Date: July 5, 2010
Title: Another Demotion for Pluto? Or is It About to Become King of the Dwarfs? Part 1
Podcaster: Marc West
Links: http://www.mrscienceshow.com
http://www.mrscienceshow.com/2010/05/ep-128-another-demotion-for-pluto.html
Description: Already demoted from the exalted planet club, Pluto could be joined by up to 50 other objects in the ever-expanding “dwarf planet” club if the new definitions of dwarf planet, recently proposed by Australian scientists Charles Lineweaver and Marc Norman, are accepted by the International Astronomical Union. On the other hand, perhaps you would rather regard Pluto as the leading player in the astronomical second division. Rather than being the smallest of the planets, Pluto is set to become the charismatic king of the dwarfs. Marc West had a fascinating chat with Dr Lineweaver about how they derived these new numbers, and also about their work defining the shape and mass of other astronomical bodies.
Part 2 of this podcast will be aired on July 26.
Bio: Marc West was a University Medallist in Chemistry at Sydney University, completed a Graduate Diploma in Science Communication at ANU and a Masters of Operations Research and Statistics at UNSW. Having grown up in Sydney, he ventured to Canberra and then London to be editor of Plus Magazine, and now works in Operations Research back in Sydney. Marc has written freelance for a number of magazines and newspapers, including G Magazine, The Canberra Times, The Helix and All Out Cricket Magazine, his article Political Music was published as one of top 50 science blogs of 2008 in The Open Laboratory 2008: The Best Science Writing on Blogs, whilst A sorry saga – the crumbling cookie made the 2009 version. Marc set up the Mr Science Show when on a trip to China, frequently talks on radio with The Diffusion Science Radio Show on 2SER, and co-founded The Beer Drinking Scientists podcast.
Today’s sponsor: “Between the Hayabusa homecoming from Itokawa and the Rosetta flyby of asteroid Lutetia, 13 June until 10 July 2010, this episode of ‘365 Days of Astronomy’ is sponsored anonymously and dedicated to the memory of Annie Cameron, designer of the Tryphena Sun Wheel, Great Barrier Island, New Zealand, a project that remains to be started.”
Transcript:
365 Days of Astronomy
Marc West
Another demotion for Pluto? Or is he about to become king of the dwarfs?
PART 1 (PART 2 will be heard on July 25)
Marc West: MW
Charles Lineweaver: CL
MW: Greetings and welcome to the 365 Days of Astronomy Podcast, my name is Marc West and I usually broadcast as part of the Mr Science Show , the podcast where science meets pop culture. You can find our website at www.mrscienceshow.com. We like to look at that mysterious grey area between science and pop culture, that area where you’re really not sure if you’re cool. One of the topics that we always like to tackle is astronomy,y and space. Our website again, www.mrscienceshow.com.
And this week we’re talking about another demotion for everyone’s favourite astronomical body, Pluto. Not that long ago, Pluto was a fully paid-up member of the planet club, however a few years ago, the International Astronomical Union decided to demote Pluto to a new class of astronomical body called dwarf-planet. This week I spoke to Dr Charles Lineweaver, a senior fellow at the planetary science institute at the ANU in canberra – he has recently studied the definition of dwarf planet, and by looking up at the stars and also at some fundamental physics, he has come up with a new definition of dwarf planet that may mean another 50 dwarf planets, so Pluto will hardly be an individual anymore. I started by asking Dr Lineweaver about the controversy surrounding the definition of dwarf planet and Pluto.
THIS IS THE FIRST PART OF AN INTERVIEW WITH DR LINEWEAVER – THE SECOND PART CAN BE HEARD ON THE JULY 25 EDITION OF 365 DAYS OF ASTRONOMY
MW: I recently came across your fantastically titled report “The potato radius, a lower minimum size for dwarf planets” and there’s been some controversy about dwarf planets and what is a planet, and Pluto was famously demoted from planet to dwarf planet, what is this debate about?
CL: Well, I guess we scientists, we look up into the sky and see things. In 2006, the IAU, astronomers from all over the world who meet every once every 3 years who give names to things and classify things, and in 2006 there was a big debate about Pluto because a guy at Caltec named Mike Brown discovered an object named Eris which is bigger than Pluto, and not too much further away far from the Sun than Pluto – so, was this going to be a planet or something else? So instead of calling it a planet, they decided to demote Pluto to something called a dwarf-planet. Now this is not unprecedented as about 200 years ago, the largest asteroids in the asteroid belt were found – Ceres for example. And people said “woah, we’ve found another planet” – but then as they found more and more asteroids, they said “well this isn’t a planet, its one of many bodies at the same orbit and so we’ll call them asteroids and not planets”.
