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Solar System Planets: Order of the 8 (or 9) Planets

The planets of the solar system as depicted by a NASA computer illustration. Orbits and sizes are not shown to scale.
The planets of the solar system as depicted by a NASA computer illustration. Orbits and sizes are not shown to scale.
Credit: NASA

Ever since the discovery of Pluto in 1930, kids grew up learning about the nine planets of our solar system. That all changed starting in the late 1990s, when astronomers began to argue about whether Pluto was a planet. In a highly controversial decision, the International Astronomical Union ultimately decided in 2006 to call Pluto a “dwarf planet,” reducing the list of “real planets” in our solar system to eight. But many kids (and adults) cling to the notion of nine planets.

Regardless of your view, here’s the order of the eight larger planets, starting nearest the sun and working outward through the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.

[Solar System Pictures: A Photo Tour]

If you insist on including Pluto, then that ninth world would come after Neptune on the list; Pluto is truly way out there, and on a wildly tilted, elliptical orbit (two of the several reasons it got demoted). Interestingly, Pluto used to be the eighth planet, actually. More on that below.

Terrestrial planets

The inner four worlds are called “terrestrial planets,” because, like Earth, their surfaces are all rocky. Pluto, too, has a solid surface (and a very frozen one) but has never been grouped with the four terrestrials.

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Jovian planets

The four large outer worlds — Jupiter, Saturn, Uranus, and Neptune — are known as the “Jovian planets” (meaning “Jupiter-like”) because they are all huge compared to the terrestrial planets, and because they are gaseous in nature rather than having rocky surfaces (though some or all of them may have solid cores, astronomers say). These worlds are also frequently called the gas giants, but that’s not a great way to describe them, some astronomers say, because Uranus and Neptune are more ice than gas. All four contain mostly hydrogen and helium.

Dwarf planets

The new IAU definition of a full-fledged planet goes like this: A body that circles the sun without being some other object's satellite, is large enough to be rounded by its own gravity (but not so big that it begins to undergo nuclear fusion, like a star) and has "cleared its neighborhood" of most other orbiting bodies. Yeah, that’s a mouthful.

The problem for Pluto, besides its small size and offbeat orbit, is that it shares its space with lots of other objects in the Kuiper Belt, beyond Neptune. Still, the demotion of Pluto remains controversial.

The new IAU planet definition puts other small, round worlds in the dwarf planet category, including the Kuiper Belt objects Eris, Haumea, Makemake.

Also now a dwarf planet is Ceres, a round object in the Asteroid Belt between Mars and Jupiter. Ceres was actually considered a planet when discovered in 1801 and then later deemed to be an asteroid. Some astronomers like to consider Ceres as a 10th planet (not to be confused with Nibiru or Planet X), but that line of thinking opens up the possibility of there being 13 planets, with more bound to be discovered.

The planets

Below is a brief overview of the eight primary planets in our solar system, in order from the inner solar system outward:

Mercury temperature swings are the greatest in the solar system, despite being the closest planet to the Sun. There is not enough atmosphere on the planet to entrap heat.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Mercury

The closest planet to the sun, Mercury is only a bit larger than Earth's moon. Its day side is scorched by the sun and can reach 840 degrees F (450 C), but on the night side, temperatures drop to hundreds of degrees below freezing. Mercury has virtually no atmosphere to absorb meteor impacts, so its surface is pockmarked with craters, just like the moon.

  • Discovery: Known to the ancients and visible to the naked eye
  • Named for: Messenger of the Roman gods
  • Diameter: 3,031 miles (4,878 km)
  • Orbit: 88 Earth days
  • Day: 58.6 Earth days

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Cloud Cities On Venus?
Venus' southern hemisphere, as seen in the ultraviolet.
Credit: ESA

Venus

The second (2nd) planet from the sun, Venus is terribly hot. The atmosphere is toxic. The pressure at the surface would crush and kill you. Scientists describe Venus’ situation as a runaway greenhouse effect. Its size and structure are similar to Earth, Venus' thick, toxic atmosphere traps heat in a runaway "greenhouse effect." Oddly, Venus spins slowly in the opposite direction of most planets.

The Greeks believed Venus was two different objects — one in the morning sky and another in the evening. Because it is often brighter than any other object in the sky — except for the sun and moon — Venus has generated many UFO reports.

  • Discovery: Known to the ancients and visible to the naked eye
  • Named for: Roman goddess of love and beauty
  • Diameter: 7,521 miles (12,104 km)
  • Orbit: 225 Earth days
  • Day: 241 Earth days

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Earth from Russian Satellit
An image of the Earth taken by the Russian weather satellite Elektro-L No.1.
Credit: NTsOMZ

Earth

The third (3rd) planet from the sun, Earth is a waterworld, with two-thirds of the planet covered by ocean. It’s the only world known to harbor life. Earth’s atmosphere is rich in life-sustaining nitrogen and oxygen. Earth's surface rotates about its axis at 1,532 feet per second — slightly more than 1,000 mph — at the equator. The planet zips around the sun at more than 18 miles per second.

  • Diameter: 7,926 miles (12,760 km)
  • Orbit: 365.24 days
  • Day: 23 hours, 56 minutes

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NASA Scientist: 'Mars Could be Biologically Alive'
Mars researchers are focusing both Earth-based and planet orbiting sensors to better understand sources of methane on the red planet. Image
Credit: Space Telescope Science Institute

Mars

The fourth (4th) planet from the sun, is a cold, dusty place. The dust, an iron oxide, gives the planet its reddish cast. Mars shares similarities with Earth: It is rocky, has mountains and valleys, and storm systems ranging from localized tornado-like dust devils to planet-engulfing dust storms. It snows on Mars. And Mars harbors water ice. Scientists think it was once wet and warm, though today it’s cold and desert-like.

