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How Many Planets are in the Solar System?

by Fraser Cain on July 16, 2008

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Planets and the dwarf planets. Image credit: IAU
Are you wondering how many planets are in the Solar System? Until just a few years ago, there were 9 planets in the Solar System. However, the International Astronomical Union decided that Pluto is no longer a planet, so there are now only 8 planets in the Solar System.

Why is Pluto not a planet? We have a whole article that discusses this in great detail, but it happened because astronomers discovered a new object larger than Pluto out beyond its orbit. This object is called Eris, and it’s at least a few hundred kilometers bigger than Pluto. The IAU wondered if this object should be considered a planet too?

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At that, there could have been 10 planets in the Solar System, and maybe more.

The IAU met in decided that planets need to have 3 things:
- they need to orbit the Sun
- they need to have enough gravity to pull themselves into a ball
- they need to have cleared out all other objects from their orbit

Although it orbits the Sun, and has pulled itself into a sphere, Pluto has failed to clear out all the other objects from its orbit. Earth more than 1 million times more massive than all the other material in its orbit combined, while Pluto is just a tiny fraction of the rest of the icy material around it.

The IAU decided that Pluto and Eris should be reclassified as dwarf planets. The asteroid Ceres meets those requirements, so it’s a dwarf planet too.

So, how many planets in the Solar System? Until a new, large object is discovered out beyond Pluto, there are only 8 planets, and 3 dwarf planets.

If you want to read the full news about why Pluto isn’t a planet, read the story here. You can also read the announcement from the IAU describing what a dwarf planet is.

Here’s a link to an overview of the Solar System from the Nine Planets, and NASA’s news about the Solar System. Here’s an article that theorizes that there are 12 planet.

We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.

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About

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

  • Laurel Kornfeld

    There are 12 planets in the solar system–Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, MakeMake, and Eris. It’s not the International Astronomical Union but four percent of that group that voted on excluding Pluto in a sloppy, nonsensical definition that states a dwarf planet is not a planet at all. The requirement that an object clear its orbit to be considered a planet is vague and if applied literally, could exclude all the planets because none has fully cleared its orbital field of nearby asteroids, and Neptune has not cleared its orbit of Pluto.

    Most of the 424 IAU members who voted on this are not planetary scientists. No absentee voting was allowed. Also, many planetary scientists are not members of the IAU and therefore had no say in this matter.

    Interestingly, almost as many astronomers as those who voted at the IAU General Assembly in 2006 signed a petition saying they will not use the new definition, and a better one is needed. The lead scientist rejecting the IAU decision is Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission and one of the foremost experts in the world on Pluto and the Kuiper Belt.

    An object orbiting the sun with enough self-gravity to pull itself into a round shape (a condition known as hydrostatic equilibrim) is a planet. With this definition, we keep the term planet as broad as possible while allowing for multiple subcategories such as terrestrial planet, gas giant, ice giant, dwarf planet, hot Jupiters, super Earths, etc.

    There is no problem with defining a dwarf planet as an object that does not dominate the neighborhood of its orbit. The problem comes in stating that a dwarf planet is not a subtype of planet. This is where the IAU decision falls short and needs to be revised.

    This matter will be further discussed in August 2008 at the Great Planet Debate in Laurel, Maryland. More information on this conference, which is open to the public, is available at http://gpd.jhuapl.edu/

    It is a disservice to readers to portray the planet question as a done deal as opposed to an ongoing debate. I urge you to present both sides of this debate as opposed to choosing one view and presenting it as a foregone conclusion.

  • patience

    The explanation based on the nine planet:but i suggest that there was jst nine planet in the solar system.how come the eight now in the solar system.joining the three dwarf planet that is the ceres,eris,and the makemake plus the pluto which is not part of the planet agin making twelve planet in the solar system.can u pls give more explanation based on that.send any information to my email box provided below.

  • M. Afzal

    I like the comments of Laurel Kornfeld, please let me know the present status of number of planets after the Aug 2008 meeting or any new development in this topic.

  • Laurel Kornfeld

    Thank you, M. Afzal. You can find the proceedings of the Great Planet Debate–some audio and some video–at this site:
    http://gpd.jhuapl.edu/

    For those of us who believe dwarf planets should be counted as a subclass of planets, the latest status is that our solar system now has 13 planets: four terrestrial planets, four jovian planets, and five dwarf planets. Haumea, formerly known as 2003EL61, was recognized as a dwarf planet in September. The current order of the planets from the sun is Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake and Eris–with more dwarf planets likely to come.