Now that happened 200 years ago and people have forgotten about that but its very analogous to whats happening to Pluto right now. You find the largest body in a certain region and call it a planet, and then say “wait a minute” there are a whole bunch of other objects in that region as well. And thats what happened in 2006.
Now the paper we just published is not about the boundary between planets and dwarf planets, but the lower boundary of dwarf planets – how big are you if you want to be a dwarf planet. Essentially this comes down to how big you need to be to be a ball – that is called hydrostatic equilibrium. And thats what we did. We analysed that boundary and called it the potato radii as essentially objects smaller than the potato radius are shaped like potatoes and objects larger are shaped like spheres. And so this boundary / transition from potatoes to sphere, I just called it the “potato radius” as I thought was was cool and catchy!
MW: It is very catchy, I love it!
And how did you go about defining the potato radius, you can look up into the sky and see potato shapes, but you’ve gone into the maths of it to derive when something becomes a sphere.
CL: Well, we did both. The initial thing we did was to look at the icy moons of the gas planets, so the moons of Saturn and Uranus, Jupiter and Neptune, and look at specifically the ones that have a radius 100-400 km and see how round they look. Most of them had already had pictures taken of them in which they are resolved as a disk. You need to see that disk – if the disk looks like a potato, then its not in hydrostatic equilibrium and should not be called a dwarf planet.
This is a little bit of a misnomer as Im talking about moons. In terms of hydrostatic equilibrium, they should set the scale, whether they’re orbiting a planet or not, so thats why they can be used as a calibration of the potato radius. We found out that if they have a radii of over 200 km, they’re round, they’re bald and if they’re smaller then they look like potatoes. So this tells us the potato radius should be about 200 km
And so based on this quick and dirty look at these things, we said the PR should be about 200km – that was one part of the study. The other part of the study was to look at the physics – and so we figured out what the pressure if as a function of depth, and the density of these things and hose strong they are – they are all factors that go into the calculation – and lo and behold the calculation came out with about 200 km as well. And so here we have the icy moons telling us thats the potato radius – 200 km – we have the calculations telling us 200 km and then my coauthor pointed out that “you know what, the radii thats now being used is twice this big.” And I said “oh is that right” and i talked to a colleague of mine whose job it is to monitor trans-Neptunian objects, and she is working with Mike Brown who discovered this object larger than Pluto, and who has discovered a lot of other objects 100, 200, 400 km in diameter, and it turns out if this suggestion were making is taken seriously, then 50 new bodies already known would be classified as dwarf planets.
And so if this is the case – how many dwarf planets are known now? Only 5. And so I said “that’s inconsistent with the IAUs classification system, if we’re right about the potato radius”
And so what we’ve found is that the classification radius should be half as big as that being used now.
MW: “Its a fall from grace for Pluto, isn’t it, from fully fledged paid-up member of the planet club!”
CL: Well you know, there are 2 ways to look at it. THe first is that the classification of a dwarf planet is that of an elite club and as long as there are few members, its elite. Its kinda like the qantas club. You’re a member and think you’re elite and boom the price comes down and there are lots of members!
MW: Its a great analogy! I always thought it would be great to be a member of the qantas club, until they invited me and then I thought do I want to be a member of a club that invites me!
CL: If they’re making me a member they’re making everyone else a member too!
MW: Thats right!
CL: Whats the quality of the food going to be if they’re letting everyone in! But thats one way of looking at it, but there another way to look at it. You could think Pluto is kinda lonely out there with 4 other dwarf planets, but now its got 50 new friends its got a lot of other things to do. You can lookout it that way too. These emotions are really inaccurate ways of understanding the solar system.
MW: So would all these new dwarf planets be trans-Neptunian objects? Are they all in the far reaches of the solar system?
CL: Yes yes
MW: And do you think there would be a better way to classify DP or is this just the way it is?
CL: Well, I think its just the way it is, but its not something that has been accepted yet. I should mention that the trans Neptunian objects that have been discovered, their sizes are not well known, we just have estimates of them, because as far as our telescopes are concerned, we can see them as points of light and we estimate their sizes from how bright they are, not how big they are in our telescopes as essentially they are just points. So we can get estimates of how big they are, and using those estimates, classify them. And thats what Im talking about when I say that theory are 50 known objects who have a radius of over 200km rather than 400km.
