This script accompanies a powerpoint presentation. It is intended for free presentations given for educational purposes. For other uses contact: John Davies, Royal Observatory, Blackford Hill, Edinburgh UK. 44-131-668-8100 jkd@roe.ac.uk John: Good evening, ladies and gentlemen, my name is Dr John Keith Davies and I'm here tonight to explain that Pluto is definitely a planet. Suzie: Good evening, ladies and gentlmen, my name is Suzie and I'm here tonight to explain that Pluto is definitely NOT a planet. J: Oh yes it is S: Oh no it isn't J: It is S It isn't..... J IS S: Now just a minute, this is silly, why don't you explain to me why you think Pluto is a planet? J OK, well let's start back in ancient history. Up until then there were only 5 known planets (6 if you count Earth), stretching from Mercury to Saturn. They had been known since prehistoric times as star-like objects that moved slowly across the sky from night to night. Because they moved they were called planets, or wanderers, and named after Gods. No- one had any idea what they really were. Then about 500 years ago Nicolaus Copernicus realised that planets went around the Sun (slide) and not long after Johannus Kepler worked out the basic laws of planetary motion. Isaac Newton and others refined the maths and no-one had any reason to think there were any more undiscovered planets until 1781 when astronomer William Herschel discovered a new one quite by accident. It was soon called Uranus and this is what it looks like (slide). For a while everything was fine until astronomers realised that their predictions of where Uranus should be found on the sky were getting less and less accurate. Uranus was slowly drifting out of position. Since the laws of planetary motion were well understood, it meant that there must be another large planet still further out whose gravity was pulling on Uranus and modifying its orbit. In a now classic story of astronomical detective work, English and French mathematicians called John Couch Adams and Urbain Le Verrier independently calculated the position of the missing planet and sent it their respective authorities. The missing was planet was eventually discovered, quite near the predicted position, from the Berlin observatory in 1846 It was called Neptune (slide) S: So where does Pluto come in? J After a few decades it seemed that there were still problems with the motion of Uranus and these suggested that there was another distant planet yet to be found. American millionaire Percival Lowell, (slide) who was an astronomy enthusiast, set out to find it. Lowell had a private observatory in Flagstaff, Arizona and it from here that he made his famous,if erroneous, observations of Canals on Mars. Once Lowell got interested in the problem of the missing planet he started a programme to search for what he called Planet X. To make along story short, the first attempts were unsucessful and Lowell died of a stroke in 1916, his planet finding ambitions unfullfilled. He was buried at his observatory (slide) in a tomb built to resemble the planet Saturn. After a long delay, caused by a dispute over Lowell's will, the Observatory's search for Planet X was restarted. A young astronomer called Clyde Tombaugh (slide) was taken on to do the observing. Tombaugh was an amatuer astronomer who had sent some sketches to the Lowell observatory and seemed keen and entusiastic, so they gave him the job. In 1931, after about a year of searching, Tombaugh found a slowly moving object in the constellation Gemini (slide) This was pretty much were Lowell had predicted Planet X would be found and after a short delay the new object was named Pluto, after the god of the Underworld. So there you have it, Pluto is the ninth planet S: Not so fast, There are some problems here. First of all Pluto has a very unusual orbit. Look at this diagram (slide).Within a few years of its discovery it was clear that Pluto crosses the orbit of Neptune every time it goes around the Sun. This is very unusual. What is more, Pluto's orbit is quite strongly tipped relative to the other planets. Real planets don't look like that, they go around the Sun in orbits that are almost circular and pretty much all in the same plane. In fact the only reason that Pluto doesn't crash into Neptune when their orbits cross is that the two orbits have a very special relationship. They have periods that are almost exact fractions of each other. Every time Neptune goes around the Sun three times, Pluto goes around twice. Pluto and Neptune have been missing each other by about the same amount ever since the solar sytem formed four and a half billion years ago. This special situation is called an orbital resonance. The fact that Pluto was in the 2:3 resonance was originally thought to be just a fluke. As early as 1931 some people were suggesting that Pluto was not a planet, but in fact that it was just a stray asteroid or a big comet. J: Its MUCH to big to be a comet, comets are small icy things, not planet sized objects. (slide Halley Nucleus). The nucleus of Halley's comet is only about 15km across. Its just a ball of ice and dust, a dirty snowball that boils when it get near the Sun. S: Well since you've brought size into it, isn't it true that Pluto was much fainter than Lowell predicted? This meant that it was rather small. In fact the estimated size of Pluto has got steadly smaller since it was discovered. Here's a graph (slide). Here are the predicted masses based on the problems with the motion of Uranus. This is how big they thought Pluto was when they found it. As long ago as the sixties comet expert