Is Jupiter a planet? What a bizarre question.
Of course Jupiter is a planet, it’s the biggest by far in our Solar System! And yet … as our speaker, Jerry Stone of Spaceflight UK , argued at our May meeting, if you apply the definition of a planet developed by the International Astronomical Union in 2006 strictly, then Jupiter would fail to meet all three of their criteria. The speaker’s title was actually “Is Pluto a planet?” and his talk was based on the outcome of 2006 IAU meeting where the new criteria were agreed which led to Pluto being demoted to dwarf planet status. His presentation looked in detail at the new definition of a planet, using the example of Jupiter to present a case for looking at the criteria again.
The three criteria as shown on the IAU website are that a planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. They all sound perfectly reasonable to me, yet Jupiter fails each of them.
The first criterion seems the one which Jupiter most obviously fits. Come on, it orbits the Sun … doesn’t it? To decide the answer, you have to understand the idea of a barycentre. This is in effect the centre of mass of two objects and is the point around which they will orbit. For two objects of equal mass, then the barycentre is half way between them. They both orbit this point and you cannot really talk about one object orbiting the other. When one of the objects is far more massive than the other, then the barycentre will be much closer to the centre of the heavier one and if the difference is sufficiently extreme, the barycentre is within the heavier object itself. This is the case for all the planets…except Jupiter. The barycentre of the Sun-Jupiter pairing is just outside the Sun’s radius, so strictly speaking Jupiter orbits this point rather than the Sun itself.
What about the second criterion, which requires a planet to be “nearly round”? Our speaker took great exception to this as it is hardly a very precise definition. We all know that the Earth is not perfectly spherical but the difference between the equatorial diameter and that measured round the poles is very slight. For Jupiter however, given its gaseous state and very rapid rotation, the difference is about 6% . Is this “round enough” to count as “nearly round”? Maybe, maybe not.
The third criterion requires a planet to have cleared its orbit. Jupiter has definitely not done this! There are two sets of asteroids which have become trapped in its orbit – one set around 60 degrees in front of Jupiter and the other around 60 degrees behind it. Their existence was predicted by the mathematical work of the Italian Legrange and they remained just a theoretical possibility during his lifetime (he died in 1813). The first one was not observed until nearly a century later and now there are known to be thousands of the things.
Coming back to the speaker’s actual title “Is Pluto a planet?”, he didn’t really set out to give an answer Yes or No but rather to point out what he considered to be the absurdity of the present definition of a planet. Perhaps the issue is summed up by the choice of name for the object 2003 UB313 which is now designated as a dwarf planet, like Pluto and was given the name Eris after the 2006 IAU meeting. And why is that a suitable name ? Eris was the Greek goddess of strife and discord. The debate continues!
Talk given by Jerry Stone from Spaceflight UK
Post written by Katherine Rusbridge
May 2015