Since January’s post, in which I offered a comprehensive practical solution to the ongoing problem of how to classify planets, there has been some discussion revolving around the centre of gravity, or barycentre, of a system.

The suggestion has been made that, according to the current IAU definitions, even Jupiter does not count as a planet because the barycentre of the Jupiter–Sun system lies 46,000 km above the surface of the Sun.

Against this contention, I should like to say four things.

(1) The wording “X orbits Y” is commonly understood to mean that X orbits the centre of gravity of the X–Y system. So for example we speak of the Moon orbiting Earth, when in fact Earth is by no means a stationary partner, and the resulting wobble of our planet helps produce the ocean tides.

This is on a par with when we talk about the Sun, Moon and stars “rising” or “setting”, where we also use for convenience a popular term which is understood as shorthand for a more complex reality. The astronomers at the IAU will certainly have intended “orbits the Sun” as an abbreviation for “orbits [the centre of mass of the system whose most massive member is] the Sun”.

(2) Whether the barycentre is above or below the surface of the primary does not have any particular physical significance, as the barycentre is only a theoretical mathematical point. I do not see that the Jupiter–Sun system is dynamically different from the Saturn–Sun system, or from the system containing 51 Pegasi and its “hot Jupiter” planet.

(3) The statement that the Jupiter–Sun barycentre is above the surface of the Sun is only part of the story, because in reality Jupiter and the other planets, as well as the Sun itself, actually orbit the barycentre of the Solar System as a whole. But this barycentre is sometimes above, sometimes below that surface, mainly depending upon whether Jupiter and Saturn are on the same side of the Sun or on opposite sides. This is shown in the diagram above, borrowed from Wikipedia.

(4) Finally, consider the case of Epsilon Eridani. If we take the data from the Open Exoplanet Catalogue (which used to have a pretty animation, but for some reason it’s not showing now), and if my rough calculation is correct, we find that the barycentre of the planet–star system is above the stellar surface when the planet is at apoastron, below it when the planet is at periastron. (These data are still a bit uncertain, but there will surely be similar cases elsewhere.) Thus whether the barycentre is above or below the star’s surface may well change back and forth during the course of a single orbit.

Further on Pluto specifically: Centauri Dreams has posted a good animation of Pluto and Charon orbiting each other, as seen from the long-range reconnaissance camera on the approaching New Horizons spacecraft at a range of about 200 million km. Again, the barycentre is outside the body of Pluto.

Some have therefore suggested calling the Pluto–Charon system a “double planet” (by which they actually mean a double satellite, i.e. no primary in the system). But again, all planets with satellites wobble to some extent, and Pluto–Charon is hardly fundamentally different from Earth–Moon. Pluto is the primary body by almost an order of magnitude in mass. Furthermore, there seems to be a clear trend, at least in our own system, for satellites to be more massive relative to their primary the smaller that primary is. The barycentre of a planet with a large satellite is therefore more likely to be outside the body of the planet in the cases of dwarf planets and asteroids, and the distinguishing feature of such systems is therefore not the position of the barycentre but their small size relative to terran and giant planets.

Meanwhile, the nonsense about Pluto having been “demoted” will surely evaporate once New Horizons has reached it this summer and revealed a world of planetary interest and complexity!

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