Thanks to Bill at the Evil Eyebrow for inviting me to participate in his Plutopia! blog carnival. Or if “invite” isn’t really the appropriate word, at least throwing out a blanket invitation and letting me sign on.
So the whole point is that today is the 2nd anniversary of Pluto’s official demotion from the arbitrary astronomical classification “planet”. I’m not going to get into the pseudo-political world of why this is arbitrary or whether it’s particularly significant in any particular way, what I am going to say is:
Pluto is small.
Really freaking small.
Or at least really freaking small relative to anything else that we have come to know and love as a planet. According to NASA, Pluto is 1422 miles (2288 km) in diameter. So although I wouldn’t want to personally walk its entire 4467 mile circumference in one trip, it’s easy to fly that far without leaving this planet. In fact, in my flights to and from Australia, I traveled over three times that far (more if the trans-American legs are included). Granted, Oz is pretty far away in Earth terms, but can we really respect an astronomical body that can be circumnavigated in half a day by a commercial airliner? Mercury is now the smallest planet with a circumference of over 9500 miles (15000 km). Check out this graphical planet size comparison widget.
It’s not just that Pluto is smaller than our Moon, but because it’s 8 bajillion* miles (13 bajillion** km) away it’s absolutely ridiculously hard to see. In preparation for this post, I decided to pull up my handy free home planetarium software, HNSKY, and see if Pluto is actually visible. Not that I expected to walk outside, point to the sky and say “there’s Pluto”, but I thought there might be some remote possibility that I could capture it in a picture. It IS technically in the sky right now, not far from Jupiter in the sky (about 15 degrees to the right at the time of publication: 00:01 EDT). The problem is that (according to the software) Pluto is about magnitude 13.9. Do you have any sense for how many objects in the sky are brighter than that? No? Take a look at the picture below. It is the area around Jupiter and Pluto that you might see with the naked eye on a clear summer night with stars to magnitude 6 shown. Pluto should not be visible, but is marked with a small green dot and two lines.
One could reasonably argue that this is a better sky than most people will ever see, but it doesn’t matter, because THIS is what the sky would look like if you could see everything up to and including Pluto.
Obviously, you can see that the software has sort-of broken down for this many stars… and in fact, seems to only be displaying stars up to 11th magnitude. Click on the picture to see a larger rendering of this same piece of sky. My point is that even if you COULD see Pluto, you probably WOULDN’T see Pluto because it would be lost in the jumble of the other million objects in the sky which are brighter than it is. Which of course means that even if I could point my camera at the right piece of sky, AND get a few hundred thousand photons*** to travel from the Sun AND bounce off Pluto AND end up in my lens it would take a significant effort to determine which one of the non-black pixel dots contained Plutonian photons.
For reference, the two dimmest planets are Uranus and Neptune at approximate visual magnitudes of 5.7 and 7.8 respectively. So even though Uranus is within the range of human perception it wasn’t documented as discovered until 1781 AD. It was simply another dot lost in the background noise, and moved too slowly to appear to be changing in the sky.
So Pluto, we salute thee, even though you’re small and dim and probably deserved to not quite rank with the eight remaining Solar planets. The good news is that you seem to have plenty of siblings in this new realm of astronomical bodies.