Mars' atmosphere is too thin for liquid water to exist on the surface for any length of time. Scientists think ancient Mars would have had the conditions to support life, and there is hope that signs of past life — possibly even present biology — may exist on the Red Planet.

  • Discovery: Known to the ancients and visible to the naked eye
  • Named for: Roman god of war
  • Diameter: 4,217 miles (6,787 km)
  • Orbit: 687 Earth days
  • Day: Just more than one Earth day (24 hours, 37 minutes)

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Close-up of Jupiter's Great Red Spot as seen by a Voyager spacecraft.
Close-up of Jupiter's Great Red Spot as seen by a Voyager spacecraft.
Credit: NASA/JPL-Caltech

Jupiter

The fifth (5th) planet from the sun, Jupiter is huge and is the most massive planet in our solar system. It’s a mostly gaseous world, mostly hydrogen and helium. Its swirling clouds are colorful due to different types of trace gases. A big feature is the Great Red Spot, a giant storm which has raged for hundreds of years. Jupiter has a strong magnetic field, and with dozens of moons, it looks a bit like a miniature solar system.

  • Discovery: Known to the ancients and visible to the naked eye
  • Named for: Ruler of the Roman gods
  • Diameter: 88,730 miles (428,400 km)
  • Orbit: 11.9 Earth years
  • Day: 9.8 Earth hours

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Saturn Rings Cast in Rare Light
The shadow of Saturn's moon Mimas dips onto the planet's rings and straddles the Cassini Division in this natural color image taken as Saturn approaches its August 2009 equinox.
Credit: NASA/JPL/Space Science Institute

Saturn

The sixth (6th) planet from the sun is known most for its rings. When Galileo Galilei first studied Saturn in the early 1600s, he thought it was an object with three parts. Not knowing he was seeing a planet with rings, the stumped astronomer entered a small drawing — a symbol with one large circle and two smaller ones — in his notebook, as a noun in a sentence describing his discovery. More than 40 years later, Christiaan Huygens proposed that they were rings. The rings are made of ice and rock. Scientists are not yet sure how they formed. The gaseous planet is mostly hydrogen and helium. It has numerous moons.

  • Discovery: Known to the ancients and visible to the naked eye
  • Named for: Roman god of agriculture
  • Diameter: 74,900 miles (120,500 km)
  • Orbit: 29.5 Earth years
  • Day: About 10.5 Earth hours

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Uranus Rings Tilted
Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which the planet is tilted.
Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory

Uranus

The seventh (7th) planet from the sun, Uranus is an oddball. It’s the only giant planet whose equator is nearly at right angles to its orbit — it basically orbits on its side. Astronomers think the planet collided with some other planet-sized object long ago, causing the tilt. The tilt causes extreme seasons that last 20+ years, and the sun beats down on one pole or the other for 84 Earth-years. Uranus is about the same size as Neptune. Methane in the atmosphere gives Uranus its blue-green tint. It has numerous moons and faint rings.

  • Discovery: 1781 by William Herschel (was thought previously to be a star)
  • Named for: Personification of heaven in ancient myth
  • Diameter: 31,763 miles (51,120 km)
  • Orbit: 84 Earth years
  • Day: 18 Earth hours

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Neptune’s winds travel at more than 1,500 mph, and are the fastest planetary winds in the solar system.
Neptune’s winds travel at more than 1,500 mph, and are the fastest planetary winds in the solar system.
Credit: NASA/JPL

Neptune

The eighth (8th) planet from the sun, Neptune is known for strong winds — sometimes faster than the speed of sound. Neptune is far out and cold. The planet is more than 30 times as far from the sun as Earth. It has a rocky core. Neptune was the first planet to be predicted to exist by using math, before it was detected. Irregularities in the orbit of Uranus led French astronomer Alexis Bouvard to suggest some other might be exerting a gravitational tug. German astronomer Johann Galle used calculations to help find Neptune in a telescope. Neptune is about 17 times as massive as Earth.

  • Discovery: 1846
  • Named for: Roman god of water
  • Diameter: 30,775 miles (49,530 km)
  • Orbit: 165 Earth years
  • Day: 19 Earth hours

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Pluto's weird features explained in infographic.
Learn all about Pluto's weirdly eccentric orbit, four moons and more in this SPACE.com infographic.
Credit: SPACE.com/Karl Tate

Pluto (Dwarf Planet)

The ninth (9th) planet from the sun … well … Pluto is unlike other planets in many respects. It is smaller than our moon. Its orbit carries inside the orbit of Neptune and the way out beyond that orbit. From 1979 until early 1999, Pluto had actually been the eighth planet from the sun. Then, on Feb. 11, 1999, it crossed Neptune's path and once again became the solar system's most distant planet — until it was demoted to dwarf planet status. Pluto will stay beyond Neptune for 228 years. Pluto’s orbit is tilted to the main plane of the solar system — where the other planets orbit — by 17.1 degrees. It’s a cold, rocky world with only a very ephemeral atmosphere.

  • Discovery: 1930 by Clyde Tombaugh
  • Named for: Roman god of the underworld, Hades
  • Diameter: 1,430 miles (2,301 kilometers)
  • Orbit: 248 Earth years
  • Day: 6.4 Earth day

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Robert Roy Britt

Rob has been producing internet content since the mid-1990s. He was a writer, editor and Director of Site Operations at SPACE.com starting in 1999. He served as Managing Editor of LiveScience since its launch in 2004. He now oversees news operations for the TechMediaNetwork's growing suite of technology, science and business news sites. Prior to joining the company, Rob was an editor at The Star-Ledger in New Jersey. He has a journalism degree from Humboldt State University in California.
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