MUSIC:
MW: Well thats all we have time for in todays 365 days of astronomy podcast. If you liked what you heard today then of course you should keep listening to the 365 days of astronomy podcast, but you should also get over to my podcast at www.mrscienceshow.com. You can leave comments on any of the shows or blog posts we’ve got up, you can engage with me on twitter, and you ca also become a facebook fan.
I hope you enjoyed todays show, my name is Marc West and I hope to see you soon on the Mr Science Show, the podcast where science meets pop culture, and also soon on the 365 days of astronomy podcast.
THIS IS THE FIRST PART OF AN INTERVIEW WITH DR LINEWEAVER – THE SECOND PART CAN BE HEARD ON THE JULY 25 EDITION OF 365 DAYS OF ASTRONOMY
End of podcast:
365 Days of Astronomy
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You are being very unfair and doing a disservice to the public by portraying one side in an ongoing debate as reality when this is not the case. The 2006 demotion of Pluto, highly controversial, was made by only 424 out of 10,000 IAU members (four percent) and was done through a violation of the group’s own bylaws. It was immediately opposed in a formal petition by hundreds of professional astronomers led by New Horizons Principal Investigator Dr. Alan Stern. You can find that petition here: http://www.ipetitions.com/petition/planetprotest/
If you cite Mike Brown, an ardent supporter of the view that dwarf planets are not planets, it is only fair for you to cite someone representing the other side, such as Dr. Alan Stern or Dr. Mark Sykes.
Adding more dwarf planets is in no way another demotion for Pluto. The reason is that in spite of the controversial IAU decision, dwarf planets are planets too. Dr. Alan Stern, who coined the term, intended it to refer to a subclass of planets large enough to be in hydrostatic equilibrium (pulled into a round shape by their own gravity) but not large enough to gravitationally dominate their orbits. He never intended dwarf planets to be designated as not planets at all. And he said he anticipates there being hundreds of these small planets in our solar system.
Only four percent of the IAU voted on this, and most are not planetary scientists. Their decision was immediately opposed in a formal petition by hundreds of professional astronomers led by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto. Stern and like-minded scientists favor a broader planet definition that includes any non-self-luminous spheroidal body in orbit around a star. The spherical part is important because objects become spherical when they attain a state known as hydrostatic equilibrium, meaning they are large enough for their own gravity to pull them into a round shape. This is a characteristic of planets and not of shapeless asteroids and Kuiper Belt Objects. Pluto meets this criterion and is therefore a planet. Under this definition, our solar system has 13 planets and counting: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris.
Pluto was never demoted. From Pluto’s perspective, the opportunity to leave the planet club may be the best thing that ever happened to it. What would you rather be – a small-time landowner in a large but sparsely populated country [the planet club], or king of a small, densely-populated one [the kuiper belt]? Assuming that just because we live on a planet, all solar system bodies should aspire to be planets is an extremely planetocentric way of looking at things.
The group of scientists that Laurel refers are vocal, but very small in number.
It’s interesting to note that very few, if any, astronomers and scientists outside of the United States disagree with Pluto’s current status of dwarf planet. Why is this?
In the past some have dismissed the IAU as not being in the position to rule on Pluto’s status. If not them, who?
It is true, as Laurel points out, that only 4% of the IAU voted in 2006, but it was the majority of that 4% voted for its ‘demotion’. The other 96% may just have likely voted along similar lines.
And if there is such a groundswell of support for Pluto to be reinstated, why was there no motion at the last assembly in 2009? Answer, there wasn’t enough support.
I think it’s time to move on from the ‘debate’ and concentrate on more interesting aspects of planetary science, than nonclementure.
As Adrian points out, I’d rather be the mayor of a rural town, then a nobody in the city.
Laurel seems to be very passionate about Pluto. Why don’t you do a podcast on Pluto for the site?
Also I don’t think Pluto will be the king of the Kuiper Belt. There are KBO that are larger than Pluto (we have already found one). At best Pluto will be the most well known KBO.
@Patrick: “Also I don’t think Pluto will be the king of the Kuiper Belt. There are KBO that are larger than Pluto (we have already found one).”