  • planet janet

    planet shlanet. who cares? as long as no alien is trying to take over my planet then im fine

  • ese

    pls tell me how many planets when you see all of them then send it back to
    me and i will never send any bad thing to you even if i wanted to so help me out pretty pls

  • Kate

    for me its easy if it orbits the sun and it is in are planetary system making it a planet just because its past the Kuiper belt or in it and small does not make it less of a planet

    so for me Pluto Haumea, Makemake, and Eris. Ceres are planets

  • Dmitri Yatsenko

    Laurel Kornfeld I couldn’t agree more. However, what you are calling a planet is actually termed a ‘planemo’, meaning PLANEtary Mass Object. This includes perfect planets AND dwarf planets, as well as moons the size of planets and even interstellar ‘planets’. Pluto, Eris, Haumea, and other dwarf planets, including Ceres, are planemos. You are right; the ‘clearing the neighboorhood’ thing is ABSURD, and I refuse to use the IAU definition of a planet, I will use my own definition (see my second latest comment on ‘why is pluto not a planet/universe today). And I do not agree that a dwarf planet is not a planet even though it is called a dwarf PLANET! Since both planets and dwarf planets are planemos, then they are. Dwarf planets are not between planets and planetoids, they are closer to planets! However, I do believe that perfect planets and dwarf planets are different things. But, as I said, they are two of several subclasses of planemos. And also, I will just add one thing to your definition of planet/planemo: it needs to be massive enough and typically big enough to get into the planetary span (which, however, needs to be defined roughly). All bodies fullfilling this condition would be in hydrostatic equilibrium. Also, I disagree with you that it is OK to define dwarf planet by whether it has cleared its orbit. Didn’t you mention yourself that it is stupid to determine a planet by that criteria (or, rather, the opposite of it)?

  • caca head

    Laurel Kornfeld I couldn’t agree more. However, what you are calling a planet is actually termed a ‘planemo’, meaning PLANEtary Mass Object. This includes perfect planets AND dwarf planets, as well as moons the size of planets and even interstellar ‘planets’. Pluto, Eris, Haumea, and other dwarf planets, including Ceres, are planemos. You are right; the ‘clearing the neighboorhood’ thing is ABSURD, and I refuse to use the IAU definition of a planet, I will use my own definition (see my second latest comment on ‘why is pluto not a planet/universe today). And I do not agree that a dwarf planet is not a planet even though it is called a dwarf PLANET! Since both planets and dwarf planets are planemos, then they are. Dwarf planets are not between planets and planetoids, they are closer to planets! However, I do believe that perfect planets and dwarf planets are different things. But, as I said, they are two of several subclasses of planemos. And also, I will just add one thing to your definition of planet/planemo: it needs to be massive enough and typically big enough to get into the planetary span (which, however, needs to be defined roughly). All bodies fullfilling this condition would be in hydrostatic equilibrium. Also, I disagree with you that it is OK to define dwarf planet by whether it has cleared its orbit. Didn’t you mention yourself that it is stupid to determine a planet by that criteria (or, rather, the opposite of it)?

