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Planets: Facts

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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

Solar System

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?

Orbits

An orbit is the path followed by an object in space as it moves 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

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)

Saturn

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 largest shares of ices — frozen water, ammonia, methane, and carbon monoxide.

Venus
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Astronomical symbol for the planet Venus
Venus' hand mirror

Venus

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 NinePlanets.org, Windows to the Universe, and NASA.

Earth
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Astronomical symbol for the planet Earth
Globe with an equator and a meridian

Earth

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 NinePlanets.org, Windows to the Universe, and NASA.

Mars
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Astronomical symbol for the planet Mars
Mars' shield and spear

Mars

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 NinePlanets.org, Windows to the Universe, and NASA.

Jupiter
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Astronomical symbol for the planet Jupiter
Jupiter's thunderbolt

Jupiter

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 NinePlanets.org, Windows to the Universe, and NASA.

Saturn
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Astronomical symbol for the planet Saturn
Saturn's scythe

Saturn

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 NinePlanets.org, Windows to the Universe, and NASA.

Beyond Neptune

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
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Astronomical symbol for the dwarf planet Pluto
PL monogram for Pluto and the astronomer Percival Lowell

Pluto

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.

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 NinePlanets.org, Windows to the Universe, and NASA.

Kuiper Belt and Beyond

Kuiper Belt

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.

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