Pluto, distant gateway to the Kuiper Belt and Beyond!


This time we are going to take a look at Pluto and the rather novel way it came to be discovered. But first let’s take a quick look at Pluto with some basic facts about this distant member of our Solar System and I suppose I should start with a proper introduction. Ladies and gentlemen, may I present, Pluto-


This fuzzy blob is actually the best picture we have of Pluto, taken by the Hubble Space Telescope, at least until next year when the New Horizons probe will be taking a closer look. The problem is that Pluto is a long, long way away and very, very small. It’s highly eccentric orbit takes it as “close” as 30 AU from the Sun, (1AstronomicalUnit is the distance from the Sun to the Earth, about 150,000,000 kilometers), right out to 49 AU and as it is only 0.18 times the diameter of the Earth, about 1184 km across, it is not an easy target, even for the biggest telescopes we have today.

So far we have found five moons orbiting Pluto: Charon, Nix, Hydra, Kerberos and Styx but Charon is so big compared to Pluto, over half the size of Pluto in fact, that they orbit a point outside the actual body of Pluto. Here we see Pluto and it’s moons, all except Kerberos


Most of the major planets in the Solar System are visible to the naked eye and have therefore been known since the dawn of human history. All our ancestors had to do was to look up from time to time to see that a few of the shiny dots moved in the course of the year with respect to all the other shiny dots. With the invention of the telescope came a new age of astronomical discovery which gave us, amongst other things, Uranus. But observations of Uranus’ motion showed that it wasn’t moving along its orbit as it should do, something was disturbing it. Then, in the 1840’s, Urbain Le Verrier calculated the position of a planet that was causing these irregularities and this led to the discovery of Neptune. Problem solved. Well, no. You see, when they estimated its mass, they found that Neptune alone couldn’t explain the observed perturbations in Uranus’ orbit so the hunt was on again for a new planet which Percivel Lovel  dubbed “Planet X”, which was a pretty cool name as anything called “X” sounds mysterious and exciting. To cut a long story short, a young astronomer by the name of Clyde Tombaugh was given the tiresome task of looking at thousands of images through a machine called a “Blink Comparator” which basically switches back and forth between two images so that any differences between the two become visible to the human eye, which is best when it comes to spotting movement or changes. It took Tombaugh almost a year of sifting through images before he found the first evidence of “Planet X”. In fact Pluto had, somewhat ironically, already been photographed in 1916 and 1909 without anybody realizing what they had seen.

Just for good measure, this is what Tombaugh saw-


Imagine having to look through pictures like these, day after day, week after week and month after month, until you finally see this tiny little speck that has moved a bit relative to some other tiny specks!

Right from the outset there was doubt as to whether Pluto really was the long sought after “Planet X” and the following estimates of its size and later its mass cast even more doubt on this. In fact in 1992 an analysis of data from Voyager 2’s 1989 flyby of Neptune gave a more accurate measurement of Neptune’s mass, thus proving that it was indeed all that was needed to explain Uranus’ orbit, so there was actually no need for a “Planet X” at all!

Now for some dry facts. Pluto takes 248 years to go once around the Sun so since it’s discovery nearly 85 years ago, it has only completed about a third of its orbit! Pluto’s orbit is also in 2:3 resonance with Neptune’s which means that every time Neptune makes three trips around the Sun, Pluto makes 2 and this has given the name “Plutoids” to a whole group of objects in the same neighbourhood with the same orbital resonance. Pluto’s orbit is also highly inclined. Most of the Solar System moves around the Sun in an almost flat disc, with only a 5 degree discrepancy above or below the Ecliptic, the disc the planets rotate in. Pluto’s orbit takes it 17 degrees above and below and is also highly eccentric, elongated, sometimes taking it inside the orbit of Neptune.

This is what all that looks like, a kind of aliens eye view-


This makes Pluto one of a group of objects out in the Kuiper belt, with eccentric and inclined orbits. The Kuiper belt, by the way, is an area of thousands, possibly millions of objects orbiting the Sun at a distance of about 30 to 55 AU.

So now we come to the really contentious part, is Pluto a planet? Ever since its discovery there has been some serious doubt about this, first with respect to it being the expected “Planet X”, and later as to whether or not it could be described as a planet at all. Pluto is first of all much smaller than its planetary “neighbours”, the ice giants Uranus and Neptune. In fact about the only thing it has in common with them is that, like Uranus, it rotates on its side with an axial tilt of 120 degrees, compared to Earth’s 23.5 degrees. It’s orbit is highly eccentric, (elongated), and is tilted wildly above and below the plane of the ecliptic. So Pluto has, in fact, almost nothing in common with what we now call the planets, but has a lot in common with its real family the “Kuiper Belt Objects” But all this is totally irrelevant to Pluto itself and it isn’t even the first time we have had this discussion, we’ve had it before with Ceres which was first designated a planet, then an asteroid and then in 2006 a Dwarf Planet. Like Pluto it too seems to have a rocky core with an ice mantle and the possibility of a liquid water ocean between the two, which means that they could possibly be habitable. I’ll get back to this exciting prospect when I write about Jupiter and it’s moon Europa at a later date.

So don’t feel sorry for Pluto because it lost its status as a planet. There are far cooler things to be than a planet and Pluto and the rest of its family are getting more and more exciting the more we discover about them. In fact I think I will have to write about some of these other things too, but maybe that should wait until New Horizons has paid them a visit….





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