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Okay, so I am slow on this one. There were complications, okay? I was originally going to post about the expected decision after an opinion piece in a national newspaper dared express an opinion on the outcome they wanted. Anyway, it may surprise some (many?) to learn that until now there was no official definition of 'planet'. This has changed since the International Astronomical Union made one up on the 16th. Well, according to their (temporary) website it is only a draft definition.
I am pleased to say they have been mostly sensible with this one. The proposed definition runs thusly (you may have seen it in news articles the past couple of days): "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." and to clarify, "the shape of objects with mass above 5 x 1020 kg and diameter greater than 800 km would normally be determined by self-gravity, but all borderline cases would have to be established by observation."
This is, as I said, quite a sensible definition, though I should point out that despite all the noise being made in the press over the past few days, it has not yet been adopted as the official definition. That still remains to be put to the vote. Also, and I am probably not the first to point this out, the definition as it stands would include brown dwarfs as planets (a particularly massive class, perhaps). I am a bit less concerned about this than I was reading the press articles as they make it clear they are at least aware there needs to be a definition restricting the upper bound too.
What I am less excited about is the creation of the category 'plutons'. The IAU is considering defining a pluton as a planet with an orbit longer than 200 years. I don't like this category. It is as artificial and arbitrary as they were trying to avoid in defining planet in the first place and we already have suitable classification for objects like that: kuiper belt objects, or KBOs. And we have a name for KBOs in similar orbits to Pluto too, we call them Plutinos.
I do think, though, that there should be another category added. We should dust off the seldom used term 'minor planet' and use it to describe any planet which is not the dominant body in its part of the system. All of the 'new' planets and Pluto would fall into this category. A planet which is the dominant local body is a major planet. This classification is not my idea but I do mean to use it whatever the IAU decides (well, unless they come up with something genuinely better), and I am going to encourage anyone else I can to use it too.
In the solar system the major planets are (counting outwards): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The minor planets are Ceres (which has been a planet before), Pluto/Charon and 2003 UB313. 2003 UB313 is that body (only one of them actually) which is yet to receive an official name, for now it goes by the nickname Xena. It has been issued with a satellite too, as are most planets. And 2003 UB313 still doesn't get to be the tenth planet. For now it will be 12th. Pluto is 10th.
Both Pluto and Charon now qualify as planets (if this definition is adopted), as their barycentre (that point in space about which both orbit) lies beyond the surface of either. This would make the term 'double planet' no longer a nickname but an official designation. As far as I know there is no standard way of referring to such a pair so this is a good chance for me to try and slide in my own terminology and get it adopted. Pluto/Charon or Pluto-Charon? Right now I am leaning toward Pluto-Charon (the more massive of the pair should go first).
Doesn't this make things more exciting? A chance to look at familiar bodies in a new light. I have been waiting for this decision for a few years now and it is great to see the IAU proceeding in (generally) the right direction. Just for fun, here is the solar system's revised roster: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto-Charon, 2003 UB313, illustrated at the IAU site.
But wait. Aren't there a number of other large spherical or near spherical objects in the solar system too? There are, and a number of them are larger than Charon (Charon is larger than Ceres too). About another dozen are under consideration by the IAU (handy picture go!) and I expect a fair few of them will be classed as planets too. Hopefully a couple of the larger asteroids like Vesta will get in so the minor/major planet designation can have another shot at officiality.
Tricia
I am pleased to say they have been mostly sensible with this one. The proposed definition runs thusly (you may have seen it in news articles the past couple of days): "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." and to clarify, "the shape of objects with mass above 5 x 1020 kg and diameter greater than 800 km would normally be determined by self-gravity, but all borderline cases would have to be established by observation."
This is, as I said, quite a sensible definition, though I should point out that despite all the noise being made in the press over the past few days, it has not yet been adopted as the official definition. That still remains to be put to the vote. Also, and I am probably not the first to point this out, the definition as it stands would include brown dwarfs as planets (a particularly massive class, perhaps). I am a bit less concerned about this than I was reading the press articles as they make it clear they are at least aware there needs to be a definition restricting the upper bound too.
What I am less excited about is the creation of the category 'plutons'. The IAU is considering defining a pluton as a planet with an orbit longer than 200 years. I don't like this category. It is as artificial and arbitrary as they were trying to avoid in defining planet in the first place and we already have suitable classification for objects like that: kuiper belt objects, or KBOs. And we have a name for KBOs in similar orbits to Pluto too, we call them Plutinos.
I do think, though, that there should be another category added. We should dust off the seldom used term 'minor planet' and use it to describe any planet which is not the dominant body in its part of the system. All of the 'new' planets and Pluto would fall into this category. A planet which is the dominant local body is a major planet. This classification is not my idea but I do mean to use it whatever the IAU decides (well, unless they come up with something genuinely better), and I am going to encourage anyone else I can to use it too.
In the solar system the major planets are (counting outwards): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. The minor planets are Ceres (which has been a planet before), Pluto/Charon and 2003 UB313. 2003 UB313 is that body (only one of them actually) which is yet to receive an official name, for now it goes by the nickname Xena. It has been issued with a satellite too, as are most planets. And 2003 UB313 still doesn't get to be the tenth planet. For now it will be 12th. Pluto is 10th.
Both Pluto and Charon now qualify as planets (if this definition is adopted), as their barycentre (that point in space about which both orbit) lies beyond the surface of either. This would make the term 'double planet' no longer a nickname but an official designation. As far as I know there is no standard way of referring to such a pair so this is a good chance for me to try and slide in my own terminology and get it adopted. Pluto/Charon or Pluto-Charon? Right now I am leaning toward Pluto-Charon (the more massive of the pair should go first).
Doesn't this make things more exciting? A chance to look at familiar bodies in a new light. I have been waiting for this decision for a few years now and it is great to see the IAU proceeding in (generally) the right direction. Just for fun, here is the solar system's revised roster: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto-Charon, 2003 UB313, illustrated at the IAU site.
But wait. Aren't there a number of other large spherical or near spherical objects in the solar system too? There are, and a number of them are larger than Charon (Charon is larger than Ceres too). About another dozen are under consideration by the IAU (handy picture go!) and I expect a fair few of them will be classed as planets too. Hopefully a couple of the larger asteroids like Vesta will get in so the minor/major planet designation can have another shot at officiality.
Tricia
