Alan Stern: If an august body of scientists got together and all voted unanimously that the sky is now green, that wouldn’t make it so, because voting is irrelevant in science.
Planetary scientist Alan Stern is principal investigator for the New Horizons space mission now on route to Pluto. Dr. Stern disagrees with the International Astronomical Union, a body of 10,000 professional astronomers, who voted in 2006 to demote Pluto from planet status.
Alan Stern: When I was a kid – and I think most of your listeners, as children learn this – the solar system consisted of four rocky, terrestrial planets, four gas giants, and that little misfit Pluto. But with the advance of technology, we’ve discovered actually that Pluto is just the first and the brightest of a large population of small planets called dwarf planets that have been discovered since the 1990s. In fact it’s thought that there may be as many as a thousand Pluto-like planets in the solar system. So in fact, the whole equation has been turned upside down.
Stern believes strongly that these small bodies should be called planets.
Alan Stern: In fact, Pluto, and its cohorts, are planets. They have all the attributes of planets. Let me give you some examples. They have cores. They have geology. They have seasons and atmospheres. They have clouds. They have polar caps in many cases. They have moons. And I can’t think of a single distinguishing characteristic that would set apart Pluto and other things that you’d call a planet, other than its size. So I like to say, a Chihuahua is still a dog.
Dr. Stern talked more about the New Horizons mission, which he described as “a small, robotic space craft that’s making its way across the entire expanse of the solar system to explore Pluto and the Kuiper belt in 2015.”
Alan Stern: We launched New Horizons to explore Pluto and the Kuiper belt in January of 2006. It was the fastest spacecraft ever launched, and it has been making a beeline across the solar system for Pluto. The spacecraft is very healthy, but it still has five and a half years to go until we begin the Pluto encounter. It’s a very large solar system. So we travel about a million kilometers per day. And it takes about 3,500 days to reach Pluto.
Stern invoked the name of the famous astronomer Galileo when explaining why he thinks Pluto should retain its planet status.
Alan Stern: Science works through a process of successive approximation. Our whole view of the solar system, its architecture, was turned upside in the 1990s when we discovered this plethora of Pluto-like planets. The fact is that it’s our larger Earth that’s the misfit. Well, that’s jarring. And for some people, they just couldn’t handle it. Some scientists said, well, that’s too many planets. I don’t think I can remember all of the names of those, so we just have to stop. We have to find some way of limiting the number of planets. I don’t like that way of approaching science, and many don’t.
Stern speculated that a similar situation could have arisen during Galileo’s time.
Alan Stern: This is the 400th year of Galileo’s early telescope. I imagine that there must have been at the time someone in the early 17th century said, ‘You know, all those uncountable stars the new telescopes can see. We’re not going to be able to deal with those. So we won’t call those stars at all.” But in fact they are stars. And in fact, Pluto, and its cohorts, are planets. They have all the attributes of planets.
Some day when an Earth-mass body is found orbiting a gas giant planet, what will we call it? I don’t think “planet”, “double planet”, or “moon” for that matter will ever have a perfect definition that makes every object fall neatly into proper place.
I still think the IAU’s current definition is reasonable. Pluto’s demotion can be thought of as an increase in our understanding of how dominant bodies (IAU planets) evolve in a star system. If Pluto was removed from our solar system, gravitationally the other planets (and most other bodies) would not even notice. Mercury being so close to the Sun perturbs far more comets than Pluto every will.
So does a planet need to orbit a star? Does it need to dominate its orbit? All of it is somewhat arbitrary. Even spherical moons are defined by “where they are” and “what they do not dominate (the planet they orbit).”
– Kevin Heider
If an Earth-mass body is found orbiting a giant planet, it would be a secondary planet, as are the spherical moons of the solar system’s primary planets. The IAU’s definition is not reasonable because it says dwarf planets aren’t planets at all and because it defines objects solely by where they are while ignoring what they are. If Earth were in Pluto’s orbit, it would not clear that orbit and therefore would not be considered a planet either. Plus, we have already found exoplanet systems where two giant planets orbit their star in a 3:2 resonance like Neptune and Pluto. According to the IAU definition, neither of these could be considered planets. The IAU definition also precludes any binary planet systems since by definition the two planets in a binary do not clear their orbits of one another. Not only is the IAU definition not reasonable–to the point that even many dynamicists reject it–the IAU has shown its complete inflexibility on the issue by refusing to allow any additional debate on this topic. If they are really so sure they have it right, why are they afraid to reopen the discussion so that this time, more than four percent of members can take part?
