The Solar System Models Formation Inner Planets Outer Planets Other Objects in Space

Ideas About the Solar System Known since ancient times, five planets are visible to the unaided eye as bright points of light in the night sky (Mercury, Venus, Mars, Jupiter, and Saturn). Early observers noticed that the positions of the planets changed with respect to the fixed stars, and their observations and belief systems influenced their ideas about the solar system. “Planet” is Greek for “wandering star.”

Earth - Centered Model Ancient Greek scientists thought the planets (Mercury, Venus, Mars, Jupiter, and Saturn) as well as the Sun and the moon were in fixed, separate spheres that revolved around Earth. Stars were also thought to be in another fixed sphere that centered around the Earth.

Sun - Centered Model In the year 1543, the Polish astronomer Nicholas Copernicus stated that the Earth and other planets revolved around the Sun. Copernicus believed that the daily movement of the planets and the stars was due to Earth’s rotation. In the year 1610, the Italian astronomer Galileo Galilei observed through his telescope that Venus went through a full cycle of phases like the moon. This could only happen if Venus orbited the Sun. Galileo concluded that Venus and the other planets revolve around the Sun, and that the Sun is at the center of the solar system.

Modern View of the Solar System We now know that the solar system consists of eight major planets orbiting the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto and other “dwarf planets,” as well as many other smaller objects also orbit the Sun. The solar system includes a huge volume of space that stretches in all directions from the Sun. Our star (the Sun) contains 99.86% of the entire mass of the solar system. Due to the gravitational pull of the Sun, it is the central object in the solar system, and all other objects in the system revolve around it.

How the Solar System Formed Scientists theorize that 4.6 billion years ago our solar system formed from part of a nebula of gas, ice, and dust. The Ant Nebula, a cloud of dust and gas between the stars, resembles an ant when observed using ground-based telescopes. The nebula lies within our galaxy between 3,000 and 6,000 light years from our solar system.

About 4.6 billion years ago, the solar system was part of a vast, swirling cloud of gas, ice, and dust (a nebula). A nearby star might have exploded, and shock waves from this event caused the cloud to start to contract under its own gravity.

As the cloud contracted under gravity, the matter within the cloud was squeezed into less space. As the density increased and the attraction of gravity pulled more gas and dust toward the center, the contracting nebula began to rotate faster and flatten into a disk with a dense center. Temperatures at the center increased greatly, at the reaching about 10,000,000 o C. At this temperature, nuclear fusion began. In nuclear fusion, hydrogen atoms combine to form helium atoms, and a small fraction of the nuclear mass is converted to vast amounts of energy (E = mc 2 ). As nuclear fusion begins, a star is born that becomes our Sun.

Planet Formation Gas, ice, and dust particles that were not drawn into the core of the cloud and the infant Sun collided and stuck together to form planets, asteroids, and other solar system objects. Temperatures near the Sun caused easily vaporized water and ices preventing them from condensing into solids. This is why lighter elements and compounds are scarcer in the planets near the Sun than in planets further from the Sun. The formation of water and other ices are more common farther from the Sun. Rocky and iron-rich materials dominate the formation of bodies in the inner solar system. Further out, gases and ices dominate the formation of bodies in the outer solar system.

Motion of Planets Nicholas Copernicus thought that the planets orbited the Sun in exact circles. In the early 1600s, the German mathematician Johannes Kepler began studying orbits of the planets. He determined that the shapes of all the planets’ orbits are not circular, but elliptical. From Kepler, we understand that orbits are ellipses, with the Sun at one focus of the ellipse. This includes the Earth, which has a slightly elliptical orbit with the Sun slightly offset from the center. This is why we are closer to the Sun in January (147 million km) than we are in July (152 million km).

Speed of Planets Kepler found that the planets travel in their orbits at different speeds, and the planets closer to the Sun travel faster than those further away: PlanetAverage Orbital Speed Mercury48 km/s Venus35 km/s Earth30 km/s Mars24 km/s Jupiter13 km/s Saturn9.7 km/s Uranus6.8 km/s Neptune5.4 km/s Pluto (dwarf planet)4.7 km/s

Order of Planets

How Can I Remember the Order? Here’s a few ways to help you remember the order: MercuryMyMy VenusVeryVery EarthEasyExcellent MarsMethodMother JupiterJustorJust or ? SaturnSpeedsServed UranusUpUs NeptuneNamingNine PlutoPlanetsPizzas