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.
Comments: Planets: questions answered
how do plants get their shape?
Posted by liza on Tuesday 05th Dec 2006 (01:53 UTC)
why is pluto not a planet anymore? why did they change the it now when we learned that there is 9 planets? why change it now?
Posted by katie on Tuesday 12th Dec 2006 (20:10 UTC)
come one, it shoud still be a planet. it makes me sad that it isnt a planet anymore. you guys are mean
Posted by katie on Tuesday 12th Dec 2006 (20:12 UTC)
Katie, defining what is and what isn't a planet is not as easy as it sounds because the Universe provides us with so much variety that we should realise that the term 'planet' is a fairly arbitrary label. A good book on the whole subject is "Is Pluto a Planet?" by David Weintraub. Anyway, many people know that "there are 9 planets" but in the last decade over 200 planets have been found orbiting stars other than the Sun. This "fact" (that there are 9 planets) isn't a fact but was a limit of what we could detect until recently.
Just because we were told something (especially when it comes to classification) does not mean it is totally true. Classifications are based on what we know at the time and in all areas of human endeavour these can change as we find more examples or exceptions. Really, we shouldn't stick to ideas irrespective of what the Universe is actually like.
By the way, not being a member of the IAU, I didn't get a vote on the status of the definition of a planet. In fact, I don't think any of the people that commented here are members of the IAU either.
Personally, I don't care if it is or isn't a planet. It is still Pluto and interesting in its own right. If we only care about things because somebody once said it was a planet I think that would be very sad.
Posted by Stuart on Thursday 14th Dec 2006 (10:16 UTC)
Sorry to say friends, but your all mistaken. Pluto is a planet. It's Mercury thats isn't a planet. It's Moon to Venus. Using mass as the main criteria for determining planetary status is insufficeient . I've been writing a paper that explains that in order for anybody orbiting the Sun to be considered a planet it must have one moon. Venus is an identical planet to Earth each with one almost identical moon.
Posted by toby on Thursday 28th Dec 2006 (13:01 UTC)
Toby, I don't think you've solved the problem, you've only shifted the problem to how you define a moon.
One basic requirement would be for it to orbit the 'planet'. Using this definition I don't see how you can count Mercury as a moon of Venus. There are other issues to be thought of. Is a small lump of rock orbiting a body good enough to be a moon? Do artificial satellites count? I'm sure there could very easily be a small rock orbiting both Mercury and Venus. You also seemed to imply that the relative sizes is important but in that case Mars:Phobos isn't too impressive and neither is Jupiter:Io.
Posted by Stuart on Friday 29th Dec 2006 (00:42 UTC)
Actually Stuart,
If you look closely you'll see that Mercury is indeed orbiting Venus. Well Mercury is co orbiting the Sun and Venus as the Moon co orbits the Sun and Earth.It just dosen't appear as though it is. Think, Mercury & Venus are one thing similar to the Moon & Earh which another thing. Mercury and Venus behave differently because they are closer to the Sun.
Posted by toby on Saturday 27th Jan 2007 (18:46 UTC)
Toby, that isn't a correct analogy. Mercury, Venus and the Sun are NOT like the Earth, Moon and Sun. The Moon and Earth move around their common centre of mass which lies within the surface of the Earth. The Earth-Moon system orbits the Sun with a centre of mass well within the Sun. The orbit of the Moon around the Earth and Earth around the Sun are not far off circular. Mercury-Venus-Sun are very different.
Mercury and Venus orbit the Sun (the centre of mass of the Mercury-Sun or Venus-Sun systems are well within the Sun). Although you can sort of squint and say that Mercury kinda circles Venus, this "orbit" is far from circular and is only because they are both orbiting the Sun. The separation of Mercury and Venus ranges from 0.3 - 1.1 AU. Plus, they only have a tiny effect on each other. You may as well say that the Earth orbits Jupiter.
I suspect that you are just being deliberately provocative.
Posted by Stuart on Sunday 28th Jan 2007 (01:42 UTC)
I have removed many inappropriate comments from this post. It is not appropriate to post things totally unrelated to astronomy or of explicit nature as this is a family friendly site. This behaviour is not acceptable and I suspect that one person is behind this. All dodgy comments will be removed and the links will not boost your Google rankings because they have appropriate code added to them. There are also very few people reading the comments to old entries. There really is no benefit to advertising your dodgy sites here. Please stop it.
Posted by Stuart on Thursday 01st Feb 2007 (11:24 UTC)
tea bags tea bags http://bagsbargain.info/tea-bags.html tea bags wholesale fashion bags wholesale fashion bags http://bagsbargain.info/wholesale-fashion-bags.html wholesale fashion bags evening bags evening bags http://bagsbargain.info/evening-bags.html evening bags designer diaper bags designer diaper bags http://bagsbargain.info/designer-diaper-bags.html designer diaper bags cosmetic bags cosmetic bags http://bagsbargain.info/cosmetic-bags.html cosmetic bags
Posted by ujwotefakta on Monday 19th Feb 2007 (08:06 UTC)
I have solved the problem of deflecting an asteroid,( in a nut shell,)" simply land a craft onto the asteroid, the craft having a computer and large retro jets, the nozzle blast and duration can be aimed by the computer and well there it is.