Fred Whipple said that Pluto was not Lowell's planet X. In fact Whipple remarked that based on its brightness, Pluto was so small that its gravity could not disturb the orbit of Uranus even if it was made of SOLID GOLD. Then in the 70's Dale Cruikshank and others found that Pluto was covered in ice, This has been confirmed with spectroscopy (Slide). Here is a modern infrared spectrum of Pluto. By splitting up the light that arrives from an object, using exactly the sort of equipment we build here in the Astronomy Technology Centre, you can identify individual molecules in an astronomical objects. With this technique astronomers could determine the chemical composition of the surface. They found that Pluto has methane and nitrogen ice on its surface. This means that the surface is very reflective and so Pluto doesn't have to be very big to reflect a lot of light. So Pluto's estimated size shrank again. Finally in 1978 Robert Christy found that Pluto had a satellite (slide). He called the satellite Charon (SHAR- ON), which is mythologically correct and also commemorates the name of his wife, Sharlene (her friends call her SHAR). Charon, is quite big, and it goes around Pluto every 6 days or so. From the orbital period of the satellite you can estimate the mass of system and its pretty small, much less than any other planet. When a few years ago the Earth passed through the orbital plane of the Pluto-Charon system Pluto and Charon started to eclipse each other (slide), By observing during these eclipses it was possible to establish the true sizes of both objects and Pluto shrank still further. The latest estimates make it only 2300km in diameter. What is more, once astronomers knew the orbital period of Charon it was possible to calculate the mass of the two objects, and Pluto shrank again. At this rate its should have vanished completely by now (slide). J: Well thats just silly, when Pluto was discovered in the 1930's we didn't have these fantastic telescopes and people were bound to make a few mistakes. Ok, so Pluto is small, but its round, it has a satellite and it goes around the Sun. Its a planet, its just a small planet. S: Sorry but that won't do. Having a satellite doesn't make something a planet for two good reasons. Firstly Mercury and Venus don't have moons, and they are definately planets (slide Mercury + Venus). Secondly we now know that even small objects can have satellites. There are thousands of small rocky objects in a belt between Mars and Jupiter. We call this the asteroid belt and there are so many asteroids that astronomers ran out of mythological things to name them after. So they started naming them after places, musicians, scientists and a cat. They even named one after you (slide 9064). J: True, 9064 Johndavies is only about 6km across and even I wouldn't call it a planet. S: This is asteroid Ida, photographed fron the NASA Galileo spacecraft (slide). It is irregular, just a lump of rock a few 10's of kilometres long, and look (point) it has a tiny satellite called Dactyl. You're not going to tell me that Ida is a planet are you? J: Not exactly, but Dactyl is just a large boulder, its not a proper satellite. S: Well there are others. Using what is called Adaptive Optics its now possible to use ground based telescopes to take images just as sharp as can be done from the Hubble Space Telescope. These telescopes are bigger than the Hubble and using these techniques astronomers have found satellites, some of them quite big, around a number of asteroids (slide). There's even a movie! (movie). So having a satellite does not make something a planet. J: Well, OK but unlike asteroids, Pluto has an atmosphere. When Pluto passes in front of a star the star does not blink out suddenly, but fades slowly away (slide). This is due to a thin atmosphere above the surface. So there you are Pluto is round, has a satellite and it has an atmosphere. What else do you have to be to be planet? S: Well lets talk about that atmosphere for a moment. Pluto is pretty close to the Sun just now, almost as close at it ever gets, but its moving further away every year. This means its getting colder and colder. Within 20 years that so called atmosphere is going to collapse onto the surface and turn back into ice. That is why some people want to send a space mission there now (slide) to study this atmosphere before it freezes out onto the surface for the next 250 years. So Pluto is small, icy and its atmosphere turns on and off as it aproaches the Sun. That's not a planet, its a comet. By the way, while we're on the subject of comets, what about the Kuiper belt? J: What about it? S: Isn't it true that some comets come from a belt of objects just about where Pluto is? J: Yes, for quite a while people thought that comets came from a giant sphere of small icy objects which stretches half way to the nearest stars (slide). This is called the Oort Cloud, after a Dutch astronomer called Jan Oort who first suggested the idea. The theory is that comets are icy objects a few kilometres across, which spend almost all their time out in this Oort cloud, in deep freeze. From time to time one of these 'dirty snowballs' gets is orbit nudged slightly and falls in towards the Sun. Once they get near the Sun, say within a few 100 million kilometers, the ice starts to sublime and the snowball, or nucleus, develops a coma and tail. Most comets, like Hale-Bopp (slide), make a quite unexpected appearance, go around the Sun once then vanish back into the Oort cloud and are not seen again for thousands of years. A few of these comets, so the idea went, that