Big Picture Science – A Sudden Change in Planets: Alan Stern / New Horizons

by Gary Niederhoff on September 6, 2014


A Sudden Change in Planets – Alan Stern
click to listen

Part 1 of A Sudden Change in Planets, featuring Alan Stern, planetary scientist, Southwest Research Institute and Principal Investigator of the New Horizons mission, explaining why the International Astronomical Union should never have changed Pluto’s designation to “dwarf planet.”
(TRT 10:28)

{ 15 comments… read them below or add one }

avatar Adrian Morgan September 14, 2014 at 4:06 am

All of the arguments for classifying Pluto as a planet apply just as well to large moons, since a body’s shape and composition aren’t determined by what it orbits. Some Pluto fans do advocate resurrecting the term “secondary planet” as a synonym for natural satellites, at least when pressed.

But Stern and others don’t really make a case for why “planet” is the right word for Pluto and hundreds of other bodies like it. They seem to just assert it and then work backwards from that conclusion. Pluto is a WORLD. So is the moon. So is Titan. So is Triton. Isn’t that distinction enough? Why the fuss over the word “planet” particularly?

There’s nothing wrong in principle with defining “planet” in a way that includes hundreds of objects in the solar system. But equally, there’s nothing wrong with the definition rather clumsily expressed by the IAU: that a planet is a body orbiting the sun that has a significant influence on the distribution of other bodies orbiting the sun. It’s a natural distinction to make, as the dynamics of the solar system create a sharp distinction between bodies that do have such an influence and those that don’t.

In choosing between those two definitions, however, the latter is a FAR less radical change from the former status quo. Stern agrees that the earth is “weird” for being so big, so why shouldn’t we have a word for bodies that are so weird, and why shouldn’t that word be “planet”?

His assertion that planetary scientists (as compared to astronomers generally) think the IAU blundered sounds like so much propoganda.

avatar Jon Lomberg September 14, 2014 at 11:25 am

A tomato is not a vegetable and a peanut is not a nut. Technically. Common vernacular usage often is at odds with textbook definition. The whole point of the Pluto controversy is that the general public takes ownership of the solar system and wants Pluto to remain a planet. It does not care whether Ceres or Sedna are planets too. Experts can have their specialist taxonomies, but it is common usage that determines what people will say. That so many people CARE about Pluto is a wonderful thing and astronomers would be wise to encourage that, and snotty professional snobbery only alienates the public from science. The classification of Pluto is a teachable moment, but it is the IAU that ought to learn something from it.

avatar David Grinspoon September 14, 2014 at 1:40 pm

Well, its just the “rather clumsily expressed” that you rightly identified, that is the whole problem. We just need a self-consistent definition, and we need one that applies for exoplanets as well as for the planets in our own solar system. The IAU also completely punted on exoplanets. So, it just needs to be fixed, so we have a decent definition. The IAU definition will never work for exoplanets. It is linguistically awkward and silly to say “A dwarf planet is not a planet”. A definition based on intrinsic properties of a planet, rath

avatar David Grinspoon September 14, 2014 at 1:45 pm

A definition based on intrinsic properties of a planet, rather than an object’s external dynamical environment will fix these problems.

avatar Laurel Kornfeld September 14, 2014 at 3:22 pm

Those of us who argue that Pluto–and all dwarf planets–are a subclass of planets do not have to be “pressed” to consider moons large enough to be in hydrostatic equilibrium “satellite planets” or “secondary planets.” That is clearly what they are. Objects large enough and massive enough to be rounded by their own gravity are inherently distinct from tiny asteroids and comets in that the former are complex worlds, often with geology and weather. In fact, several satellite planets in our own solar system are among the top possible locations for microbial life–Europa, Enceladus, and Titan specifically.

The argument for including dwarf planets as planets is based on preference for a geophysical rather than a dynamical planet definition. This means a celestial object should be defined by its intrinsic properties, not by its location. There is no “working backwards” from anything. The IAU definition is clumsy because defining objects by location is could result in the same object being a planet in one location and not a planet in another. If Earth were in Pluto’s orbit, it would not gravitationally dominate that orbit either. Any requirement that an object dominate its orbit to be considered a planet is inherently biased against objects further from their parent stars, which have larger and larger orbits to “clear.”

