Printable Version of Topic

Click here to view this topic in its original format

Unmanned Spaceflight.com _ Pluto / KBO _ 2003 Ub 313: The Incredible Shrinking Planet?

Posted by: BruceMoomaw Jan 31 2006, 09:20 PM

(Thanks to Emily Lakdawalla):

http://sciencenow.sciencemag.org/cgi/content/full/2006/127/1

Posted by: elakdawalla Jan 31 2006, 09:36 PM

QUOTE (BruceMoomaw @ Jan 31 2006, 01:20 PM)
(Thanks to Emily Lakdawalla):

http://sciencenow.sciencemag.org/cgi/content/full/2006/127/1
*

Actually SigurRosFan deserves the credit! I must get half of my blog entry material from the sharp-eyed people on this site...
http://www.unmannedspaceflight.com/index.php?showtopic=1207&view=findpost&p=39115

Since I posted that, I've gotten an email from someone saying that there will be a publication in Nature tomorrow that flip-flops again on the size of UB313 -- hopefully another sharp-eyed watcher will post that link here as soon as it appears!

--Emily

Posted by: SFJCody Feb 1 2006, 12:19 AM

QUOTE (elakdawalla @ Jan 31 2006, 09:36 PM)
Actually SigurRosFan deserves the credit!  I must get half of my blog entry material from the sharp-eyed people on this site...
http://www.unmannedspaceflight.com/index.php?showtopic=1207&view=findpost&p=39115

Since I posted that, I've gotten an email from someone saying that there will be a publication in Nature tomorrow that flip-flops again on the size of UB313 -- hopefully another sharp-eyed watcher will post that link here as soon as it appears!

--Emily
*


There's a post on the badastronomy forums saying that the orbital period of the satellite has been found to be 15.42 days. Not sure what the source was.

Posted by: Rob Pinnegar Feb 1 2006, 02:12 AM

QUOTE (SFJCody @ Jan 31 2006, 06:19 PM)
There's a post on the badastronomy forums saying that the orbital period of the satellite has been found to be 15.42 days.

Four significant figures already, and with those fuzzed out images? Guess it could be true -- but it sure sounds like bad astronomy, alright.

Posted by: MichaelT Feb 1 2006, 10:32 AM

I just read that the http://www.mpifr-bonn.mpg.de/index_e.html in Bonn has confirmed a size of 3000 km. That was announced yesterday and can be read http://de.news.yahoo.com/060131/12/4uqnr.html. On the institute's website I could not find a press release, though. So I don't know any more details...

Michael

Posted by: SigurRosFan Feb 1 2006, 01:19 PM

Here's the detailed press release:

- http://www.unmannedspaceflight.com/index.php?showtopic=1207&view=findpost&p=39431

--- Note on reports of an HST size measurement (31.1.)

Mike Brown gave a public talk recently where he presented some preliminary results [Albedo 0.92 news] on an attempt to measure the size of UB313 with the Hubble Space Telescope. A journalist picked this up and reported it, against Mike Brown's explicit request. In response to this report Mike Brown stated on Jan 31:

"Contrary to rumors otherwise, we're just in the preliminary stages of analyzing the HST data. When we are done we should have a very precise measurement. The study that is coming out in Nature is the best info that we have for now about how big and reflective it is. The uncertainties are large, but it seems a solid result to me. I hope that we will have the HST analysis done within perhaps a month, and I'll be able to say more then." ---

Posted by: David Feb 1 2006, 01:34 PM

QUOTE (SigurRosFan @ Feb 1 2006, 01:19 PM)
Here's the detailed press release:
http://www.unmannedspaceflight.com/index.php?showtopic=1207&view=findpost&p=39431
*


Then:
"The so-called "tenth planet," announced last July to much fanfare, is a "smidge" bigger than Pluto rather than earlier estimates of 25% to 50% larger, a planetary scientist reported here on 25 January."

