Planets: questions answered

The blogosphere is now filled with complaints about the new definition by people who don't seem to have read the actual proposal and are getting second-hand news reports from reporters who don't properly understand the implications ether. I would like to point out that most of the points that people raise (many of which are pretty good questions) were discussed (and resolved) in the voting session yesterday. I'll try to list a few of the contentious points here along with some answers. Add any other good questions to the comment section.



  • It shouldn't be allowed; we can't change the definition of a planet! Yes we can. Is the Sun a planet? No, but it was until only a few hundred years ago. What about the Moon? Like the Sun it had its planet status removed. There are also a few examples of large asteroids being named planets and subsequently having their planet status removed when we found out how small they were. Ceres (discovered by an Italian) is a good example of that. We have to re-evalute things as we get more information.


  • Why isn't Pluto a planet? Pluto is one of the largest objects that reside is a huge belt of similar icy/rocky objects named the Kuiper belt. As such it hasn't cleared its orbit and would have to be much bigger to do so. And remember, Pluto has less than one percent of the mass of the Earth.


  • Ceres has cleared its neighbourhood so should be a planet rather than a dwarf planet. Ceres is in the asteroid belt and as such is together with untold millions of asteroids which it has little influence over. That does not constitute clearing its orbit


  • Jupiter has not cleared its neighbourhood. This claim goes on the basis that there are two groups of asteroids sharing an orbit with Jupiter. However, these 'trojan' asteroids sit 60 degrees behind and ahead of Jupiter in its orbit at Lagrangian points (L4 and L5 if you are interested) that exist because of Jupiter. This means that they are dominated/controlled by Jupiter so Jupiter is a planet.


  • Pluto crosses Neptune's orbit so doesn't that mean that Neptune hasn't cleared its orbit. Again, Pluto like many other objects is in a 3:2 resonance caused by Neptune so Neptune is dominating them. That makes Neptune a planet and perhaps Pluto is then an object that is dominated by Neptune (another reason not to be a planet).


  • Planets with satellites do not qualify because they haven't cleared their neighbourhood. Again, the planet is the dominant object and controls the satellites (otherwise they would fly off into space and not be satellites!).


  • If a satellite such as Titan was to leave Saturn and take its own orbit around the Sun it wouldn't be accepted as a planet. Firstly, that is incredibly unlikely to happen but if it did, and it was not dominated by another object then yes it would be a planet.


  • What about Sedna, Quaoar etc? They are on the "watch-list" for dwarf planet status. It will depend on what we find out about their shape, size and how much they control their neighbourhood.


  • Under these rules wouldn't Mercury be a dwarf planet? No. Mercury has hydrostatic equilibrium and dominates its neighbourhood so is a planet.


  • Shouldn't a planet have an atmosphere? No. As examples think of. Mercury and the limited atmosphere on Mars. How much atmosphere would be enough?


  • Extrasolar planets are no longer planets. This resolution was only concerned with "Objects in the Solar System" so doesn't declassify them. It was suggested that extrasolar planets would get their own resolution at some point in the future.


  • Planets not orbiting a star are no longer planets. Again, the resolution only covers objects in our Solar System so says nothing about them.


  • Isn't 'demoting' Pluto just international astronomers being spiteful to the only US discovered planet? No and Clyde Tombaugh would not be turning in his grave either. I should explain that politics isn't split in the same way amongst astronomers as it does amongst politicians. Many astronomers know each other and often collaborate on projects with astronomers in different countries. As individual countries only provide limited funds for projects, it is often necessary to make an international collaboration to get together enough funds. In some ways all astronomers are in a common battle against their own national funding councils so tend to be quite friendly towards each other. The 'anti-American' issue is just ridiculous as far as I can see.


  • We now have to wait three years for the group of trans-neptunian objects (TNOs) which are dwarf planets like Pluto to get a name. I'm not sure about this. The name will now be chosen by a committee of astronomers (as the suggested name was voted against) but I'm not sure if the IAU membership have to ratify this.

Posted in astro blog by Stuart on Friday 25th Aug 2006 (11:49 BST) | Permalink
Comments are no longer included on this blog as increasing numbers of people couldn't follow the ground rules and be civil.