happened to pass close to the giant planet Jupiter were captured by Jupiter's gravity. These comets were trapped into elliptical orbits (slide) that keep them in the inner solar system. Comets trapped like this re-appear regularly, every 10 years or so. These are called short period comets and they are pretty faint and hard to find. The thing was that as more and more of these short period comets, were found it was realised that there were too many to be explained by chance encounters with Jupiter. The chance of a comet crossing Jupiter's orbit at just the right time, and just the right height, to be captured by Jupiter's gravity is rather low. In fact it is too low to explain the number of known short period comets. There are just too many of these short period comets to be explained by chance captures of objects from the Oort Cloud. So, in the late 1980's, following on from ideas expressed earlier by a few other people, three Canadians called Martin Duncan, Tom Quinn and Scott Tremaine used high speed computers to track the paths of simulated comets through a simulated solar system. They found it was much easier to explain the number of short period comets if you assumed that they were coming from a disk, or belt, just outside Neptune. They called this hypothetical comet reservoir the 'Kuiper Belt' after a Dutch born, American naturalised astronomer called Gerard Kuiper (slide). This is Kuiper, and (point) here is his belt. Kuiper was a well known solar system astronomer who in 1951 had written a very detailed book chapter about the state of solar system scince as it was then understood. In this chapter he spoke about a trans-Neptunian comet belt. As it happens the name 'Kuiper Belt' is not entirely fair. The idea of a comet belt beyond Neptune had already been suggested by an Irish amateur astronomer called Kenneth Edgeworth (slide). Edgeworth published his ideas in 1943, and again in 1949, but no-one seems to have taken them seriously at the time and his work was largely forgotten for over 50 years. s: But this theoretical idea was eventually confirmed. A British astronomer called Dave Jewitt, who was by then working in the USA, and his student Jane Luu (slide) started a programme to look for distant solar sytem objects in 1988. As the technology of electronic imaging detectors was pretty crude then, they did not have much luck at first. However, after about four years they found something from this 2.2 metre telescope (slide) on Mauna Kea, in Hawaii. It was clear with a day or so that this new body was a smallish object orbiting the Sun outside Pluto (slide, QB1 detection). Was this a new planet then? J. No, it was obviously rather smaller than Pluto, about 1/6 the size, and was treated like a newly discovered asteroid. It was designated 1992 QB1, which is code name that tells about when during 1992 it was discovered. Jewitt and Luu wrote a paper in which they claimed that 1992 QB1 was the first member of the Kuiper Belt predicted by Duncan, Quinn and Tremaine. S: But Brian Marsden, who keeps track of asteroids and comets for the International Astronomical Union wasn't convinced. He hinted that maybe QB1 wasn't the first 'Kuiper Belt' object at all. He told the 'Boston Globe' newspaper that maybe Pluto was the first Kuiper Belt object, implying that Pluto was not a proper planet. Whats more, there was more to come wasn't there? J: Yes, within a few years several more Kuiper Belt objects were found and by now we know of several hundred. While quite a few of these are in circular orbits beyond Neptune (slide), pretty much what Edgeworth and Kuiper had predicted, others were in paths that crossed Neptune's orbit. These objects have only survived in these locations becuase of orbital resonances like Pluto's 3:2 resonance (slide). After a while people started calling the objects in these special resonant orbits 'Plutino's' or 'Little Plutos'. S; So now I've got you. There are dozens, probably hundreds of these Plutinos, made of the same icy stuff as Pluto and in very similar orbits. Although we can't be sure of their exact sizes, its becoming clear that several of them are similar in size to Charon, Pluto's moon (slide, relative sizes) . Its probably only a matter of time until one is found that is almost as big, or even bigger than Pluto. There is already one bigger than the biggest main belt asteroid and at least a couple have been found with satellites (slide). What is more if I take the orbital details of Pluto and stick them on a diagram like (slide, ei diagram) this only an expert could tell which one was Pluto. Here (point) is the 2:3 resonance and Pluto is in here somewhere with all the other Plutinos. Its just one of this bunch of essentially similar objects. We both know what these are, they are small icy bodies left over from when the Solar sytem was formed. Astronomers call them planetesimals. Things like this formed all over the solar system about four and half billion years ago. Most of them got incorporated into the planets. A few got left behind in the asteroid belt. Some were ejected by the gravity of the forming giant planet Jupiter to form the Oort cloud of comets. Others, which formed in the Uranus-Neptune region, got parked in this thing we now call the Edgeworth-Kuiper Belt. Fred Whipple called it a comet ring (slide) and drew it in about 1964, complete with Pluto. Out here, beyond Neptune ,the volume of space is so big that these objects have not had time to collide into each other and grow into a big planet. They are the icy rubble left over once the solar system was formed. Pluto is just the biggest