Now:
"Here we report observations of the thermal emission of 2003 UB313 at a wavelength of 1.2 mm, which in combination with the measured optical brightness leads to a diameter of 3,000±300±100 km; here the first error reflects measurement uncertainties, while the second derives from the unknown object orientation."

If 2003 UB313 is 3000 km in diameter, then it is 30% larger than Pluto, which is a bit above the going rate for smidges these days. If it is at the lowest end of the given range (2600 km) it is still 14% larger than Pluto.

Posted by: Rob Pinnegar Feb 1 2006, 02:46 PM

QUOTE (SigurRosFan @ Feb 1 2006, 07:19 AM)
A journalist picked this up and reported it, against Mike Brown's explicit request.

Yeah, you can always trust the discretion of the media. Hey, they wouldn't deliberately misinform the public just to sell newspapers!

Posted by: AlexBlackwell Feb 1 2006, 05:48 PM

QUOTE (elakdawalla @ Jan 31 2006, 09:36 PM)
Since I posted that, I've gotten an email from someone saying that there will be a publication in Nature tomorrow that flip-flops again on the size of UB313 -- hopefully another sharp-eyed watcher will post that link here as soon as it appears!

From the February 2, 2006, issue of Nature: http://www.nature.com/nature/journal/v439/n7076/edsumm/e060202-08.html.

Posted by: JRehling Feb 1 2006, 07:17 PM

Incidentally, don't forget that Pluto "shrank" throughout its lifetime, too, in at least two steps. I can remember when it was bigger than Mercury, and maybe bigger than Mars. wink.gif

It would be interesting if size estimates consistently shrank for newly-discovered objects, as though there were a regression to the mean effect, or a pro-big bias on the part of the early researchers... Who wants to think that they discovered something tiny?

PS: Titan has shrunk a bit, too... wink.gif

Posted by: TritonAntares Feb 2 2006, 12:27 PM

QUOTE (JRehling @ Feb 1 2006, 09:17 PM)
Incidentally, don't forget that Pluto "shrank" throughout its lifetime, too, in at least two steps. I can remember when it was bigger than Mercury, and maybe bigger than Mars.  wink.gif

It would be interesting if size estimates consistently shrank for newly-discovered objects, as though there were a regression to the mean effect, or a pro-big bias on the part of the early researchers... Who wants to think that they discovered something tiny?

PS: Titan has shrunk a bit, too...   wink.gif

...as Triton did.

As child I read in an old astronomy library book from the '50s - I think it was from Otto Struve - Triton's diameter should be ~6000 km:
'Wow, a moon nearly as large as Mars...' ohmy.gif
I'm not quite sure what it was before Voyager II - maybe about 3500 km,
but at the the end it came down to poorly 2720 km, probably the same size as UB313 now.

If we get more cases like Pluto, Triton, UB313, we'll find a 'shrinking law' from 'detection-diameter' to real diameter at the end... tongue.gif

Bye.

Posted by: Big_Gazza Feb 3 2006, 11:01 AM

I have heard a rumour that one of the Mormons secret beliefs is that Heaven is not on another plane of existance but is on a hidden planet past the orbit of Pluto. Yeah, I know sounds kinda wierd, but who am I to judge rolleyes.gif

Anyway, with 2003 UB313 being discovered, what is the opinion of the good folks from Salt Lake City? At least they can have the certainty of knowing they have somewhere to go, though I don't think they need to pack sunglasses and tanning lotion. A nice warm jacket might be well advised.

Posted by: Bob Shaw Feb 3 2006, 01:24 PM

QUOTE (Big_Gazza @ Feb 3 2006, 12:01 PM)
I have heard a rumour that one of the Mormons secret beliefs is that Heaven is not on another plane of existance but is on a hidden planet past the orbit of Pluto. Yeah, I know sounds kinda wierd, but who am I to judge  rolleyes.gif

Anyway, with 2003 UB313 being discovered, what is the opinion of the good folks from Salt Lake City?  At least they can have the certainty of knowing they have somewhere to go, though I don't think they need to pack sunglasses and tanning lotion. A nice warm jacket might be well advised.
*


Do you think they'd like to sponsor some KBO missions, or would that take the fun out of things for them?

