Now that Pluto is not longer a Planet, we have a new Solar System: the Sun, 8 planets and 4 dwarf planets.
The new Solar System looks mostly like the old Solar System, but there are a few significant differences. The Sun stays the same. Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune all remain planets. Pluto has been downgraded from a planet to a dwarf planet. The asteroid Ceres has been upgraded to a dwarf planet as well. So has the newly discovered Kuiper Belt objects, Eris and Makemake.
Why isn’t Pluto a planet anymore? We’ve got an article on Universe Today that goes into the decision in great detail – it’s called, Why Pluto is Not a Planet?
But here’s the short answer. Astronomer Mike Brown from Caltech recently announced the discovery of a new object out beyond the orbit of Pluto. Like Pluto, this new object – now called Eris – was part of the Kuiper Belt, but it was actually larger than Pluto. Since Eris was bigger than Pluto, astronomers had to decide if this, and any other icy objects would also be considered planets. Would we have 10 planets, 11 planets – 20 planets?
The International Astronomical Union met in 2006 to decide on the matter once and for all. The members voted to pass a proposition that for an object to be a planet, it must do the following:
It’s on this third requirement where Pluto falls down. Even though Pluto orbits the Sun and has a spherical shape, its orbit is filled with many other objects of similar size. In fact, Pluto is just a fraction of the mass of all the material in its orbit, while the other planets in the Solar System have millions of times more mass than anything else nearby.
The IAU threw Pluto a bone recently, creating a brand new category called Plutoids. Pluto, Eris and other objects like this will receive this classification.
And that’s the new Solar System. I know it’s hard to tell your kids that there are only 8 planets, and the ignore what all there books tell them, but that’s science in action.
We have recorded a whole series of podcasts about the Solar System at Astronomy Cast. Check them out here.