Planet Facts

Planets ['plă-nəts]

Celestial bodies moving in an elliptical orbit around a star.

Stars in the sky. The traveler looks at the starry sky

Long ago, ancient people noticed "wanderers" among the stars, heavenly bodies that moved their position over time. Today, we know they were observing planets. Planets are bodies in space that move in orbit around a star.

There are over 4,000 known exoplanets beyond our solar system, and more are being discovered all the time. But the planets we are most familiar with are the ones that revolve around our own star, the Sun. We live on planet Earth, but there are seven other planets that are all a part of our solar system. These planets have a special relationship to each other because they all revolve around the sun.

Our Solar System

Planet parade model

Our solar system has:

  • One central star called the Sun
  • Eight planets:
    • Mercury (closest to the sun)
    • Venus
    • Earth
    • Mars
    • Jupiter
    • Saturn
    • Uranus
    • Neptune (farthest from the sun)
  • More than 200 moons
  • More than a million rocky asteroids
  • Thousands of icy comets
  • Five dwarf planets:
    • Pluto
    • Ceres
    • Makemake
    • Eris
    • Haumea

Questions and Answers

How old is the solar system? About 4.6 billion years old.

How was it formed? All of the solar system except the sun are loose particles left over from the formation of the Sun. Find out more at Amazing Space.

How big is the solar system? The Oort Cloud, a collection of comets and icy objects surrounding the solar system, is considered the boundary between our solar system and deep space. It lies about 50,000 astronomical units away from the sun (an astronomical unit is the distance between Earth and the Sun).

Do the planets have the same shaped orbit? No! All the planets have their own unique paths around the sun.

Can you see the planets? You can see Mercury, Venus, Mars, Jupiter, and Saturn without a telescope but not Uranus or Neptune.

Where do the planet names come from? Every planet, except for Earth, was named for an ancient Roman god or goddess.

Wasn't Pluto once called a planet? Yes, but scientists reclassified it as a dwarf planet in 2006. Scientists currently use three criteria to define a planet: (1) it must orbit a star, (2) it must have enough gravity to force a spherical shape, and (3) it must have cleared away all similar-sized objects from its orbit. When scientists began to detect other distant objects in Pluto's neighborhood, they found that Pluto did not meet the third criteria for a full-sized planet. Read more about this change at the Library of Congress's Q & A page.

What Is An Orbit?

An orbit is the path followed by an object in space as it moves around another object. Read more about orbits from NASA!

Earth takes 365 days to go around the sun, while Neptune takes 164 years. Makemake, a dwarf planet, takes 310 years to complete one orbit around our sun.

The Rocky Planets

The rocky planets are the four closest ones to the Sun — Mercury, Venus, Earth, and Mars.

The Rocky Planets are small and similar in composition to Earth — they all have a solid, rocky surfaces and hot, molten cores. They do not have rings. Earth and Mars have moons.

A look at how the early solar system formed will help explain how the inner, rocky planets came to differ from the outer, gaseous ones.

The Gaseous Planets (or Gas Giants)

The gaseous planets are the four furthest away from the Sun — Jupiter, Saturn, Uranus, and Neptune.

The Gas Giants are much larger than the rocky planets and are made mostly of hydrogen, helium, frozen water, ammonia, methane, and carbon monoxide. They all have rings and moons.

Jupiter and Saturn contain the largest percentages of hydrogen and helium. Uranus and Neptune contain the largest shares of ices — frozen water, ammonia, methane, and carbon monoxide.



Mercury has almost no atmosphere and can be very, very hot and very, very cold.

The Messenger space probe orbited Mercury from 2011-2015 and helped us learn more about the closest planet to the sun. The European Space Agency launched BepiColombo, a mission to explore Mercury, in 2018.

Check out this gallery of Mercury images.

Learn more about Mercury at, Windows to the Universe, and NASA.


Venus is the hottest planet and the brightest object in the sky after the Sun and the Moon.

The Soviet space program successfully landed 10 probes on Venus that sent back pictures of the surface. Currently, Japan has a spacecraft studying Venus from orbit.

Check out this gallery of Venus images.

Learn more about Venus at, Windows to the Universe, and NASA.


Earth is the only planet with liquid water on its surface, and the only planet with known life!

Earth has one moon. It has an atmosphere of nitrogen and oxygen in the perfect balance to protect and support life.

We use spacecraft to learn about Earth in much the same way we explore other planets.

Check out this gallery of Earth images.

Learn more about Earth at, Windows to the Universe, and NASA.


Mars has enormous volcanoes, canyons and dust storms. Olympus Mons is the largest volcano in the solar system.