True, some exo-planets are is resonance, but a lot of exo-planet simulations also seem to show that like the Solar System, most of these systems are full. This means that there is not enough room to drop in another large body without making the system unstable. Removing Pluto from our solar system would not cause a noticeable gap to open up and the 8 dominant planets would not behave differently.
I do not think the IAU’s definition is perfect. But I do think having 3 classes of objects “planets, dwarfs, and asteroids (belt objects)†is not a bad way to go at this time. I don’t think the IAU should bother with the definition until 2018, when we have had a few years to digest Dawn & New Horizons, not to mention any other new knowledge on the solar system and exo-systems. Certainly the general public does not care that we have over 225,000 “belt objects†with known orbits. Large dwarf planet candidate 2007 OR10 (nickname: Snow White) was just recently assigned as Minor Planet #225088.
– Kevin Heider
cool
Kevin, in the grand scheme of things, I don’t suppose it really matters just how we class Pluto.
However, within our own context, *how* we make scientific decisions matters very much indeed. And in that context, I agree with Dr. Stern that the reclassification of Pluto by a tiny number of scientists voting is a mockery of science.
The only possible scientific basis I can see for even considering reclassifying Pluto was its not clearing its orbit. Could it be possible tiny objects are sweeping along behind Earth in its orbit, perhaps truly tiny — say a few inches across — distant enough we can’t detect them? If so, and if we learn that they’re there, wouldn’t that suggest “promoting” Pluto again?
And why won’t the IAU allow any further discussion with most of the members present? That smacks of legislative tricks we see in our own Congress and other legislative bodies.
Further, why was Pluto’s status of such apparently burning importance to a handful of scientists?
But the main point science does not operate by decree or vote. Except, for instance, during the Inquisition.
The idea behind the IAU definition is to clear the orbit or control the orbit (of similarly sized objects). In this case there are 6 scientifically spherical objects (based on Spitzer diameter estimates) in a 2:3 resonance with Neptune. Objects in the 2:3 resonance are known as Plutinos. These 6 spherical objects in the 2:3 resonance are: Pluto, 90482 Orcus, 28978 Ixion, (84922) 2003 VS2, 38628 Huya, and (208996) 2003 AZ84.
Any definition can be nitpicked. It would also be weird to call Pluto a planet based on geophysical characteristics and then exclude Triton, Titan, Europa, etc.
I still prefer to classify planets as either special (the 8 major dynamical bodies), or as a very inclusive group including spherical secondary planets. In the latter case we would already have about 70+ planets in the Solar System. We would have 8 primaries, ~18 secondaries, and 40-60 likely dwarfs.
– Kevin Heider
I don’t think we have enough information to conclude that most exoplanet systems are full. Simulations are one thing, but have we seen the entirety of even one exoplanet system? We’re seeing the most massive planets and in places where we didn’t think these could exist. The IAU definition still has the same problem of defining two identical objects differently based on where they are. Put Mercury where Pluto is, and it would not be a planet. Put Pluto where Mercury is, and it would be a planet. This is problematic. Ultimately, classification does not depend on the IAU. If they take no action, by default, other scientists and groups can create and use their own definitions. Waiting until 2018 is ridiculous. At least let’s keep the debate active with both views regarded as legitimate. By 2015, we should have enough data from Dawn and New Horizons.
There is also an intermediate class of objects almost in hydrostatic equilbrium but not quite there, for example Pallas and Vesta. These are very unlike asteroids, which are mostly rubble piles. We need an additional classifiction for these objects as well.
Indeed the demotion of Pluto was an unfortunate maneuver which added little value, drew undue attention by the media, and confused the general public. It was certainly not the finest hour for the science of astronomy. It does however illustrate that we still have a long way to go in our understanding of the solar system. The word planet comes from the Greek meaning “wanderer”. Pluto certainly qualifies as that! Whether or not there is an adjective modifying the noun is not terribly important. However, the tendency of people to take sides on the matter can be a great source of entertainment.
Thank you for your comment! Interesting perspective …
Deborah
Nice discussion and yes interesting.
I’ve often wondered whether the IAU didn’t save themselves a lot of time & hassle by simply saying that a planet is a planet if its aphelion is less than 50AU. More than 50AU its an TPO, KBO whatever!