Morgan seems to contradict himself, on the one hand calling the IAU definition “clumsy,” but on the other hand saying there is “nothing wrong” with that definition. He also seems motivated by the same unscientific argument that we cannot have “too many planets” in our solar system in spite of saying there is “nothing wrong” with defining “planet” in a way that includes hundreds of solar system objects.

As we learn more about the outer solar system and about other solar systems, chances are we will learn enough about the unique characteristics of these bodies to further divide them into additional subcategories.

There is no “former status quo” because this issue is a matter of ongoing debate. One can cherry pick qualities to argue that every planet in our solar system is unique. That is why we use adjectives in front of these bodies to describe them. The IAU definition is the one that makes the least sense because it groups unlike objects together while excluding objects that are similar. How can anyone justify putting Earth and Jupiter in the same category but excluding Pluto? Earth has far more in common with Pluto than with Jupiter. Both have solid surfaces and are rocky; both have large moons formed via giant impact; both are geologically differentiated into core, mantle, and crust; and both have nitrogen in their atmospheres. Jupiter, on the other hand, is composed largely of hydrogen and helium, like the Sun. It has no known solid surface and its own “mini-solar system” of rings and moons.

At a time when we have discovered close to 2,000 exoplanets, how can anyone argue that a definition that requires a planet to orbit the Sun rather than a star is the better one? Many very large exoplanets would not meet the IAU definition even if that wording was changed, as we have discovered exoplanets in resonant 3:2 resonant orbits like Neptune and Pluto, planets in comet-like orbits around their stars, planets orbiting their stars backward, two planets sharing an orbit, etc. We have also discovered rogue planets that float freely in space and don’t orbit any stars. Naturally, they cannot dominate or clear their orbits if they don’t have any orbits!

The fact is, the IAU did blunder in not waiting for data from a mission to Pluto already underway when they hastily adopted a poorly worded planet definition. By calling Stern’s arguments “propaganda,” Morgan reveals he already has a pre-existing bias toward the dynamical view.

Some planets gravitationally dominate their orbits, and some do not. The latter are called dwarf planets. They are still planets by virtue of their inherent characteristics. The “dwarf” adjective tells us these are smaller versions of the larger bodies and therefore not large enough to influence other bodies. Such a definition fits well with the use of the term “dwarf” in astronomy, where dwarf stars are still stars, and dwarf galaxies are still galaxies.

avatar P Edward Murray September 14, 2014 at 3:43 pm

First of all there is something to be said of folks who are young and want to change the world simply because they want to change it.

Change is something that is inevitable but not all change is good.

For instance, the change in the definition of planet by the IAU a few years ago does not bear up under scrutiny.

Let me focus on one item:

The object must “clear” it’s orbit.

Well that really means that nothing can cross it’s orbit.

Earth, under that definition isn’t even a planet because it is regularly crossed by meteor showers and asteroids.

Actually, what the IAU did bears more resemblance to Jr High School antics as most of the astronomers had already left the meeting when it was voted upon and as such there wasn’t even a quorum under Roberts Rules of Order..

Bad Science and worse Astronomy:(

avatar Adrian Morgan September 14, 2014 at 4:47 pm

See my comment on Part 5 of the podcast.

