Why isn't Pluto a planet anymore?

11 Answers

  • 8 years ago
    Favorite Answer

    For you to understand this, you need to know what the definition of the term planet is.

    1. It revolves around the sun

    2. Size

    Lets talk about size first. Back in 1801, a celestial object was discovered between Jupiter and Earth. Since it revolved around the sun, it was designated as a planet and named it Ceres. This was followed by Pallas (1802), Juno(1803) and Vesta (1804) around the same place as where Ceres was, that is between earth and jupiter. So for many years, people learned that the solar system contained 11 planets (Mercury, Venus, Earth, Mars, Ceres, Pallas, Juno, Vesta, Jupiter, Saturn and Uranus) But with each year, more and more planets were discovered at the same place. So, naming them became a big problem. As, they were mostly similar in size to each other and very different from the planets on the either side of them (Earth and Jupiter), a new term 'asteroid' was termed for them. This was a good decision as now astronomers have found 1000s of such small celestial bodies and learning their names would have been a pain (:P). All of these bodies make up the present Asteroid belt between earth and jupiter.

    Pluto was discovered in 1930. Since it was initially estimated to be the of the size of neptune and it revolved around the sun, it was designated as a planet. However, as the telescopes became more and more powerful, the size was more accurately found to be lesser and lesser. In fact, some of the moons in our solar system like our moon, Callisto, titan, Ganymede, Triton, Europa and Io are larger than the Pluto. At the same time, like in the case of Ceres, other celestial bodies were discovered near pluto too. This made the rule tilt in the favour of pluto causing some astronomers to start arguing whether it should be designated as a planet. All was okay, until one day, a celestial body bigger than pluto was discovered in the same zone, named Eris. This led the scientists to remove pluto from the list of planets in order to reduce the confusion and the arguments.

    Now, Pluto is a dwarf planet named 134340 pluto, along with Eris and other celestial bodies that make up the Kuiper belt beyond Neptune.

  • 8 years ago

    Because they discovered lots of similar objects, and adding all of those as new planets would make the solar system very messy. Therefore, they set up some criteria that a celestial body must fulfill in order to count as a full blown planet. These are that the object must orbit the Sun directly (i.e. not be a moon), be massive enough to become spherical under its own weight, and to have cleared the general area it orbits in.

    These objects were all rather small, but they fulfilled all criteria except the third one, as they are part of the Kuiper belt. Therefore, it was decided that these new objects would not be added as planets, so they invented a new category: Dwarf planets. Some of these objects were also bigger than Pluto, so it made no sense to keep Pluto as a planet while not adding the others as well. Therefore, Pluto was demoted.

    Sad story, maybe, but the same thing happened about 200 years ago to the asteroid Ceres. It was thought to be another planet, but then they started to discover that it had lots and lots of smaller space buddies. It was decided that these rocks would be called asteroids instead of planets, and that applied to Ceres as well. However, Ceres later got promoted to a dwarf planet at the same time as Pluto got demoted.

    You don't miss Ceres, do you? Probably not.

  • 8 years ago

    Because in 2006, Scientist had discovered Eris. Which looked the same like when they saw it in 2003. It was larger than Pluto. So scientist wonder that should they make Eris as another planet? That way, the Solar System would have 10 planets(Excluding Ceres, Haumea, Makemake.) So Scientist say that they should make both Pluto and Eris dwarf planets.

  • 8 years ago

    Pluto is not a planet because it does not satisfy the third of the three criteria for a planet as defined by the International Astronomer's Union:

    1.It is in orbit around the Sun.

    2.It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).

    3.It has "cleared the neighbourhood" around its orbit.


    Explanation of "clearing the neighborhood":



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  • 8 years ago

    The new definition of planet excludes Pluto because a planet must have cleared its orbit. Pluto is a part of the Kuiper belt, and has not absorbed most of the cometary bodies into itself, or captured them as satellites. Pluto is therefore a dwarf planet. &

    The rules of a planet are:

    It orbits a star or the remnants of a star

    It is large enough for gravity to squash it into a sphere-like shape

    It must have cleared its orbit

  • Anonymous
    8 years ago

    Since pluto does not revolve round the sun in the fixed particular path as the other planets. Also it shows the properties that belongs to the satellites. So pluto is not planet anymore.

  • Anonymous
    8 years ago

    Pluto isn't massive enough to be rounded by its own gravity and hasn't cleared its neighboring region. This is the main feature that every planet requires. It is considered a dwarf planet and is the seconds biggest dwarf planet in our solar system.

  • Anonymous
    8 years ago

    Well, lets look at the criteria that makes a planet:


    This article should help.

  • Anonymous
    8 years ago

    its not considered a planet because it doesnt orbit around the sun, or have sufficient mass to maintain a nearly round shape, and doesnt clear the neighborhood around its orbit

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