Bob Shaw

Posted by: AndyG Feb 3 2006, 03:35 PM

QUOTE (Bob Shaw @ Feb 3 2006, 01:24 PM)
Do you think they'd like to sponsor some KBO missions, or would that take the fun out of things for them?
*

Not at all. Though with their endless interest in genealogy, it's likely the instrument package would be ditched for thousands of names-on-DVDs... wink.gif

Andy G

Posted by: Rob Pinnegar Feb 3 2006, 07:18 PM

QUOTE (TritonAntares @ Feb 2 2006, 06:27 AM)
As child I read in an old astronomy library book from the '50s - I think it was from Otto Struve - Triton's diameter should be ~6000 km.

If I recall right, that was because of Triton's unexpectedly high reflectivity.

The 6000-km figure was still around in the 1970s; I'm pretty sure it was included as an upper limit in Ludek Pesek's book "Solar System" which was from about 1979.

Posted by: ljk4-1 Feb 10 2006, 04:52 PM

For what this is worth, Sky & Telescope is now including 2003 UB313
in its online section "This Week's Planet Roundup" right after Pluto:

2003 UB313 (magnitude 19, in Cetus) is getting low in the southwest after dark. This is the "tenth planet" discovered last year; see our articles on its discovery and on the finding of its moon. Advanced amateurs with good CCD setups have been imaging 2003 UB313 and tracking its motion. The discovery team is informally calling the object and its moon Xena and Gabrielle, for the TV warrior princess and her longtime companion. The official names they will get are still tied up in committees of the International Astronomical Union. According to the February 1st Nature, "Xena" is unquestionably larger than Pluto: 3,000 kilometers in diameter compared to Pluto's 2,300.

http://skyandtelescope.com/observing/ataglance/article_110_1.asp

Posted by: JRehling Feb 10 2006, 05:50 PM

QUOTE (Big_Gazza @ Feb 3 2006, 03:01 AM)
I have heard a rumour that one of the Mormons secret beliefs is that Heaven is not on another plane of existance but is on a hidden planet past the orbit of Pluto. Yeah, I know sounds kinda wierd, but who am I to judge  rolleyes.gif
*


http://en.wikipedia.org/wiki/Kolob

More sensational and also more fun:

http://nowscape.com/mormon/mormons5.htm

Posted by: punkboi Feb 10 2006, 05:54 PM

QUOTE (JRehling @ Feb 10 2006, 10:50 AM)
http://en.wikipedia.org/wiki/Kolob

More sensational and also more fun:

http://nowscape.com/mormon/mormons5.htm
*


I thought heaven was on Venus. According to Scientologists.

Oh wait, the leader supposedly came on a spaceship from Venus. Now, I'm confused. huh.gif

biggrin.gif

Posted by: Katie Feb 11 2006, 08:30 AM

QUOTE (JRehling @ Feb 10 2006, 10:50 AM)
http://en.wikipedia.org/wiki/Kolob

More sensational and also more fun:

http://nowscape.com/mormon/mormons5.htm
*


I've been Mormon my whole life and I must say I never heard any of that. ohmy.gif

I always sorta figured Kolob was more a figurative place than a literal one. Well that's what I always figured anyway.

They apparently keep the crazy space travel beliefs of the early Mormons out of church these days but still I laughed when reading that second link.

I hate to disappoint people but most of that stuff is no longer taught in the church and whether it was truly official Mormon doctrine at one time I really don't know.

Only a few things on there are legitamately true and official Mormon doctrine to the best of my knowledge. However Mormons truly do believe there are other earth-like planets inhabited by humans. As for the other stuff about space aliens impregnanting women well I never heard that. laugh.gif

I personally don't believe that Kolob is a real physical place that a space craft could go to and I have never heard it preached that way. But I think that site was meant to be funny and not true. tongue.gif

Posted by: BruceMoomaw Feb 11 2006, 08:56 AM

QUOTE (Big_Gazza @ Feb 3 2006, 11:01 AM)
I have heard a rumour that one of the Mormons secret beliefs is that Heaven is not on another plane of existance but is on a hidden planet past the orbit of Pluto. Yeah, I know sounds kinda wierd, but who am I to judge  rolleyes.gif


*


I hope that isn't the same one Lovecraft was talking about...