Everyone agrees that the IAU definition is clumsily worded, particularly the requirement for a planet to clear its orbit. It’s stupid wording — we agree on that — but that does not invalidate the concept that it was trying to express. The “one item” you chose to focus on is irrelevant to the question of whether the 8-planet solar system is the right one in principle.

avatar Mike Wrathell September 14, 2014 at 7:02 pm

Dr. Stern only had ten minutes and had to ask the questions asked, thus he did not present his case for Pluto’s planethood in a point by point, or PowerPoint presentation, as it were. He has expressed other good points before in other articles, if you look him up. The case for dwarf planets to be a third subcategory of planet is a strong one. Pluto has all the characteristics of a planet that Earth has, viz., a core, mantle, and crust. Not to mention that if Earth was in Pluto’s orbit, it would not be able to clear its path. The IAU def is not merely clumsily worded, it is unworkable, arbitrary, and capricious. It was made in order to exclude Pluto. The executive committee had it out for Pluto and there is evidence of that. It was more political than scientific and I know of one IAU member who was pressured to vote to demote using threats of having his academic career destroyed.

avatar Laurel Kornfeld September 14, 2014 at 9:12 pm

The 8-planet solar system is not the right one because our solar system has more than 8 planets. It’s that simple. Some of those planets dominate their orbits; others do not. Some are in primary orbits around the Sun; others orbit other planets. The point here is none of these differences trumps the fact that all these objects have very similar intrinsic properties, which is why they should be subsumed under one broad category and distinguished via multiple subcategories.

avatar P Edward Murray September 15, 2014 at 3:34 pm

Perhaps it’s about time to ask Mr Adrian Morgan what credentials he has?

I’m an Amateur Astronomer who actually discovered something on the moon..

And you sir, apart from your opinion, what have you done?

It’s fine that you want to question but going head to head with luminaries like Jon Lomberg and Dr David Grinspoon , who have resumes while you do not isn’t something to be congratulated my friend.

avatar P Edward Murray September 15, 2014 at 3:48 pm

I think what Jon Lomberg said is best…

People taking ownership of our Solar System !

It belongs to all of us not just to members of
The IAU!

avatar Adrian Morgan September 16, 2014 at 1:28 am

Despite what Laurel Kornfeld says, I have never heard any member of the “Pluto is a planet” brigade freely explain to an audience, without being pressed in some way, that if Pluto is a planet, then the moon and other large satellites are planets too. The point is always raised by supporters of the 8-planet system.

I will respect the Pluto fans’ position when I hear them shouting it from the rooftops. But I don’t see that happening, because advocates of Pluto’s planethood enjoy the illusion of popular support, and there is no popular support for calling the moon a planet.

(BTW, Pluto itself, I am sure, will be spectacular. Triton is spectacular, and that’s the most Pluto-like object we’ve seen up close. Really looking forward to seeing what New Horizons discovers.)

The only point in answering P Edward Murray’s request for my credentials is to mock it. To that end: I once wrote a song about the solar system. Pttttth!

avatar P Edward Murray September 16, 2014 at 7:23 am

Well Mr Morgan, your response was, shall we say, pretty much expected…I saw you coming from 100 miles or even 100 Astronomical Units away:)

You did not say that you are a fellow astronomer or member of any astronomical group or even a science teacher perhaps.

I can only gather that you are here just to make controversial remarks only.

Nice try but no dice.

Oh and btw, try looking at the stars:)

I do wish you well.

avatar Mike Wrathell September 16, 2014 at 5:00 pm

I would love to shout it out from multiple rooftops. You have a point there, Adrian. We Pluto huggers should mobilize like other activists of other issues do. Regarding satellite moons like Luna and Triton that have achieved hydrostatic equilibrium, they, too, should have a subcategory within the broader category of planet.

avatar Laurel Kornfeld September 16, 2014 at 5:10 pm

I second the shout from the rooftops that spherical moons are satellite planets or secondary planets. Adrian Morgan, just because you never heard an advocate of the geophysical planet definition publicly state that spherical moons should be referred to as such does not mean it has not happened. Dr. Stern has advocated it publicly on numerous occasions, including his talk at this year’s Northeast Astronomy Forum. No one had to “press” him. This view was also expressed at the 2008 Great Planet Debate, held at the Johns Hopkins University Applied Physics Lab in August 2008, held in response to the 2006 IAU vote. I have expressed it myself in writings and public talks. This point is not raised only by supporters of the 8-planet system. Members of the public who are not involved with astronomy are less likely to be familiar with the concept of satellite or secondary planets, which is why those on the geophysical side of this debate often have to introduce it to them.

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