CFA Asks “What is a Planet?”
Artist’s impression of Pluto. Courtesy New Horizons mission.
It’s been a few years since the great “Define a Planet” vote took place at the International Astronomical Union meeting. That 2006 event touched off what still remains a firestorm of controversy about what is or isn’t a planet, and in particular, whether Pluto fits the definition set out in the vote. I’ve blogged about this a few times, and my viewpoint has remained pretty constant: Pluto is a dwarf planet. That’s still a planet, but the term “dwarf” tells us something about its place in the size hierarchy of planets in our solar system. However, my opinion is just that: my opinion, based on what my understanding of the issues involved.
This Planet Endlessly Fascinates Viewers
Saturn as seen by Cassini from a distance, with the planet “occulting” the Sun. we can see the rings in this mosaic. NASA/JPL/Space Science Institute
When I was a little kid, I found a book in the Time-Life series about planets. I could barely understand most of the words, but the pictures spoke loudly. In particular, one image of a strange-looking planet with a ring around it caught my imagination. It seemed like the most alien thing, the embodiment of alien worlds and far-off places. I later learned that was the planet Saturn. Wow! I couldn’t even think of how such a thing could exist. And I had SO many questions about it. How a planet with rings around it. How did it come about? How far away was it? Could we live on it? Would we ever get to explore it?
Later, of course, I found out much more about Saturn and its rings and moons. The Voyager 2 mission flyby of Saturn was the first one I ever covered as a science writer, and I was like that little kid all over again. I still have the images I carefully collected during the press conferences, and somewhere in my archives, I have a tape of me interviewing some of the project scientists. It was my first “up close” encounter with the Saturn People and the planet that so fascinated me as a kid.
Saturn was the first REALLY alien world I “discovered” on my own, and so I still look at it with some mixture of that old awe and childlike wonder. This is why the constant stream of images from the Cassini Solstice Mission is such a delight. They are, like the images from the rovers and orbiters at Mars (my other most favorite planet), providing an almost real-time look at a distant world. The rings, in particular, still boggle my mind. They stretch out from Saturn so far that if you plopped the planet between Earth and the Moon, they’d cover nearly 3/4 of the distance between the two. Yet, this expansive set of rings is less than a mile thick! The rings are made of countless particles of ice and dust, and they’re threaded by the orbits of small moons.