Posted by: ljk4-1 Feb 11 2006, 07:12 PM

I like the idea that when you die you get to evolve into higher and higher levels of beings until you can become the God of your very own Universe! Kinda like moderating a forum, I bet, only with a few extra perks.

But the whole not drinking coffee and listening to the Osmonds thing - I'm still traumatized by the Donny and Marie Show from the 1970s.

http://www.timewarptv.com/Default.aspx?tabid=143

Posted by: ljk4-1 Feb 11 2006, 09:53 PM

Looks a lot like Europa! cool.gif


UB 313: Larger than Pluto

Illustration Credit & Copyright: Thierry Lombry

Explanation: What do you call an outer Solar System object that is larger than Pluto? Nobody is yet sure. The question arose recently when 2003 UB313, an object currently twice as far out as Pluto and not in the plane with the rest of the planets, was verified recently to be 30 percent wider than Pluto. UB313's size was measured by a noting its distance from the Sun and how much infrared light it emits. Previous size estimates were based only on visible light and greatly affected by how reflective the object is.

Whether 2003 UB313 is officially declared a planet will be answered shortly by the International Astronomical Union. In the above picture, a scientific artist has imagined UB313 in its distant orbit around the Sun coupled with a hypothetical moon.

http://antwrp.gsfc.nasa.gov/apod/ap060207.html

Posted by: BruceMoomaw Feb 12 2006, 06:08 AM

If they declare Pluto a planet but 2003 UB313 a non-planet -- a proposal I have heard seriously made by one member -- I am officially going to give up on Western civilization.

Posted by: David Feb 12 2006, 03:48 PM

QUOTE (ljk4-1 @ Feb 11 2006, 09:53 PM) *
In the above picture, a scientific artist has imagined UB313 in its distant orbit around the Sun coupled with a hypothetical moon.

http://antwrp.gsfc.nasa.gov/apod/ap060207.html


That would have to be an incredibly long-distance zoom shot to get the sun that size -- at the distance 2003 UB313 is, the sun ought to be tiny. And where does the back-illumination that's brightening the back side of "Xena" (but not, curiously, "Gabrielle") come from?

Posted by: tty Feb 12 2006, 07:31 PM

QUOTE (ljk4-1 @ Feb 11 2006, 08:12 PM) *
I like the idea that when you die you get to evolve into higher and higher levels of beings until you can become the God of your very own Universe! Kinda like moderating a forum, I bet, only with a few extra perks.
http://www.timewarptv.com/Default.aspx?tabid=143


Reminds me of an episode in the old "B C" comic strip. Curls, the local philosopher is preaching reincarnation, and explains that everything will be reborn in a higher form.

"Then what are humans reborn as?"

[brief silence]

"Money, I suppose"

tty

Posted by: Katie Feb 13 2006, 04:06 AM

QUOTE (Big_Gazza @ Feb 3 2006, 03:01 AM) *
I have heard a rumour that one of the Mormons secret beliefs is that Heaven is not on another plane of existance but is on a hidden planet past the orbit of Pluto. Yeah, I know sounds kinda wierd, but who am I to judge rolleyes.gif

Anyway, with 2003 UB313 being discovered, what is the opinion of the good folks from Salt Lake City? At least they can have the certainty of knowing they have somewhere to go, though I don't think they need to pack sunglasses and tanning lotion. A nice warm jacket might be well advised.