You wouldn’t expect the King of the Kuiper Belt to be the largest Kuiper Belt object, any more than the greatest warrior on an ancient battlefield is necessarily the strongest man. There are other attributes that come into play, like grasp of tactics.
We can set up the metaphor any way we want, so let’s decide that the two attributes which make a kuiper belt object fit for kingship are (a) size, and (b) proximity to the Sunward side of the belt (analagous to the social ladder). On balance of the two, Pluto is the clear winner.
@Adrian: LOL! Yes, but until we have an official vote on the “Official definition of KOB kingship”by some official astronomy group, there with be ambiguity.
I like your definition, but by that definition I think Neptune’s Moon Triton wins kingship (if it actually is a captured KBO).[Since Pluto is sometimes closer to the Sun than Neptune, one could argue the Sunward side of the Kuiper Belt encompasses the orbit of Neptune. But Neptune is closer to the Sun for a bigger percentage of the time.]
So we will have to add a third attribute: (c) has the Sun located at one of the foci of its elliptical orbit (i.e. orbits the Sun, not a planet).
Captured warriors very rarely get to be king. 🙂
I would be happy to do a podcast on Pluto once I learn how podcasts are done. As of now, I am not sure of the specifics; however, I would appreciate it if the organizer of this project sends me instructions via email.
So far, only one Kuiper Belt Object larger than Pluto has been found, and that is Eris. Even if there are many, that does not preclude the ones large enough to be rounded by their own gravity to be dually classed as both small (dwarf) planets and KBOs.
There is no matter of assuming that “all solar system objects aspire to be planets.” The reason for classifying Pluto and all spherical objects orbiting the Sun (and stars in general) as planets is the use of a geophysical definition in which any object that orbits a star and is in hydrostatic equilibrium is a planet. The different types of planets can be distinguished by use of subcategories. Why do some insist that if an object is not a terrestrial or a jovian, it is not a planet? Maybe there are just more types of planets than we previously thought.
It is absolutely not time to “move on” from this debate, which remains just that, a debate with two sides supported by roughly equal numbers of astronomers. There is no scientific basis for assuming the 96 percent of the IAU who did not vote in 2006 would have voted for demotion. In fact, most of the 2,500 members who initially attended the 2006 General Assmbly and left early did so under the assumption that a different resolution, one that added Ceres, Pluto, Charon, and Eris, would be voted on on the General Assembly floor. They had no way of knowing that a small, organized group would quickly throw together an alternate resolution in violation of the IAU’s own bylaws, under which it is illegal to put a resolution to the floor of the General Assembly without first vetting it by the appropriate committee. Most who voted saw that resolution only on the morning of the vote. No absentee voting was permitted, so if one was not in that particular room on that particular day, he or she could not vote.
The statement that it is largely Americans who oppose the demotion of Pluto is a fallacy, especially when one counts astronomers who are not members of the IAU and the large number of amateur astronomers who take part in clubs and forums on the Internet. I have personally heard from such people from all over the world opposing the IAU definition. These are people who are very knowledgable about and vested in astronomy, not just members of the general public, and they make very rational, scientific arguments. It is also true that most of the professional astronomers who oppose the demotion are planetary scientists, and a large number of planetary scientists are American. This is due to the fact that the US, through NASA, has led the world in exploration of the solar system’s planets and moons.
It is not true that “only a small number of scientists” oppose the IAU decision. What is true is, many have given up on the IAU altogether. The reason there was no motion at the 2009 General Assembly to reopen the debate is that planetary scientists petitioned the IAU leadership in advance to do just this at last year’s General Assembly. The IAU leadership adamantly refused–without even putting the issue to a vote of all members. As a result, the group of scientists who asked for a reopening of this issue boycotted the General Assembly entirely. Discussions continue about founding a new, alternate planetary science organization to better address such issues.
The fact that the IAU leadership refuses to reopen the debate shows an underlying insecurity with the 2006 decision. If one is certain about one’s position, why fear more discussion? Interestingly, both the American Geophysical Union and the European Geophysical Union continue to conduct discussions and debates on what is a planet. Maybe, at least for the next five years, until New Horizons gets to Pluto, the best thing to do is acknowledge this as an ongoing debate with two legitimate sides. Even Neil de Grasse Tyson has come to this conclusion.
It seems to me that everyone is comfortable with the term “Dwarf Planet” – even Lauren is. I don’t think anyone disputes this. Anyway, part 2 out today (26th) so expect more comments of a similar vein.