  • caca head

    Laurel Kornfeld I couldn’t agree more. However, what you are calling a planet is actually termed a ‘planemo’, meaning PLANEtary Mass Object. This includes perfect planets AND dwarf planets, as well as moons the size of planets and even interstellar ‘planets’. Pluto, Eris, Haumea, and other dwarf planets, including Ceres, are planemos. You are right; the ‘clearing the neighboorhood’ thing is ABSURD, and I refuse to use the IAU definition of a planet, I will use my own definition (see my second latest comment on ‘why is pluto not a planet/universe today). And I do not agree that a dwarf planet is not a planet even though it is called a dwarf PLANET! Since both planets and dwarf planets are planemos, then they are. Dwarf planets are not between planets and planetoids, they are closer to planets! However, I do believe that perfect planets and dwarf planets are different things. But, as I said, they are two of several subclasses of planemos. And also, I will just add one thing to your definition of planet/planemo: it needs to be massive enough and typically big enough to get into the planetary span (which, however, needs to be defined roughly). All bodies fullfilling this condition would be in hydrostatic equilibrium. Also, I disagree with you that it is OK to define dwarf planet by whether it has cleared its orbit. Didn’t you mention yourself that it is stupid to determine a planet by that criteria (or, rather, the opposite of it)?Laurel Kornfeld I couldn’t agree more. However, what you are calling a planet is actually termed a ‘planemo’, meaning PLANEtary Mass Object. This includes perfect planets AND dwarf planets, as well as moons the size of planets and even interstellar ‘planets’. Pluto, Eris, Haumea, and other dwarf planets, including Ceres, are planemos. You are right; the ‘clearing the neighboorhood’ thing is ABSURD, and I refuse to use the IAU definition of a planet, I will use my own definition (see my second latest comment on ‘why is pluto not a planet/universe today). And I do not agree that a dwarf planet is not a planet even though it is called a dwarf PLANET! Since both planets and dwarf planets are planemos, then they are. Dwarf planets are not between planets and planetoids, they are closer to planets! However, I do believe that perfect planets and dwarf planets are different things. But, as I said, they are two of several subclasses of planemos. And also, I will just add one thing to your definition of planet/planemo: it needs to be massive enough and typically big enough to get into the planetary span (which, however, needs to be defined roughly). All bodies fullfilling this condition would be in hydrostatic equilibrium. Also, I disagree with you that it is OK to define dwarf planet by whether it has cleared its orbit. Didn’t you mention yourself that it is stupid to determine a planet by that criteria (or, rather, the opposite of it)?Laurel Kornfeld I couldn’t agree more. However, what you are calling a planet is actually termed a ‘planemo’, meaning PLANEtary Mass Object. This includes perfect planets AND dwarf planets, as well as moons the size of planets and even interstellar ‘planets’. Pluto, Eris, Haumea, and other dwarf planets, including Ceres, are planemos. You are right; the ‘clearing the neighboorhood’ thing is ABSURD, and I refuse to use the IAU definition of a planet, I will use my own definition (see my second latest comment on ‘why is pluto not a planet/universe today). And I do not agree that a dwarf planet is not a planet even though it is called a dwarf PLANET! Since both planets and dwarf planets are planemos, then they are. Dwarf planets are not between planets and planetoids, they are closer to planets! However, I do believe that perfect planets and dwarf planets are different things. But, as I said, they are two of several subclasses of planemos. And also, I will just add one thing to your definition of planet/planemo: it needs to be massive enough and typically big enough to get into the planetary span (which, however, needs to be defined roughly). All bodies fullfilling this condition would be in hydrostatic equilibrium. Also, I disagree with you that it is OK to define dwarf planet by whether it has cleared its orbit. Didn’t you mention yourself that it is stupid to determine a planet by that criteria (or, rather, the opposite of it)?

