10 Facts about the Sun, Planets, Comets, and Asteroids

An artist's conception of our solar system, set against the larger galaxy and its deep-sky objects. NASA

The solar system is where our Sun and planets exist. Long before astronomers could use telescopes to look at objects in the sky, people thought that the planets were simply wandering stars and had no concept of an organized system of worlds orbiting the Sun. The first astronomer to look at another planet with a telescope was Galileo Galilei, and he used his tiny instrument to look at Jupiter. He discovered its four largest moons.

That one night's observations and his subsequent follow-ups changed humanity's view of our place in space.

Today, we know that planets are other worlds and not stars.They have all been actively explored since the beginning of the space age. There are currently spacecraft at Venus, Mars, Jupiter, Saturn, and the dwarf planet Ceres.Another one just passed Pluto and is on its way to explore even more distant worlds.

So, what have we learned from all our explorations? Let's look at the top 10 facts to know about the solar system. 

Top Solar System Facts

1. The Sun and planets 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 leftovers of that formation are called asteroids and comets. 

2. The Sun contains 99.8 percent of the mass of the solar system. The planet Jupiter is the next most massive object and it comprises 2.5 times the mass of all the other planets combined.

Contrary to popular belief, Jupiter is NOT a failed star. It never had enough mass for that. 

3. The four inner planets, Mercury, Venus, Earth, and Mars, are called the "terrestrial" or "rocky" planets. Only Earth has an ocean, although there is evidence that both Venus and Mars once had flowing water on their surfaces.

Mercury has water ice at its poles. It got there when comets smashed into areas around the north and south poles. 

4. Jupiter, Saturn, Uranus, and 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". 

5. 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.

6. 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. It is very distant and the worlds out there are likely icy and frozen.

7. The outermost region of the solar system is called the Oort Cloud. It contains chunks of ice that become comets when they orbit very close to the Sun. 

8. 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. 

9. 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. 

10. 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. 

These are just a few facts to know about our solar system. It's a fascinating place and the scientists who study it—called planetary scientists—are learning new things every day.