its “orbital period,” and the distance from the Sun to the planet. If the orbital period of a planet is known, it is possible to determine the planet’s distance from the Sun. This is how astronomers without modern telescopes could determine the distances to other planets within the solar system.

Distances in the solar system are often measured in astronomical units (AU). One astronomical unit is defined as the distance from Earth to the Sun. 1 AU equals about 150 million kilometers or 93 million miles. The table below shows the distances to the planets (the average radius of orbits) in AU. The table also indicates how long it takes each planet to spin on its axis (the length of a day) and how long it takes each planet to complete an orbit (the length of a year); in particular, notice how slowly Venus rotates relative to Earth.

Planet Average Distance from Sun Length of Day Length of Year
Mercury 0.39 Astronomical Units (AU) 56.84 (In Earth Days) 0.24 (In Earth Years)
Venus 0.72 243.02 0.62
Earth 1.00 1.00 1.00
Mars 1.52 1.03 1.88
Jupiter 5.2 0.41 11.86
Saturn 9.54 0.43 29.46
Uranus 19.22 0.72 84.01
Neptune 30.06 0.67 164.8

The Role of Gravity

Isaac Newton was one of the first scientists to explore gravity. He understood that the Moon circles the Earth because a force is pulling the Moon toward Earth’s center. Without that force, the Moon would continue moving in a straight line off into space. Newton also came to understand that the same force that keeps the Moon in its orbit is the same force that causes objects on Earth to fall to the ground.

Newton defined the Universal Law of Gravitation, which states that a force of attraction, called gravity, exists between all objects in the universe. The strength of the gravitational force depends on how much mass the objects have and how far apart they are from each other. The greater the objects’ mass, the higher the force of attraction; also, the greater the distance between the objects, the smaller the force of attraction.

The distance between the Sun and each of its planets is substantial, but the Sun and each of the planets are also gigantic. Gravity keeps each planet orbiting the Sun because the star and its planets are enormous objects. The force of gravity also holds moons in orbit around planets. There are two additional key features of the solar system that helps us understand how it formed: 1) All the planets lie in nearly the same plane, or flat disk-like region, 2) All the planets orbit in the same direction around the Sun.

Formation of the Solar System

The most widely accepted explanation of how the solar system formed is called the nebular hypothesis. According to this hypothesis, the Sun and the planets of our solar system formed about 4.6 billion years ago from the collapse of a giant cloud of gas and dust, called a nebula.

The nebula was drawn together by gravity, which released gravitational potential energy. As small particles of dust and gas smashed together to create larger ones, they released kinetic energy. As the nebula collapsed, the gravity at the center increased, and the cloud started to spin because of its angular momentum. As it collapsed further, the spinning got faster, much as an ice skater spins faster when he pulls his arms to his sides during a spin.

Much of the cloud’s mass migrated to its center, but the rest of the material flattened out in an enormous disk. The disk contained hydrogen and helium, along with heavier elements and even simple organic molecules.

Formation of the Sun and Planets

As gravity pulled matter into the center of the disk, the density and pressure at the center became intense. When the pressure in the center of the disk was high enough, nuclear fusion within our star began, and the blazing star stopped the disk from collapsing further.

Meanwhile, the outer parts of the disk were cooling off. Matter condensed from the cloud, and small pieces of dust started clumping together to create ever bigger clumps of matter. Larger clumps, called planetesimals, attracted smaller clumps with their gravity. Gravity at the center of the disk attracted more massive particles, such as rock and metal, and lighter particles remained further out in the disk. Eventually, the planetesimals formed protoplanets, which grew to become the planets and moons that we find in our solar system today.

The gravitational sorting of material with the inner planets, Mercury, Venus, Earth, and Mars, dense rock and metal formed. The outer planets, Jupiter, Saturn, Uranus, and Neptune, condensed farther from the Sun from lighter materials such as hydrogen, helium, water, ammonia, and methane. Out by Jupiter and beyond, where it is frigid, these materials formed solid particles.

The nebular hypothesis was designed to explain some of the essential features of the solar system:

  • Orbits of the planets lie in nearly the same plane with the Sun at the center
  • Planets revolve in the same direction
  • Planets mostly rotate in the same direction
  • Axes of rotation of the planets are mostly nearly perpendicular to the orbital plane
  • Oldest moon rocks are 4.5 billion years

The two videos below, from the European Space Agency (ESA), discusses the Sun, planets, and other bodies in the Solar System and how they formed. The first part of the video explores the evolution of our view of the solar system, starting with the early Greeks, who reasoned that since some points of light, which they called planets, moved faster than the stars, they must be closer.




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Physical Geography and Natural Disasters Copyright © 2020 by R. Adam Dastrup, MA, GISP is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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