  • caca head

    Laurel Kornfeld I couldn’t agree more. However, what you are calling a planet is actually termed a ‘planemo’, meaning PLANEtary Mass Object. This includes perfect planets AND dwarf planets, as well as moons the size of planets and even interstellar ‘planets’. Pluto, Eris, Haumea, and other dwarf planets, including Ceres, are planemos. You are right; the ‘clearing the neighboorhood’ thing is ABSURD, and I refuse to use the IAU definition of a planet, I will use my own definition (see my second latest comment on ‘why is pluto not a planet/universe today). And I do not agree that a dwarf planet is not a planet even though it is called a dwarf PLANET! Since both planets and dwarf planets are planemos, then they are. Dwarf planets are not between planets and planetoids, they are closer to planets! However, I do believe that perfect planets and dwarf planets are different things. But, as I said, they are two of several subclasses of planemos. And also, I will just add one thing to your definition of planet/planemo: it needs to be massive enough and typically big enough to get into the planetary span (which, however, needs to be defined roughly). All bodies fullfilling this condition would be in hydrostatic equilibrium. Also, I disagree with you that it is OK to define dwarf planet by whether it has cleared its orbit. Didn’t you mention yourself that it is stupid to determine a planet by that criteria (or, rather, the opposite of it)?Laurel Kornfeld I couldn’t agree more. However, what you are calling a planet is actually termed a ‘planemo’, meaning PLANEtary Mass Object. This includes perfect planets AND dwarf planets, as well as moons the size of planets and even interstellar ‘planets’. Pluto, Eris, Haumea, and other dwarf planets, including Ceres, are planemos. You are right; the ‘clearing the neighboorhood’ thing is ABSURD, and I refuse to use the IAU definition of a planet, I will use my own definition (see my second latest comment on ‘why is pluto not a planet/universe today). And I do not agree that a dwarf planet is not a planet even though it is called a dwarf PLANET! Since both planets and dwarf planets are planemos, then they are. Dwarf planets are not between planets and planetoids, they are closer to planets! However, I do believe that perfect planets and dwarf planets are different things. But, as I said, they are two of several subclasses of planemos. And also, I will just add one thing to your definition of planet/planemo: it needs to be massive enough and typically big enough to get into the planetary span (which, however, needs to be defined roughly). All bodies fullfilling this condition would be in hydrostatic equilibrium. Also, I disagree with you that it is OK to define dwarf planet by whether it has cleared its orbit. Didn’t you mention yourself that it is stupid to determine a planet by that criteria (or, rather, the opposite of it)?Laurel Kornfeld I couldn’t agree more. However, what you are calling a planet is actually termed a ‘planemo’, meaning PLANEtary Mass Object. This includes perfect planets AND dwarf planets, as well as moons the size of planets and even interstellar ‘planets’. Pluto, Eris, Haumea, and other dwarf planets, including Ceres, are planemos. You are right; the ‘clearing the neighboorhood’ thing is ABSURD, and I refuse to use the IAU definition of a planet, I will use my own definition (see my second latest comment on ‘why is pluto not a planet/universe today). And I do not agree that a dwarf planet is not a planet even though it is called a dwarf PLANET! Since both planets and dwarf planets are planemos, then they are. Dwarf planets are not between planets and planetoids, they are closer to planets! However, I do believe that perfect planets and dwarf planets are different things. But, as I said, they are two of several subclasses of planemos. And also, I will just add one thing to your definition of planet/planemo: it needs to be massive enough and typically big enough to get into the planetary span (which, however, needs to be defined roughly). All bodies fullfilling this condition would be in hydrostatic equilibrium. Also, I disagree with you that it is OK to define dwarf planet by whether it has cleared its orbit. Didn’t you mention yourself that it is stupid to determine a planet by that criteria (or, rather, the opposite of it)?Laurel Kornfeld I couldn’t agree more. However, what you are calling a planet is actually termed a ‘planemo’, meaning PLANEtary Mass Object. This includes perfect planets AND dwarf planets, as well as moons the size of planets and even interstellar ‘planets’. Pluto, Eris, Haumea, and other dwarf planets, including Ceres, are planemos. You are right; the ‘clearing the neighboorhood’ thing is ABSURD, and I refuse to use the IAU definition of a planet, I will use my own definition (see my second latest comment on ‘why is pluto not a planet/universe today). And I do not agree that a dwarf planet is not a planet even though it is called a dwarf PLANET! Since both planets and dwarf planets are planemos, then they are. Dwarf planets are not between planets and planetoids, they are closer to planets! However, I do believe that perfect planets and dwarf planets are different things. But, as I said, they are two of several subclasses of planemos. And also, I will just add one thing to your definition of planet/planemo: it needs to be massive enough and typically big enough to get into the planetary span (which, however, needs to be defined roughly). All bodies fullfilling this condition would be in hydrostatic equilibrium. Also, I disagree with you that it is OK to define dwarf planet by whether it has cleared its orbit. Didn’t you mention yourself that it is stupid to determine a planet by that criteria (or, rather, the opposite of it)?Laurel Kornfeld I couldn’t agree more. However, what you are calling a planet is actually termed a ‘planemo’, meaning PLANEtary Mass Object. This includes perfect planets AND dwarf planets, as well as moons the size of planets and even interstellar ‘planets’. Pluto, Eris, Haumea, and other dwarf planets, including Ceres, are planemos. You are right; the ‘clearing the