I’ve always been very fond of Pluto’s status of a planet, like in… after Neptune there’s this tiny rocky frigid and silent world in a sea of spacious blackness… has always inspired my imagination, Gustav Holst could have written a set for this one as well. I guess oversized rocks deep space far can be as captivating to our hearts as the Disney dog.:)
I am thankful for the information.Could you please give information about pluto as aplanet?
Picking Galileo Galilei out of so many other more or less well-known scientists is not very scientific either, as is arguing by use of authority (which Galilei, bye the way, were among the first real scientists to pursue the ending of), and speculating about what this or that person might have thought, said or acted like is, exactly what is says, only speculation. It contributes nothing significant to this discourse, and it only helps to undermine the seeming intention of the author to underscore the importance of being a true scientist by not letting any heartfelt issues inflict on the (more rational?) categorization of the part of nature that is under inspection. As it turns out, it only displays his own reluctance to accept the choice of the majority of people whose area of expertise this is and whose say in what names they give the objects they work with should prevail. And yes, their reasons may be heartfelt too. But it can also be argued, as seem to have been the case throughout history, that each time science stumbles upon a large range of objects, the reasonable way to deal with them seems to make divisions, sub-categories or whatever in order to achieve a higher degree of nuance, which in turn adds to the accuracy with which one may refer to these objects unambigously. When Alan Stern says that “a Chihuahua is still a dog” this seems to suggest that should one ever wish to purchase a dog, this level of nuance is sufficient to meet all the needs that a future dog-owner may have regarding the properties of the dog s/he wants, i.e. size, aestethics, etc. as well as degree of maintainance required; a somewhat long and rather unprecise way of describing the kind of dog one is considering. The name Chihuahua implies that we are talking about at least a small kind of dog, not a grand danoir. In precisely the same way the category “dwarf planet” implies a certain kind of object which, indeed, is rather different in size, mass, chemical constituents, etc. than, say, Jupiter. In short, Alan Stern only seems to show his own lack of (a need of) a better accuracy when talking qualitatively about the objects of his research, but, then again, in his own un-nuanced way of looking at the world, an un-nuanced scientist is also a scientist.
To JM: You state, regarding Stern, “As it turns out, it only displays his own reluctance to accept the choice of the majority of people whose area of expertise this is and whose say in what names they give the objects they work with should prevail.”
But the IAU decision is NOT the choice of a majority of planetary scientists. Nor is it the choice of a majority of IAU members. It is the choice of 424 people in a particular room on a particular day that has been opposed formally by just as many scientists. The IAU view is only one view in an ongoing debate, and the majority of those who voted for it do not have expertise in planetary science–they study other areas of astronomy such as galaxies and cosmology.
You also say, “The name Chihuahua implies that we are talking about at least a small kind of dog, not a grand danoir. In precisely the same way the category “dwarf planet” implies a certain kind of object which, indeed, is rather different in size, mass, chemical constituents, etc. than, say, Jupiter.”
This analogy is flawed. Stern’s point is not that all dogs must be like chihuahuas, but that in spite of their small size, chihuahuas are still a subclass of dog. In contrast, the IAU definition very specifically states that dwarf planets are NOT a subclass of planets, in fact are not planets at all. If this one issue were changed and dwarf planets brought back into the broad umbrella of planets, much of the controversy would dissipate.
To: Laurel Kornfeld
Thank you for you reply which made two points clear to me:
I seem to have had too much confidence to the vote having been made under circumstances where everyone professionally concerned with the subject would have had the opportunity to elect how these objects are to be classified. So by my own statement, and if your numbers are correct, which I shall have to check by myself, the process does not seem to have been ideally carried out.
Another point that I did not make entirely clear to myself, is that Alan Stern would have the category dwarf planet be a subset of the set of planets (which thus requires another definition of what planet implies).
I want to elaborate more on my view on this, but, first, I need to clarify the point I was trying to make in the second statement that you quote from me. Paraphrasing this, you turn the argument upside down: I am saying that “Chihuahua” and “Grand Danoir” are labels given to certain, well-defined subsets (here defining race) of the larger set of the species dog. This means that all chihuahuas are dogs, but not, as you state, that all dogs ‘should’ (or, rather, are?) chihuahuas.