Camie Gionet, 03/07/07
Posted by Camie Gionet on Thursday 08th Mar 2007 (03:30 UTC)
Why can't an object in the Kuiper Belt that has achieved hydrostatic equilibrium be considered both a planet and a KBO? It would make much more sense to keep "planet" as a broad general category with multiple subcategories, one of which would be the ice dwarfs. I believe the IAU made a mistake with this decision. The process they used is also questionable, as only four percent of members voted; no Internet or absentee voting was allowed; and most of those who did vote are not planetary scientists. Also, the definition they created makes no linguistic sense in that it claims a "dwarf planet" is not a planet at all. Personally, I do not accept the IAU's definition and am reasonably confident it will be overturned.
Posted by Laurel Kornfeld on Tuesday 04th Sep 2007 (20:06 UTC)
Laurel, who are your comments directed to?
Posted by Stuart on Wednesday 05th Sep 2007 (08:11 UTC)
father of the pride Now lower the soldier barked drawing close. The.
Posted by fat on Friday 07th Sep 2007 (20:55 UTC)
muscular woman gallery All emotional guards down, but no one another.
Posted by gallery on Sunday 09th Sep 2007 (01:07 UTC)
why isnt pluto a planet please can you reply over the next theree days??
thanks
p.s. can you give me all the reoson that pluto isnt a planet!?
Posted by jake warner on Friday 28th Sep 2007 (16:44 UTC)
I like the instresting facts!
Posted by Rose on Wednesday 03rd Oct 2007 (23:53 UTC)
I like the instresting facts!
Posted by Rose on Wednesday 03rd Oct 2007 (23:53 UTC)
What year was Mercury found?
Posted by Sloan on Thursday 11th Oct 2007 (19:58 UTC)
What year was Mercury found?
Posted by Sloan on Thursday 11th Oct 2007 (19:58 UTC)
What year was Mercury found?
Posted by Sloan on Thursday 11th Oct 2007 (19:58 UTC)
I live in northern England.
Does anyone know the identity of a very bright star I see early in the morning.
It is in the east , if 90 degrees was directly above I would say it is at about 25 degree elevation.....not very high above the horizon.
I don`t know if it`s a star or a planet.
Thanks,
Mike
Posted by Michael Kershaw on Tuesday 30th Oct 2007 (15:39 UTC)
Mike, if you mean directly east at about 20 degrees you are looking at the star Arcturus which is in the constellation Bootes. However, it is more likely that you are actually looking towards the south-east as that is where the planet Venus is in the early morning. Venus is very bright at the moment.
Posted by Stuart on Tuesday 30th Oct 2007 (17:05 UTC)
Thanks Stuart,
Come to think of it,it is more south east so I guess it must be Venus.
Posted by Michael Kershaw on Wednesday 31st Oct 2007 (11:55 UTC)
Flying at 30,000 feet last night at midnight a bright object rose in the southeast. Close to the horizon it appeared to glisten green and red. It looked like a planet, but was off the lower left tip of orion, not close to where I could see mars. At first it was so bright we thought it was another aircraft. Any ideas?
Posted by liam mcguinness on Thursday 29th Nov 2007 (15:54 UTC)
Liam, if the three stars that make Orion's belt were roughly pointing at it, I think you were looking at Sirius. Sirius - the Dog Star - is the brightest star visible in the sky except for the Sun.
When stars are low down on the horizon the light from them has to pass through a lot of our atmosphere which causes it to twinkle wildly. The twinkling affects different colours differently, so Sirius (when low) can often appear to change colour just as you describe.
Posted by Stuart on Thursday 29th Nov 2007 (17:09 UTC)
Stuart,I have noticed another bright star in the morning sky and wondered if you could tell me what it is.
It is almost directly opposite to Venus (ie NW)but about twice as high and not as bright.I could be wrong but it seems to have an orangish or pinkish tint.
Posted by Michael Kershaw on Thursday 20th Dec 2007 (14:43 UTC)
Michael, that is probably planet Mars. You can see it over in the east after sunset and through the night it moves around through south and sets in the north west at around 8:40 am or so (as seen from the north of England). Just the other day Mars was at its closest approach to Earth of the year (the orbits are slightly elliptical) so it is looking quite large in a telescope and is a bit brighter.
By the way, it might be worth your while downloading Stellarium for your computer. It is an excellent (and free) planetarium program which will show you the view of the sky from where you live for any time of day. It points out stars and planets.
Posted by Stuart on Thursday 20th Dec 2007 (17:10 UTC)
Oops, here is the correct link to Stellarium.
Posted by Stuart on Thursday 20th Dec 2007 (17:11 UTC)
Thanks Stuart,
It definitely is Mars as you say.
Thanks a lot for the stellarium link it is brilliant..... perfect for my needs.
Posted by Michael Kershaw on Wednesday 02nd Jan 2008 (14:16 UTC)
Thanks Stuart,
It definitely is Mars as you say.
Thanks a lot for the stellarium link it is brilliant..... perfect for my needs.
Posted by Michael Kershaw on Wednesday 02nd Jan 2008 (14:17 UTC)
Thanks Stuart,
It definitely is Mars as you say.
Thanks a lot for the stellarium link it is brilliant..... perfect for my needs.
Posted by Michael Kershaw on Wednesday 02nd Jan 2008 (14:17 UTC)
Sorry about that triple posting.
Posted by Michael Kershaw on Wednesday 02nd Jan 2008 (14:19 UTC)