Mars is the most intensely studied planet beyond Earth. Scientists have been sending spacecraft to Mars since 1960. NASA has been landing rovers on Mars since 1997. As of 2020, the rover Curiosity and the lander Insight remain active on the planet. Learn more about missions and what they have discovered.

Check out this gallery of Mars images.

Learn more about Mars at, Windows to the Universe, and NASA.


Jupiter is so big that it is twice as massive as all the other planets combined. It has a thick atmosphere, visible bands, and a great red spot, which is a giant storm.

In 1610, when Galileo first pointed a telescope at the sky, he discovered Jupiter's four moons: Io, Europa, Ganymede, and Callisto. Today scientists have identified 79 moons orbiting Jupiter.

Check out this gallery of Jupiter images.

Learn more about Jupiter at, Windows to the Universe, and NASA.


Saturn has 82 moons and the most spectacular set of rings in the solar system. The rings are made of chunks of ice and rock, are very thin sheets, and there are lots of them!

The Cassini mission to Saturn lasted from 2004 to 2017, sending back spectacular images and data that increased scientists' understanding of the solar system.

Check out this gallery of Saturn images.

Learn more about Saturn at, Windows to the Universe, and NASA.


Uranus as seen by NASA's Voyager 2

Uranus rotates on its side. Because of this, daytime on Uranus lasts a whole summer and the seasons are very different from ours.

The Voyager Two spacecraft has provided us with our best information about Uranus to date.

Check out this gallery of Uranus images.

Learn more about Uranus at, Windows to the Universe, and NASA.



Each season on Neptune lasts 40 years. Its blue color is caused by methane in its atmosphere.

Neptune has dark spots, which are anti-cyclones in the planet's freezing clouds.

After it flew by Uranus, the Voyager Two spacecraft went past Neptune. It is the only human-made object to have flown by this planet.

Check out this gallery of Neptune images.

Learn more about Neptune at, Windows to the Universe, and NASA.

Beyond Neptune

Eris dwarf planet

Past Neptune there are a class of objects known the Trans-Neptunians or Kuiper Belt Objects. The most well-known of these is the dwarf planet, Pluto. But there are at least three more dwarf planets in this region, Eris, Makemake, and Haumea. Ceres, the only dwarf planet located in the inner solar system, is found between Mars and Jupiter.


Pluto is so small that some moons in the solar system are bigger than this dwarf planet. Pluto is usually farther away from the sun than Neptune, but its unique orbit sometimes brings it closer than the eighth planet. Pluto's surface is covered by ice made from frozen nitrogen.

Pluto has five moons — Charon, Nix, Hydra, Kerberos, and Styx.

Artist's Illustration of New Horizons Spacecraft

In July 2015, the New Horizons spacecraft visited Pluto, the first human spacecraft to do so.

Pluto was changed from a planet to a dwarf planet on August 24, 2006.

Check out this gallery of Pluto images.

Learn more about Pluto at, Windows to the Universe, and NASA.

Kuiper Belt and Beyond

Kuiper Belt Object - Artists Concept

Pluto is just one of the many thousands of objects that make up the Kuiper Belt. These are the icy remnants of the solar system's formation 4.5 billion years ago.

Astronomers believe that past the Kuiper Belt is the Oort Cloud, a collection of icy debris that surrounds the solar system at almost a light-year away. This marks the edge of our solar system.

Find Out More!

Learn more about the solar system by visiting these Science Trek pages:

Top 10 Questions

December 2008

Thanks to Jason Barnes, Professor of Astrophysics at the University of Idaho; and Dr. Gary L. Bennett, retired scientist and program manager for NASA for the answers.

  1. Why is Pluto not a planet anymore?

    When we first discovered Pluto in 1930 we thought it was a planet because it was the brightest thing we found in that orbit. It turns out that in 1993 we started finding other, what seemed to be planets, about the same size as Pluto in similar orbits to Pluto. Instead of Pluto being the only planet in its part of the solar system, it turns out there are hundreds of icy planets in a big swarm. So like the asteroid belt, when the first asteroid was found, it was called a planet as well. As we found more and more planets that were in the same or similar orbits, we realized it wasn't a full planet itself, only a member of a swarm of minor planets. We find the same thing is true for Pluto; it's only the largest and brightest element of a larger swarm of what we call corporate belt objects. (From Jesse at Wapello Elementary School in Blackfoot.)

