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Asked by catherinemaguire to Amy, Drew, Julia, Kimberley, Sara on 13 Jun 2011. This question was also asked by eviej26, melissamartin123, fatimaxxo, eimearconnolly, mccro123.
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Drew Rae answered on 13 Jun 2011:
That’s a good question, and it is an interesting story.
When they first found Pluto around 1930, they thought it was a lot bigger than it was – bigger than Mercury in fact. Eventually they found out that it was actually two planets (Pluto and Charon) going around each other. Being able to see this also helped them work out how big it was, and it was smaller than they thought. It was by a long way the smallest planet.
Not long after, they realised that there are actually quite a few things (around 40) almost as big as Pluto and in the same general part of the solar system.
What really put the nail in the coffin, though, was that one of these things, Eris, was HEAVIER than Pluto, and possibly BIGGER. Two others, Quaoar and Sedna, were about the same size.
It wouldn’t be fair to call Pluto and planet, and not call Eris, Quaoar and Sednar planets. Either we had to call Pluto something else, or we had to have three whole new planets.
They decided to have a new category called “Dwarf Planet”, and put Pluto, Eris, Quaoar and Sednar in that category. The biggest asteroid, Ceres, is in the same category.
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Amy MacQueen answered on 13 Jun 2011:
Like Drew said Pluto is now a “Dwarf Planet” because the dudes who make up the rules about what a planet is (the International Astronomical Union (IAU)) in 2006 stated that in the Solar System a planet is a celestial body that:
1.is in orbit around the Sun,
2.has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3.has “cleared the neighbourhood” around its orbit.Pluto is too small to qualify for all of these so its no longer a planet! 🙂
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Kimberley Bryon answered on 13 Jun 2011:
This is an excellent question and Drew has answered it really well. Personally, I am very sad that Pluto is no longer a planet even though I understand why. What do you think? Were you shocked when Pluto stopped being the ninth planet in our solar system?
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Julia Griffen answered on 13 Jun 2011:
Did not know this… and I call myself a scientist…. Epic Fail!
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Sara Imari Walker answered on 13 Jun 2011:
Ooooo, good question! Looks like Drew has provided a great answer too =) with Amy covering the IAU’s definition of a dwarf planet. The basic story is that its a classification issue. Recently, due to advances in technology, we have been finding more and more objects in our solar system that are similar to Pluto – Eris, Ceres, Quaoar, and Sednar to name a few (most of these far beyond the orbit of Pluto so they have been hard to find, that is why they have only been discovered recently). So either we end up with lots and lots of planets, too many to keep track of, or we make a new classification. Viola! Dwarf planets were defined. A similar thing actually happened back in the 1800’s – we had the 8 planets we have now, but started discovering other objects such as really large asteroids in the asteroid belt. People started naming these as planets, but it soon turned to mayhem! with more and more planets being discovered all the time. So, the objects were reclassified leaving us with just 8 planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune). So, maybe this will happen again if we start discovering even more objects!!
Instead of thinking pluto was demoted from being a planet to a dwarf planet, I like to think of Pluto as the most famous dwarf planet. It’s certainly my favorite!
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