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Our Solar System

by Ron Kurtus (revised 9 September 2006)

A solar system is defined as a sun with a number of planets rotating around it. In our solar system, there are 8 planets that rotate around our Sun. There used to be considered 9 planets, but in September 2006, Pluto was downgraded to a mini-planet. Since there are billions of other suns or stars in the universe, it is assumed that many of them also have planets rotating around them as solar systems. Besides the eight planets, there are also asteroids and comets that rotate around the Sun.

Questions you may have include:

• What the other planets besides the Earth are in our solar system?
• What are asteroids?
• Are comets dangerous?

This lesson will answer those questions. There is a mini-quiz near the end of the lesson.

Useful tools: Metric-English Conversion | Scientific Calculator.

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Time = 6 min. 19 sec.

Eight planets

Besides the Earth, there are 7 other planets that orbit the Sun. The planets in our solar system consist of—starting with the closest to the Sun—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto is no longer considered a planet.

Edge of the Sun and the nine planets showing their relative sizes
(NASA montage)

The following is an overview of the planets.

Mercury

Mercury is the closest to the sun. It is much smaller than the Earth and extremely hot.

Picture of Mercury taken from Mariner 10 space probe

Venus

Venus appears to be the brightest planet in the night sky.

Cloud cover on Venus reflects most of the sunlight

Earth

The Earth is the only planet in our solar system that can sustain life, as we know it. The other planets are either too hot or too cold to sustain life, or they have poisonous atmospheres.

Earth

The size of the Earth compared to the Sun is similar as the comparison of a pea to a basketball.

Mars

Mars has a reddish glow. It is 1/2 the size of Earth. Early astronomers thought possibly was life on Mars, because planet has an icecap and appeared to have canals that could be seen from the distance.

Mars has an icecap of frozen carbon dioxide

Mars also has two moons.

Jupiter

Jupiter is the largest planet in the solar system. The Earth is the size of a pea compared to a baseball with respect to Jupiter's size.

Jupiter is known for its big red spot

Saturn

Saturn is characterized by its colorful rings around the planet.

Saturn and its rings
(NASA picture)

Uranus

Uranus is large but very far away.

Uranus

Neptune

Neptune has a blue-green color. It is also very large and far.

You can see one of Neptune's moons

Neptune has a surface of frozen hydrogen and helium.

Mini-planets and asteroids

A mini-planet is an object revolves in a orbit around the Sun but is too small to be considered a planet. Asteroids are rocky objects that also orbit the Sun, but since they are much smaller than the planets and sometimes irregular in shape, they too are not considered planets. Some are as large as a small moon, while many are simply pebbles and small rocks. Most asteroids travel in an band that exists between the orbits of Mars and Jupiter.

Pluto

For many years Pluto was listed as a planet in our Solar System. But after careful considerations, astronomers decided to downgrade Pluto to what some call a mini-planet. This is because Pluto is closer to the size of a moon than a planet. Also, Pluto's orbit is considerably different than all the other 8 planets. Some scientists think that pluto was an object from another solar system that got caught in our system's gravity field.

Pluto is very small and far away such that it is difficult to see even with the most powerful telescopes on Earth. Since it is so far from the Sun, it is extremely cold.

Pluto no longer a planet

Meteors

Some asteroids have orbits that bring them near the Earth and may get close enough to be attracted by the Earth's gravity. Since they are then no longer in orbit around the Sun, but instead falling toward the Earth, they are called meteors.

When a meteor starts falling through the Earth's atmosphere, friction increases its temperature until it starts to burn up. When a burning meteor is seen in the night sky, it looks like a rapidly moving star. Thus people call it a "shooting star." Surprisingly, most of meteors we see are only the size of a grain of sand.

Occasionally a larger meteor enters our atmosphere, and when that happens it is news, because of the large fireball it produces. One extremely large meteor hit the ground in Arizona, leaving a crater several miles in diameter. An extremely large meteor hit near Mexico and changed the climate of the Earth for many years. It is thought to be responsible for the extinction of the dinosaurs.

When a meteor strikes the Earth, it is called a meteorite. Don't ask me why they make the distinction.

Comets

Comets are objects made of gases, dust, and small rocks that move in a large elliptical path around the sun. A tail of glowing gas may be seen on a comet. Halley's comet is the most famous, and it comes within view every 77 years.

In 1996, the comet Hyakutake was seen. The comet was over 2070 miles (3340 km) wide. In 1997 the comet Hale-Bopp was seen. Both of these comets outshone Haley's comet.

Hyakutake comet

Summary

Our solar system consists of our Sun with 8 planets rotating around it. Most of these planets have at least one moon rotating around them. There are also Pluto, asteroids and comets in orbit around the sun and are included in our solar system.

Answers to Readers' Questions

Set high goals for yourself and "shoot for the stars"

Resources

The following resources provide information on this subject:

Websites

Solar System Simulator - Many pictures from the Jet Propulsion Laboratory (JPL)

Nine Planets - A great multimedia tour of the Solar System from the University of Arizona

Asteroid Fact Sheet - Facts on known asteroids

Meteors, Meteorites and Impacts

Hubble Probes Inner Region of Comet

Comet Hale-Bopp: The Great Comet of 1997 - Pictures from JPL

Comet Fact Sheet - Facts on known comets

Astronomy Resources

Books

Top-rated books on the Solar System

Miscellaneous

Mini-quiz to check your understanding

1. What planet is known for its rings?

2. Which planets besides the Earth sustain life?

3. What is another name for a shooting star?

If you got all three correct, you are on your way to becoming a Champion in Physical Science. If you had problems, you had better look over the material again.

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