Wednesday, 5 May 2010

Ep 128: Another demotion for Pluto? Or is it about to become King of the Dwarfs?

Is Pluto is set to become an also-ran in the astronomical world?

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.

The research, entitled The Potato Radius: a Lower Minimum Size for Dwarf Planets, suggests that the number of objects in the solar system classed as dwarf planets could grow by more than a factor of ten. One of the definitions of dwarf planet relates to its ability to exist in hydrostatic equilibrium - that is, the body must have sufficient mass for its self-gravity to pull it into a round shape. This is generally accepted to happen at a radius of around 400 km. Lineweaver and Norman found that the point at which a body loses its rugged and unshapely 'potato' appearance and becomes round depends on what the material that the body is made from. Icy objects form spheres at roughly 200 km radius, whilst rocky spheres form at 300 km. As these radii are considerably less than the generally accepted minimum radius for a dwarf planet - 400 km - the researchers believes that a whole new crop of trans-Neptunian objects should be classified as dwarf planets.

I had a fascinating chat with Dr Lineweaver about how they went about deriving these numbers, and also about their work defining the shape and mass of other astronomical bodies. To listen to this show, tune in here (or press play below):



Lineweaver derived these radii from first-principles physics, and compared the results to astronomical observation. Bodies are held together by gravitational and electronic forces, and the research showed a strong correlation between the mass and shape of astronomical objects. For example, at a radius of ~ 200 km - 300 km, moons and rocky asteroids transition from a rounded potato shape to a sphere.

The work also looked at other shapes in the Universe, and Lineweaver divided these into five basic shapes - dust, potatoes, spheres, disks and halos. Each of these shapes correlates with the size and mass of the object.



Most of the new dwarf planets will be out past Neptune – these are called trans-Neptunian objects. Pluto has a radius of 1,150 km, but it is not the largest known trans-Neptunian object - Eris has a radius of ~1,250 km.

The International Astronomical Union General Assembly, which makes the decisions re astronomical nomenclature, next meets in Beijing in August 2012. No matter what they decide, some people will always call Pluto a planet.

6 comments:

  1. 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.

    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: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris.

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  2. Thanks for your comments Laurel. I notice from a quick Google search that clearly you are attached to Pluto and have been pasting these exact comments all over the place! I hope your pagerank increases...

    I would recommend you listen to the show, and read this article a bit closer. This article and podcast are about "hydrostatic equilibrium" - this research probably supports your position.

    In any case, under your definition, our solar system would have over 50 planets - not the 13 you mention here.

    Thanks also for the almost exact same comments on the 365 days of astronomy show I produced, and our other show on Pluto. I'll leave them up as the promote debate, but they are copy and pasted and show that you didn't listen to the shows.

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  3. Marc, you are such a b*tch.

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  4. Thanks for the update - I did not know that Pluto had been demoted! Poor Pluto :(

    That's great timing as J just started learning about constellations and planets.

    We are having a space/astronaut party for his birthday. You should come - got any space tricks?

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  5. Our solar system definitely has more than 13 planets. Thirteen is the current number if one counts those objects officially designated as dwarf planets among the total planet count. Advocates of counting dwarf planets as a subclass of planets fully expect that when all the dwarf planets are properly designated as such, the planet count in our solar system will be 50, 100, or more. The point is, we have no problem with that.

    I'm "attached" to Pluto? It's more like I'm committed to overturning the wrongful IAU demotion, which was a political rather than a scientific decision. I am committed to promoting a geophysical definition of planet although I respect dynamics as well. I don't care about my pagerank; in fact, I don't even know how to calculate it or if LiveJournal, where I have my Pluto Blog, has a provision for doing this. The only thing I care about is making sure people know that there are still two legitimate views on the Pluto issue.

    Media articles are often republished, along with repeats of the same statements. In this case, the media for some reason jumped on the fallacy that having more planets equals a further demotion for Pluto. It does not, and I will do whatever I can to counter that spin.

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  6. Spin and counter-spin on how we view Pluto? We know so little of the Universe now, that once we know more, this debate over Pluto may seem whimsical and totally unimportant.

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