Solar System

Our Solar System: One in a Hundred Billion?

Planet Earth is part of a vast space neighborhood called the solar system. Our solar system is an amazing place. It’s even more amazing that at least 70 other solar systems have been discovered. And there could be lots more. Scientists estimate there could be hundreds of billions of solar systems in our Milky Way galaxy.

Milky Way

Our solar system is one of many groups of planets (planetary systems) that call the Milky Way Galaxy home.


We may never know the actual number, but all solar systems work the same way. Planets and other space objects revolve around a star. In our system, the star is the sun. Eight major planets and their moons, three “dwarf” planets, and countless asteroids, meteoroids, comets, and other objects whiz around it. Each object travels in its own egg-shaped orbit. The shape is called an ellipse

It Began with a Bang

About 15-20 billion years ago, there was an enormous explosion called the Big Bang. Whirling clouds of dust and gas filled the Universe. Over time, gravity made some clouds start to clump together. Big clumps formed galaxies. Smaller clumps became stars.

Then, about 4.5 billion years ago, the clouds that formed our solar system spun so fast they formed a disk. The center of the disk became so hot it dropped out. That center became the sun. The dust and gas that didn’t get sucked into the center stuck together in clumps. These clumps became planets. Our solar system was born. 

The Big Star of Our Show


The eight planets of our solar system revolve around a large star called the sun.


The sun is the center because it’s HUGE. More than one million Earths could fit inside it. It contains more than 99.8% of all the mass in the solar system. Its size gives it the gravity it needs to hold the solar system together. The sun’s gravity pulls the objects down while they keep trying to move away. It’s a never-ending tug-of-war that keeps the planets in their orbits instead of flying off into space.

Let’s Take a Spin

It takes Earth 366.6 days to make one revolution around the sun. That’s about a year. But a year on Earth is not the same as a year on another planet. The length of a planet’s “year” depends on its size and its distance from the sun. Smaller planets travel faster. Planets far from the sun have a longer trip than those close in.

While the planets spin around the sun, each one also spins around its own axis. The time it takes a planet to make one complete turn, or rotation, is called a day. A day on Earth is 23 hours and 56 minutes. Days on other planets are longer or shorter.

Mars Rocks. Jupiter Is a Gas.

Terrestrial Planets

Earth is one of four planets in our solar system that have solid, rocky surfaces. These planets are called terrestrial planets.

©Lunar and Planetary Institute

Mercury, Venus, Earth and Mars are called the “terrestrial” planets. Terrestrial comes from the Latin word terra, which means “earth.” All these planets have solid, rocky surfaces. They’re also called the “inner” planets.

Jupiter, Saturn, Uranus and Neptune are called the “gas giants.” They do not have solid surfaces. They’re made of dense clouds of gases. These planets are also called the “outer” planets.

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Sorry, Pluto!

Pluto was once the ninth planet in our solar system. But in 2006, scientists kicked it out of the lineup. Why? They had come up with new rules about what makes a “real” planet. Pluto just didn’t measure up.

The rules are:

  • A planet must be shaped like a sphere.
  • It must orbit the sun.
  • It must have enough gravity to keep it from being pulled into another planet’s orbit.
  • It must have enough gravity to pull in rocks and other space “junk” orbiting around it.

Pluto is one of three known dwarf planets in our solar system.


Pluto is shaped kind of like a potato. It has a weird orbit that sometimes crosses into Neptune’s. And it has a lot of debris orbiting around it. So scientists reclassified Pluto as a “dwarf” planet, along with Ceres and Eris.



There are more than hundreds of thousands of asteroids in our solar system and most of them are found within the main asteroid belt.


What else orbits the sun? Asteroids are loose chunks of rock that scientists think may be left over from the formation of the solar system. They may be pieces of a planet that broke apart, or just a small clump that never merged with bigger ones. Most of the asteroids orbit the sun in a region between Mars and Jupiter known as the Asteroid Belt. 


Meteoroids are smaller than asteroids.  They may be pieces of planets that flew off during a space collision. They could be pieces of dust left behind in the trail of a comet. Or they could be pieces of asteroids that broke apart.

Meteoroids burn up when they enter the Earth’s atmosphere.  The bright trails they leave behind them are called meteors, or “shooting stars.”   Larger pieces that don’t burn up completely in the atmosphere and make it to the surface of the Earth are called “meteorites.”


Comet McNaught was the brightest comet to shoot across the sky in the Southern hemisphere in 40 years.

©Jens Hackmann


Comets are sometimes called “dirty snowballs.” They’re made of icy material mixed with rocks and dust.  Scientists think that comets, like asteroids, are left over from when the solar system formed. 

Watchers and Wanderers

The earliest astronomers studied the sun, stars, and planets as the complete universe. They believed Earth was the center of everything. They noticed that stars did not change positions except to move as one across the sky. They noticed other star-like objects, too. These objects seemed to wander among the stars.  So they named the objects “planets,” after the Greek word for “wanderers.” 

In the early 1500’s, astronomer Nicholas Copernicus had an idea that made people very angry. He said that the sun, not Earth, was the center of the solar system. People didn’t like the idea one bit. How could the sun be more important than Earth? Copernicus’s theories were unpopular, but he started modern astronomy. Later, Galileo Galilei was among the first astronomers to observe the planets with a telescope.

Twinkle, Twinkle, Little . . . Planet?

Today powerful telescopes let us see things early astronomers could only dream of. Spacecraft and satellites send back astonishing images from the outer edges of the solar system. Still, you can study the planets with your own eyes. But how can you tell if you’re looking at a star or a planet?


Astronomers use powerful telescopes to study the sun, stars, moons, planets, and other bodies in space.

©Greg Piepol

The sun is our closest star. Its light is close enough to brighten and warm our world. But light from other stars comes from very, very far away. By the time it reaches us, it’s just a tiny pinpoint. As we look through the atmosphere at that point of light, it gets shifted around by all the “stuff” going on in the atmosphere.  That’s what makes it twinkle.  Planets are closer. Their light isn’t so “pointy.” It doesn’t get shifted around as much.  So if you watch a star for a while and it doesn’t twinkle, it just may be a planet!