• Question: How many dwarf planets are there in our solar system

    Asked by dizzyg12 to Dilwar, Lou, Rachel, Simon, Susan on 18 Nov 2013.
    • Photo: Simon Langley-Evans

      Simon Langley-Evans answered on 18 Nov 2013:

      Great question dizzyg

      Astronomers think that our solar system has more than 50 dwarf planets, but so far have formally identified only 5. Pluto, which we used to classify as a planet is the second biggest of these (the biggest is Eris) and like most of the planets has moons of its own (5 at the latest count).

      These objects are all fairly new from a scientific discovery point of view. Some of them have been known for a long time, but used to be called asteroids (Ceres is an example of this- it was discovered in 1801). The other two know dwarf planets are Makemake and Haumea.

    • Photo: Susan Skelton

      Susan Skelton answered on 19 Nov 2013:

      A dwarf planet is an object that is big enough for its shape to be controlled by gravity but not big enough for gravity to have pulled in all the other space junk floating around near it.

      The category of dwarf planet was only introduced in 2006. Now that telescopes are getting much more powerful, we’re discovering more and more planets and space objects. When other objects were discovered that were pretty small but bigger than Pluto, the category of dwarf planets was introduced instead of making all these objects proper planets.

      Right now, Pluto is a dwarf planet. In the Kuiper Belt over a thousand other objects have been observed that are Pluto-like, and one of them, Eris, might be even bigger than Pluto.

      This changing of definitions isn’t new though. If you walked into a library in 1850, pulled out the solar system book, went through it and began to memorise the planets, you would have found that there were 23 planets! Everything that was found in the asteroid belt was being called a planet. By 1852, they realised that they had seen so many of these in the asteroid belt and they began to characterise them better so the number of planets went down from 23 to 8. Pluto was discovered in the 1930s, and the number went up to 9.

      Astronomers think that there are more than 50 dwarf planets, that fit the current size definition, but so far only 5 have been identified.

    • Photo: Dilwar Hussain

      Dilwar Hussain answered on 19 Nov 2013:

      I was once taught that there were only 9 planets in our solar system. Then there was a confusion in my A-level Physics class as to whether dwarf planets were included and introduced in 2006 as Susan pointed out. Astronomers think there are some 50 dwarf planets and possibly more yet to be discovered. The pens used to write this chapter of Physics is still being written so exciting times ahead for astronomers.

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