Welcome to the solar system! This is where you'll find the Sun, the planets, and humanity's sole home in the Milky Way Galaxy. It contains planets, moons, comets, asteroids, one star, and worlds with ring systems. Although astronomers and skygazers have observed other solar system objects in the sky since the dawn of human history, it has only been in the past half-century that they've been able to explore them more directly with spacecraft.
Historical Views of the Solar System
Long before astronomers could use telescopes to look at objects in the sky, people thought that the planets were simply wandering stars. They had no concept of an organized system of worlds orbiting the Sun. All they knew were that some objects followed regular paths against the backdrop of the stars. At first, they thought these things were "gods" or some other supernatural beings. Then, they decided that those motions had some effect on human lives. With the advent of scientific observations of the sky, those ideas vanished.
The first astronomer to look at another planet with a telescope was Galileo Galilei. His observations changed humanity's view of our place in space. Soon, many other men and women were studying the planets, their moons, asteroids, and comets with scientific interest. Today that continues, and there are currently spacecraft doing many solar system studies.
So, what else have astronomers and planetary scientists learned about the solar system?
Solar System Insights
A journey through the solar system introduces us to the Sun, which is our nearest star. It contains an amazing 99.8 percent of the mass of the solar system. The planet Jupiter is the next most-massive object and it comprises two and a half times the mass of all the other planets combined.
The four inner planets-tiny, cratered Mercury, cloud-shrouded Venus (sometimes called Earth's Twin), temperate and watery Earth (our home), and reddish Mars-are called the "terrestrial" or "rocky" planets.
Jupiter, ringed Saturn, mysterious blue Uranus, and distant Neptune are called "gas giants". Uranus and Neptune are so cold and contain a great deal of icy material, and are often called the "ice giants".
The solar system has five known dwarf planets. They are called Pluto, Ceres, Haumea, Makemake, and Eris. The New Horizons mission explored Pluto on July 14, 2015, and is on its way out to visit a small object called 2014 MU69. At least one and possibly two other dwarf planets exist in the outer reaches of the solar system, although we do not have detailed images of them.
There are probably at least 200 more dwarf planets in a region of the solar system called the "Kuiper Belt" (Pronounced KYE-per Belt.) The Kuiper Belt extends out from the orbit of Neptune and is the realm of the most distant worlds known to exist in the solar system. It is very distant and its objects are likely icy and frozen.
The outermost region of the solar system is called the Oort Cloud. It probably has no large worlds but does contain chunks of ice that become comets when they orbit very close to the Sun.
The Asteroid Belt is a region of space that lies between Mars and Jupiter. It is populated with chunks of rocks ranging from small boulders up to the size of a big city. These asteroids are left over from the formation of the planets.
There are moons throughout the solar system. The only planets that do NOT have moons are Mercury and Venus. Earth has one, Mars has two, Jupiter has dozens, as do Saturn, Uranus, and Neptune. Some of the moons of the outer solar system are frozen worlds with watery oceans beneath the ice on their surfaces.
The only planets with rings that we know of are Jupiter, Saturn, Uranus, and Neptune. However, at least one asteroid called Chariklo also has a ring and planetary scientists recently discovered a tenuous ring around the dwarf planet Haumea.
The Origin and Evolution of the Solar System
Everything that astronomers learn about solar system bodies helps them understand the origin and evolution of the Sun and planets. We know they formed about 4.5 billion years ago. Their birthplace was a cloud of gas and dust that slowly contracted to make the Sun, followed by the planets. The comets and asteroids are often considered the "leftovers" of the birth of the planets.
What astronomers know about the Sun tells us that it will not last forever. Some five billion years from now, it will expand and engulf some of the planets. Eventually, it will shrink down, leaving behind a very changed solar system from the one we know of today.