IAU Website: NEWS – The IAU draft definition of “planet” and “plutons”

IAU Website: NEWS


The above image is a picture of what the proposed new solar system will ‘look’ like, if recent attempts to widen the definition of a planet go forward. The link at the top of this piece links to a press release by the International Astronomical Union that attempts to bring more clarity to the definition of planet, and announces a new definition that includes objects like Charon (formerly Pluto’s moon) and Ceres (an asteroid).

The part of “IAU Resolution 5 for GA-XXVI” that describes the planet definition, states “A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.” Member of the Planet Definition Committee, Richard Binzel says: “Our goal was to find a scientific basis for a new definition of planet and we chose gravity as the determining factor. Nature decides whether or not an object is a planet.”

That’s the new definition for planet given by the IAU, and on the surface, it seems to be pretty accurate. Using it, we would certainly classify all the ‘traditional’ planets as planets still, and it does seem to add some more leeway for other objects. Essentially, this definition specifies that size and orbital parent are the factors that determine the “planet-ness.”

As is often the case, however, the Devil is in the details, and this is no different. The first criteria … that a planet be large enough such that gravity tends to compress it into a mostly-spherical object … is pretty straight-forward. One of the man reason we call the objects in the asteroid belt asteroids and not planets has always had to do with size. In this case, size definitely does matter … even if it is orbiting the parent star, an orbiting boulder is still just an orbiting boulder.

The second criteria is where I have more issue, the notion that planets circle a star, by definition. This SEEMS a self-evident point, but one of the new candidates, to my mind, puts the lie to it. By including Charon, scientists have now said that there is at least one planet that doesn’t orbit its parent star directly.

In the old days (say, pre-1990) Charon was called a moon of Pluto. While their sizes are comparable, its VERY clear that there are two bodies there. As is the case in ALL multi-body gravitational systems, its not a simple case of one orbiting the other … both Pluto and Charon have ALWAYS orbited a common point in space somewhere between the two of them. Earth’s moon, for example, doesn’t orbit Earth, strictly … instead, the Earth and Moon both orbit a point that lies between the two objects (the location of the point is based on the relative masses of the two bodies … and in the case of the Jovian system, the point they all orbit is actually inside Jupiter … but NOT Jupiter’s centre, and thats the key issue here). The same is true of the Pluto/Charon system, and this means that Pluto and Charon orbit each other first, and then the parent star.

Now, the current theory holds that the Pluto/Charon system is a ‘two planet system’ not a planet and moon system, but other than the relative sizes of the bodies, I can see no real difference between Pluto/Charon and Earth/Moon … and yet, science comes to a different conclusion for each. In Pluto/Charon, they call it 2 planets both orbiting the parent star … in Earth/Moon, they call it a planet and a moon, with the Moon CLEARLY orbiting Earth, and not the Sun. Why the difference? Does the size difference alone justify the different classifications?

There’s another problem with this “orbits the parent star” criteria as well. Moons of the gas giants often rival planets in size, and Titan, for example, is larger than both Pluto and Charon (and even Mercury for that matter). Titan has a complex atmosphere, and is certainly large enough to hold its own spherical shape. The ONLY thing that keeps it from being called a ‘planet’ is the fact it orbits Saturn, and not the Sun. Yet, as described above in the simple 2-body system, Titan, Saturn, and the rest of its satellites actually ALL orbit a common point between them … strictly speaking, NONE of the objects in the Saturn system directly orbit the Sun, or they ALL do.

So, orbital parent aside, I’d argue that bodies like Titan have FAR more reason to be called planets than either Pluto or Charon, and certainly more than Ceres. And it strikes me that, by the IAU definition itself, there seems to be some contradiction about the definitions. The Charon/Pluto system, to me, contradicts itself in this definition. Its not logically possible for both Charon and Pluto to be directly orbiting the Sun, unless you also concede that both the Earth and Moon directly orbit the sun, and concede that all of Jupiter’ and Saturn’s satellites orbit the Sun directly and not their parent planet. Given that conclusion, that both Pluto and Charon orbit the Sun directly, it seems hard to deny objects like Titan, or even our own moon, status as ‘planets.’

On the other hand, if we accept that satellites of planets orbit the planet directly and not the parent star, then it seems impossible to conclude both Pluto and Charon are planets. One MUST be orbiting the other by this scenario, as the Moon orbits the Earth, or Titan orbits Saturn. That’s generally been accepted as Charon orbiting Pluto because Charon is slightly smaller … but that would effectively remove Charon from consideration as a planet.

In the end, it comes down to a logical problem to me. Either, we say that a planet must orbit the parent star, or we say it doesn’t have to. Both statements have consequences for what we can call a planet when we are done. The problem for me here is that it seems the criteria haven’t been applied rigorously to the Pluto/Charon system. Either Charon is a moon, or moons like Titan need to be reclassified as planets. If being a planet is about orbiting a star, then drop Charon from the lists. But if we insist on calling Charon/Pluto a 2 planet system, then I fail to see how we can escape calling the Earth/Moon system a 2 planet system as well, and calling the Saturn system a multi-planet system. It strikes me that including Charon on the list of planets means that we will need to broaden the definition, and ‘moons’ like Titan will no doubt end up on the list as well.

In the end, I don’t really care what they are called. I am a bit concerned with poorly defined logic, however, and the way the IAU have defined planet, then used it to include Charon, seems definately suspect from a logical perspective. Either Charon is Pluto’s moon, and therefore doesn’t qualify, or else many other objects in the solar system formerly thought to orbit a planet will need to be redefined as orbitting the Sun instead. Its a small point perhaps, but in science, logical inconsistencies can’t be tolerated, IMO … without logic, all of science collapses like a house of cards.


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