neighboorhood’ thing is ABSURD, and I refuse to use the IAU definition of a planet, I will use my own definition (see my second latest comment on ‘why is pluto not a planet/universe today). And I do not agree that a dwarf planet is not a planet even though it is called a dwarf PLANET! Since both planets and dwarf planets are planemos, then they are. Dwarf planets are not between planets and planetoids, they are closer to planets! However, I do believe that perfect planets and dwarf planets are different things. But, as I said, they are two of several subclasses of planemos. And also, I will just add one thing to your definition of planet/planemo: it needs to be massive enough and typically big enough to get into the planetary span (which, however, needs to be defined roughly). All bodies fullfilling this condition would be in hydrostatic equilibrium. Also, I disagree with you that it is OK to define dwarf planet by whether it has cleared its orbit. Didn’t you mention yourself that it is stupid to determine a planet by that criteria (or, rather, the opposite of it)?Laurel Kornfeld I couldn’t agree more. However, what you are calling a planet is actually termed a ‘planemo’, meaning PLANEtary Mass Object. This includes perfect planets AND dwarf planets, as well as moons the size of planets and even interstellar ‘planets’. Pluto, Eris, Haumea, and other dwarf planets, including Ceres, are planemos. You are right; the ‘clearing the neighboorhood’ thing is ABSURD, and I refuse to use the IAU definition of a planet, I will use my own definition (see my second latest comment on ‘why is pluto not a planet/universe today). And I do not agree that a dwarf planet is not a planet even though it is called a dwarf PLANET! Since both planets and dwarf planets are planemos, then they are. Dwarf planets are not between planets and planetoids, they are closer to planets! However, I do believe that perfect planets and dwarf planets are different things. But, as I said, they are two of several subclasses of planemos. And also, I will just add one thing to your definition of planet/planemo: it needs to be massive enough and typically big enough to get into the planetary span (which, however, needs to be defined roughly). All bodies fullfilling this condition would be in hydrostatic equilibrium. Also, I disagree with you that it is OK to define dwarf planet by whether it has cleared its orbit. Didn’t you mention yourself that it is stupid to determine a planet by that criteria (or, rather, the opposite of it)?Laurel Kornfeld I couldn’t agree more. However, what you are calling a planet is actually termed a ‘planemo’, meaning PLANEtary Mass Object. This includes perfect planets AND dwarf planets, as well as moons the size of planets and even interstellar ‘planets’. Pluto, Eris, Haumea, and other dwarf planets, including Ceres, are planemos. You are right; the ‘clearing the neighboorhood’ thing is ABSURD, and I refuse to use the IAU definition of a planet, I will use my own definition (see my second latest comment on ‘why is pluto not a planet/universe today). And I do not agree that a dwarf planet is not a planet even though it is called a dwarf PLANET! Since both planets and dwarf planets are planemos, then they are. Dwarf planets are not between planets and planetoids, they are closer to planets! However, I do believe that perfect planets and dwarf planets are different things. But, as I said, they are two of several subclasses of planemos. And also, I will just add one thing to your definition of planet/planemo: it needs to be massive enough and typically big enough to get into the planetary span (which, however, needs to be defined roughly). All bodies fullfilling this condition would be in hydrostatic equilibrium. Also, I disagree with you that it is OK to define dwarf planet by whether it has cleared its orbit. Didn’t you mention yourself that it is stupid to determine a planet by that criteria (or, rather, the opposite of it)?Laurel Kornfeld I couldn’t agree more. However, what you are calling a planet is actually termed a ‘planemo’, meaning PLANEtary Mass Object. This includes perfect planets AND dwarf planets, as well as moons the size of planets and even interstellar ‘planets’. Pluto, Eris, Haumea, and other dwarf planets, including Ceres, are planemos. You are right; the ‘clearing the neighboorhood’ thing is ABSURD, and I refuse to use the IAU definition of a planet, I will use my own definition (see my second latest comment on ‘why is pluto not a planet/universe today). And I do not agree that a dwarf planet is not a planet even though it is called a dwarf PLANET! Since both planets and dwarf planets are planemos, then they are. Dwarf planets are not between planets and planetoids, they are closer to planets! However, I do believe that perfect planets and dwarf planets are different things. But, as I said, they are two of several subclasses of planemos. And also, I will just add one thing to your definition of planet/planemo: it needs to be massive enough and typically big enough to get into the planetary span (which, however, needs to be defined roughly). All bodies fullfilling this condition would be in hydrostatic equilibrium. Also, I disagree with you that it is OK to define dwarf planet by whether it has cleared its orbit. Didn’t you mention yourself that it is stupid to determine a planet by that criteria (or, rather, the opposite of it)?

  • pluto lover

    i think that pluto sould be a planet. the reason it was in our solar system ones just let it say ontill it leaves thats when we should onsederite a lost planet.

  • Mitchell

    I didnt know that pluto was a dwarf planet I thoght it was normal planet.What else is about pluto?

  • Anonymous

    In my opinion, however, I beleieve that there are more that 8 planets and 3 dwarf planets. I think that there are thousands of other planets out there, just that scientists haven’t gotten all the information they need, and we can’t give them all the money that they need to research these planets. I don’t know if I believe aliens or not, but I do believe that there is some sort of life up there.

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