Back when I first went to school and someone asked the number of planets in the Solar System, the answer was simple. Nine. From sun-baked Mercury and cloud-shrouded Venus out to frozen little Pluto. Jupiter had the most moons and Saturn had the rings. Not much was known about Uranus or Neptune, other than each had multiple moons and that both were very cold gas giants.
In today’s world we no longer know the number of planets in the Solar System. For years astronomers have been arguing about the definition of a planet with some of them suggesting that Pluto is simply too small to qualify. Of course, with modern technology, we now know that Pluto has three moons, two of them small and one of them large enough to have a name, Clarion.
To complicate the issue, our explorations of the Solar System with robotic probes and the Hubble Space Telescope has taught us that three planets besides Saturn have rings. Jupiter has four and both Uranus and Neptune have some sort of ring structures. Both Saturn and Jupiter have complex systems of moons. Earth, it seems is unique in that it has but a single satellite, no rings, and oceans full of liquid water.
So, we’ve learned that the Solar System, as it was originally introduced to me back in elementary school is no longer what we thought it was. More planets with rings and many more satellites than we thought. And maybe many more planets than thought. The changes are significant.
So how many planets are in the Solar System?
We know, now, that three objects, out beyond Pluto in what is called the Kuiper Belt have been found. They are about the size of Pluto, maybe a little larger or maybe a little smaller. The diameters are estimated to be around a thousand miles. They are smaller than our moon and smaller than many of the other satellites around other of the planets. The difference is these objects orbit the sun rather than a planet.
So, if Pluto is thought to be too small to be a planet, then the answer is eight. If it is large enough to be a planet, then the other objects are also planets and the answer is twelve.
Thinking about this, I don’t know of another time when there would be this sort of astronomical confusion. Yes, our ancestors originally knew only of those planets visible to the unaided eye. The invention of the telescope increased the number until there were thought to be nine. Now our technology has shown us other large planet-like objects so far from the sun to be nearly invisible and almost outside the Solar System. But our definitions have changed, or are changing with these new discoveries, and that is the area of confusion. Eight or twelve, and possibly even more that have yet to be found. The Solar System is now believed to extend out a light year or more and is discovered to be an increasingly complex structure.
What this really teaches us that sometimes progress leads to confusion and that someone should have defined a planet a long time ago. Until we have a couple of answers, we won’t know how many planets are in the Solar System. To me, that makes astronomy a lot more fun.