Salt and Pepper in our Solar System

So why isn’t Pluto a planet anymore?

In August 2006 the International Astronomical Union had a meeting in Prague, Czechoslovakia to come up with the first real definition of a planet. They decided that to be called a planet, an object must pass three tests: it must orbit the Sun, have enough mass and self-gravity to pull itself into a round shape and be big enough to sweep its orbit free of other objects in its path.

Pluto failed the third test and hence was downgraded to a “dwarf planet”. But Pluto wasn’t alone in its new classification and was joined by Ceres, the largest body in the asteroid belt lying between Mars and Jupiter and Eris, a distant icy body located beyond Pluto in the Kuiper Belt. All three of these objects were deemed dwarf planets because they passed the first two tests but failed the third.

Dwarf planets are not planets because they are not big enough to have cleared the neighborhood around their orbit!

How small then is a dwarf planet? We always knew Pluto was the tiniest of all the planets. In fact, a fun way to visualize Pluto’s relative size to the other (now eight!) planets is what I like to call a solar system supper. In order of distance from the sun, Mercury is a green pea (about 1/3 inch), Venus is an unshelled walnut (less than 1 inch), Earth is a pearl onion (slightly less than 1 inch), Mars is a cherry tomato (almost one-half inch), Jupiter is a large head of lettuce (roughly 10 inches), Saturn is a head of cabbage (roughly 6 inches), Uranus is a grapefruit (slightly bigger than 3.5 inches) and Neptune is a large orange (slightly less than 3.5 inches).

Now let’s add the dwarf planets, Pluto, Ceres and Eris into this food fest. In our solar system supper model, Pluto, with a diameter of 2,300 kilometers (1,430 miles), is a peppercorn (less than 1/5 inch wide). Ceres however is less than half the size of Pluto with a diameter of 960 kilometers (590 miles). Can you think of what to add to our solar system supper that is smaller than half a peppercorn? Maybe a poppy seed or large grain of salt? The most distant dwarf planet, Eris, is actually larger that Pluto with a diameter of 2,400 kilometers (1,500 miles). Maybe we just find a slightly larger peppercorn than we did for Pluto to represent Eris.

So far we’ve identified three dwarf planets and probably more to come. We know that they’ll all orbit around the Sun and be round, but I wonder how small they will be? I’m guessing they’ll be somewhere between a grain of salt and a peppercorn in our solar system supper model!