Kuiper belt object and it should never have been called a planet in the first place. J: Well it did get called a planet and it should remain so, you can't just go around 'demoting' planets. S. Actually you can. When the first asteroid, Ceres, was discovered in 1801 it was called the eighth planet (remember Neptune had not then been found). The next few asteroids discovered were called planets too. They got proper planet-like mythological names and symbols (slide). Even in the 1800's astronomy text books referred to the 11 main planets. It was only when astronomers realised that the main belt asteroids were bits of rock that never got assembled into a proper planet, that they stopped calling them planets. The situation with Pluto and the Kuiper belt is just the same. Its time to put right a mistake that was made 70 years ago J . No No No. Astronomy is full of rich traditions. We still use the idea of constellations (slide) even though they mean nothing about how the stars are distrubuted through space in real terms. We also still call the basaltic lava plains on the Moon 'seas' even though there has never been any water there. Astronauts were wearing boots, not flippers, when they walked on the Moon (slide). Pluto's been a planet since the last century, let's leave it that way. S. Perhaps it would help if we tried to be scientific about this. What exactly is the definition of a planet? J Well now you mention it, there isn't an official one, planets were named before there were committees of elderly astronomers to make up rules. I suppose a planet is something that goes around a star (slide). S Well Pluto passes that test at least, and that rules out some of the larger satellites like Jupiter's moon Io (Slide). Io is pretty big and it only seems small because it goes around Jupiter. In fact our own Moon would make a pretty good planet if you took it away from the Earth and let it float feely around the solar system. But we also have to distinguish between planets and brown dwarf stars. In the last few years a number of very small, and faint, stars have been found. Some of these are in binary systems so they go around a star, does that make them planets? J. No, lets rule out objects which generate heat (slide). S Fine, but lets not forget that Jupiter (slide) emits more heat than it recieves from the Sun. This is trapped heat released by gravity as Jupiter contracted, so lets say that planets do not, and never did, generate heat by nuclear reactions. That way we can rule out very old, dead stars from contention. J and planets are round (slide). S Well, fairly round, most planets are oblate since rotation gives them a bulge at the equator, but lets say that they are objects which have collapsed to near spherical shape under their own gravity. The trouble is that round, non nuclear burning objects in orbit around the Sun includes not just the traditional planets but several main belt asteroids and some of the larger Kuiper belt objects. In fact, its very hard to make a definition of a planet that allows the solar system to have nine of them. J. Well lets make planets objects that are bigger than 1000 km to get rid of the asteroids (slide) S. That works, but its very arbitrary. Why use 1000 kilometers? Why not use 1000 miles or 1000 cubits or 1000 furlongs? J Ok, but times getting short, The International Astronomical Union has stated that it has never made a ruling, or considered a motion to remove Pluto's status as a planet. I suggest we ask the audience for a vote, who thinks Pluto is a planet? (slide- is Pluto a planet) (counts) S: and who doesn't. Slide. (Counts starting at opposite side from JKD). Hey, you voted twice. J: Yes, because actually it doesn't matter what we call Pluto. It, and its icy cousins out there in the Edgeworth- Kuiper Belt, are really interesting whatever you call them. They tell us an amazing amount about the formation and evolution of the solar sytem. If you want to study small, icy worlds, like Neptune's moon Triton (slide), or Pluto, Charon and the big Edgeworth-Kuiper belt objects. Then you can think of Pluto as a planet. If on the other hand you want to study how the big planets were built up when the solar system formed, and to model how planetessimal oribts evolved over billions of years (slide) then you should regard Pluto as just one of an ensemble of objects in the trans-Neptunian region. Some people have even suggested giving Pluto a sort of dual nationality, planet and asteroid. All asteroids get numbers, but everytime a round number comes up astronomers try to think of a special name. Here are some examples. How about a nice round number like 10,000 Pluto (although its too late for that number now) or 100,000 Pluto S. Then if you are changing the rules I demand a recount. J OK, bit its getting late and I can't be doing with counting hands again. Lets do what do on the radio when they put a question to the audience. When I ask the question again I want everyone who thinks that Pluto is a planet to clap really loudly, and those who disagree to cheer. Then, since this is the Astronomy Technology Centre, we'll use this high tech clapometer (slide) designed and built here to decide the result. So, Ladies and Gentlemen, (slide Yes=Clap, No=Cheer) is Pluto a planet? Appluase (hopefully) J: Well, that's the best round of applause we've ever had for a lecture here at the observatory, thanks very much indeed! (slide) S. So it goodnight from Me J and its goodnight from her J+S Goodnight!!!!!!!!!!!! (slide) (Thunderous applause, flowers thrown from the floor, lucative film and Tv contract to follow.)