Regardless of this, and I think this was your point, it is true that it is not correct to draw an analogy between a set with a certain subset and then what consitutes two totally separate sets (planets and dwarf planets). My point, however, was not this, but to make clear my position that Alan Stern seems to have no problem in having to juggle with all the elements in one (to me not very unambigous) set (the set of all planets, including what is now called dwarf planets) instead of wanting to make the categorization more nuanced so as to avoid having to specify a lot of details in any discussion of the phenomenon in order to make clear, more specifically, what kind of planet one is refering to.
To return, then, to what you mention is Alan Stern’s position: that we should have a set, called planets, with a subset, suitably labeled, which account for what is now officially called dwarf planets.
But I can not see that this makes it a more elegant solution to the problem of classification than what has already been elected (disregarding any protest one may have against the process) the best one. I do not claim to add anything new to the discussion of what the definition of a planet should be, but it is clear that the current one precludes a subset with the properties that are shared by the dwarf planets. To me, then, the elegance lies in the fact that the definition also exludes relatively smaller objects like asteroids of which there are presumably millions in our Solar system alone. It makes our known number of planets smaller but simpler (which arguably has some significant educational value as well) to handle. But it also makes a separate set of objects of which we have yet to find the total number (if ever). To my knowledge, the number of planets (following the current official definition) is unlikely to increase, so the definition allows us to have this closed set of objects which we happen to know extremely well compared to what might be discovered beyond the farthest known dwarf planets.
My argument is still one against the seeming lack of nuance that a much broader definition of ‘planet’ entails, and I hope that the above notes help to clarify my justification for this conviction.
Sincerely,
JM
I don’t care who thinks what. Pluto will always be a planet to me. No discussion needed. God gave us the ability of free thought and He created the heavens and the earth for us to marvel in His greatness. Pluto is a celestial body in deep space that make up a tiny point in space and time that we should enjoy as an object created by God.
Robert,
Keep your close minded comments to yourself. We all know that God is a hoax, your not converting anyone here. This site is dedicated to science, discovery, and learning. All which requires an open mind. Science takes us space, religion takes us to war. I denounce your God.
Sean,
I just found this site this morning. As an amateur lover of the skies, I’ve enjoyed and learned from this planetary give-and take. I’ve thought that vote in 2004 pretty bizarre.
As I reached the end of the discussion, I was startled at your hugely harsh response to the earlier expression of wonder and awe above, couched in religious terms. “Everyone knows that” there is no God is an emotional statement, and comes off as silly an awkward. Because nearly half of scientists are also believers in a Diety. You evidenty have a religious affirmation yourself, with your 100% affirmation of a negative. And that war is prevented by atheism (check your history). I must have missed your mathematical proof. Civility and broadmindedness can work for atheists too. But anyway, all the best.
Amen to that Sean!
Thank God for atheism………
Hai. Thank u.. very much for infrmation.
This nature is wonderful.amaizing, beautiful.S searching about this nature is so interesting
Alan Stern thinks Pluto should be referred to as a ‘dwarf planet’:
“But with the advance of technology, we’ve discovered actually that Pluto is just the first and the brightest of a large population of small planets called dwarf planets that have been discovered since the 1990s.”
— and so does the IAU. Perhaps he is shaving hairs on the definition of “dwarf planet”(?).
The other thing I find strange is that it was Alan Stern who came up with the concept of “clearing out the neighborhood” in a 2002 paper with Harold Levison advising NASA of how to define “planets”! In it he and Levison came up with several categories of planet. Among them:
“we define an uberplanet as a planetary body in orbit around a star that is dynamically important enough to have cleared its neighboring planetesimals….And we define an unterplanet as one that has not been able to do so.”
Look it up and read for yourself:
http://adsabs.harvard.edu/abs/2002HiA….12..205S
And just to drive my point, above, further…a few paragraphs down from the above quote (referenced above) Stern & Levison say:
“our Solar System clearly contains 8 uberplanets and a far larger number of unterplanets, the largest of which are Pluto and Ceres.”
(paper was written in 2002 before discovery of Eris)
yes, pluto is a planet, butit is just a dwarf plant in our belt of dwarf planets and should not be and isn’t part od our major planet system,
though, cerus, the dwarf planet between mars and jupiter,
is.
[...] Principal Investigator for New Horizons, Alan Stern, explained in an interview with EarthSky: Pluto, and its cohorts, are planets. They have all the attributes of planets. They have cores. [...]