  2. There are other galaxies, so are there other planets that sustain life?

    What we do know right now is that there are at least 300 planets outside of our own solar system. Astronomers have detected these by various mechanisms, primarily looking at the wobble of the parent sun. Most of these planets are what have been described as super jupiters. They're much larger than our own planet Jupiter, and many of them are in weird positions, very close to their parent star. We're still looking for earth-type planets, and with the dust clouds that have been seen around some stars plus these planets that have been found, the odds are pretty good that we're going to find planets that are like the earth. As to other galaxies, those are a bit too far away for us to really tell anything, but there are at least 300 extra solar planets that have been discovered. (From Jennings in Mrs. McCamish Cameron's class at Cynthia Mann Elementary in Boise.)

  3. What color are the planets?

    Mercury looks a lot like the moon, based on the images that have been taken by the two spacecraft that have visited mercury. Mercury is the closest planet to the sun. Venus, at least in some of the images that came back from spacecraft, actually has a yellowish cast; that could be the result of how they processed the images because other images show it not quite looking like that. Perhaps it is because they wanted to show that it has sulfuric acid in the atmosphere. Our own planet is a beautiful blue and white. Mars has an orangish red color. Jupiter and Saturn have got bands, very colorful bands, and they're different colors. Saturn tends to be more yellow while Jupiter looks gray with a giant red spot. Uranus and Neptune are in the bluish range. Neptune is a beautiful blue, Uranus is a greenish-blue, and Pluto - we'll find out when the New Horizon spacecraft gets there (it may just be covered with ice). (From Andrew in Twin Falls, who is at home school.)

  4. How is a planet formed?

    Planets are thought to have been formed at the same time that stars are formed. Our sun was formed 4.5 billion years ago from the collapse of a giant cloud of gas and dust. As the gas and dust collapsed under its own gravity the planets formed out of a disk, a flat part of the formation process that's orbiting the sun and is very thin, and composed of dust and rocks. Slowly the dust and rocks grew on to bigger and bigger objects that became rocky planets like the earth and mars. (From Karl in Mrs. McCamish Cameron's class at Cynthia Mann Elementary in Boise.)

  5. How hot is the sun?

    It is believed that it is 6,000 degrees on the surface but at the center it's on the order of 15, perhaps 20, million degrees on the metric scale. Very hot, but at those temperatures it allows the nuclear fusion to occur, which gives us the light and the warmth that we need. (From James in Mrs. Rice's class at Millcreek Elementary School in Middleton.)

  6. What is a meteor?

    A meteor is an asteroid and as asteroids are flying around the inner solar system, they're on crazy, random orbits. Sometimes a asteroid crashes into earth and as it's flying through the atmosphere it burns up on the way through. When you look up and see a big flash of light, that's this little asteroid burning up in the earth's atmosphere and that's a meteor. (From Timmer in Mrs. Schweitzer's class at Riverside Elementary School in Boise.)

  7. Is Planet "X" a planet or a moon?

    We don't know much about Planet "X." in fact, at one point it was thought that perhaps there was another additional planet beyond Neptune. This was about 100 years ago. people looked for that planet and actually found Pluto. There is an idea that maybe there are additional planets further out in our solar system, but by looking at the orbits of the planets we do know about, if there were one out there, we'd be able to see it because the motion of the other planets would change just slightly. The planet Neptune was found and as predicted, people saw the orbit of Uranus change; they knew Neptune was there before they even saw it. Planet "X" was an idea we thought might be there, but we no longer feel that is true. (From Callan at the Northwest Children's Home Education Center in Lewiston.)

  8. What is the coldest planet?

    The coldest planet would be Uranus or Neptune; they are about the same temperature. They are so far out they only receive a small amount of the light we get on earth. (From Rachel in Mrs. Hunt's class at Cynthia Mann Elementary in Boise.)

  9. Why does Saturn have rings around it?

    We found rings around Jupiter, Saturn, Uranus and Neptune. There's even speculation that earth may have a ring early in its history. The rings appear to be a combination of things such as debris which the planets have collected and in many cases it may simply be the result of very small satellites that were in orbit around the planet, such as Saturn. They may have collided and broken apart and then the dust just got spread out and formed rings around the planet. Saturn has 19 rings around. We have a spacecraft there right now which is looking at the rings amongst other things. (From Audrey in Mrs. Woodall's class at Hayden Meadows Elementary in Hayden.)

  10. How far away is the sun and the moon from Earth?

    The moon is about 250,000 miles from the earth; if you were to walk around the earth that would be about 25,000 miles. So if you have to walk around the earth 10 times, the sun is farther away still. The sun is 93 million miles away. It's almost 400 times farther away than the moon is. (From Audrey at the Northwest Children's Home Education Center in Lewiston.)