It's not true. wink.gif

I'm a life long Mormon but until you posted it here I'd never heard anything about Heaven being a space planet. I know it sounds entertaining but the truth is Mormons believe the same creation story as most Christians found in genesis and that Heaven is a spiritual plane of existance rather than a planet. Sorry if I crushed anyone's beliefs of us as crazy space people. tongue.gif

QUOTE (ljk4-1 @ Feb 11 2006, 11:12 AM) *
I like the idea that when you die you get to evolve into higher and higher levels of beings until you can become the God of your very own Universe! Kinda like moderating a forum, I bet, only with a few extra perks.

But the whole not drinking coffee and listening to the Osmonds thing - I'm still traumatized by the Donny and Marie Show from the 1970s.

http://www.timewarptv.com/Default.aspx?tabid=143


Being a single woman I don't get a planet. sad.gif Probably my biggest issue with the mormon church is how they treat single people.

I was born in 1980 so I guess I got lucky and missed the Donny and Marie show. My dad always called it the Donny and Mafreak show. Not sure why.

Posted by: ljk4-1 Feb 13 2006, 04:55 AM

QUOTE (Katie @ Feb 12 2006, 11:06 PM) *
Being a single woman I don't get a planet. sad.gif Probably my biggest issue with the mormon church is how they treat single people.

I was born in 1980 so I guess I got lucky and missed the Donny and Marie show. My dad always called it the Donny and Mafreak show. Not sure why.


That is deeply unfair. You have every right to be a goddess as much as the next Mormon female - or non-Mormon female, for that matter.

If you can, track down a few episodes of Donny and Marie. You will fully understand the reactions of your father and myself after watching them, trust me.

Posted by: Bob Shaw Feb 13 2006, 12:05 PM

QUOTE (ljk4-1 @ Feb 13 2006, 04:55 AM) *
That is deeply unfair. You have every right to be a goddess as much as the next Mormon female - or non-Mormon female, for that matter.

If you can, track down a few episodes of Donny and Marie. You will fully understand the reactions of your father and myself after watching them, trust me.


No! No! Not the white, shiny teeth! And Jimmy, the poisoned dwarf Osmond...

...when I read about 'heaven' being a *real* planet I could go to, I nearly joined up on the spot, but if it's just that spiritual malarkey then I'll stick with the Old Deep Ones, thanks. You know where you stand with unspeakable evil from the depths of space and time, which is basically in the queue to be eaten.

Aiiii!

Bob Shaw

Posted by: BruceMoomaw Feb 13 2006, 11:37 PM

Actually, it's kind of flattering that the Great Old Ones (did Lovecraft ever realize that the acronym was "GOO"?) consider us tasty enough to be eaten. Great Cthulhu seems to have considered us a positive delicacy -- you know how hard it is to stop after just one human...

Posted by: Bob Shaw Feb 14 2006, 08:57 AM

QUOTE (BruceMoomaw @ Feb 13 2006, 11:37 PM) *
Actually, it's kind of flattering that the Great Old Ones (did Lovecraft ever realize that the acronym was "GOO"?) consider us tasty enough to be eaten. Great Cthulhu seems to have considered us a positive delicacy -- you know how hard it is to stop after just one human...


Bruce:

Er, not personally, never been able to finish a whole one. Is there something you're trying to tell us?

Hmm...

Bob Shaw

Posted by: edstrick Feb 14 2006, 10:41 AM

"I can't believe I ate the ***WHOLE*** THING!

Posted by: dvandorn Feb 14 2006, 03:34 PM

Personally, I'd be more worried about your stuffed animals!

http://www.logicalcreativity.com/jon/plush/01.html

-the other Doug

Posted by: BruceMoomaw Feb 14 2006, 10:07 PM

QUOTE (Bob Shaw @ Feb 14 2006, 08:57 AM) *
Bruce:

Er, not personally, never been able to finish a whole one. Is there something you're trying to tell us?

Hmm...

Bob Shaw


Nope. Most of my enemies are, unfortunately, not fit for human consumption.

Posted by: SFJCody Feb 19 2006, 12:34 PM

Brown to discuss 'Xena' discovery

http://www.pasadenastarnews.com/news/ci_3525265

Posted by: SFJCody Feb 21 2006, 10:22 AM

Amateurs spot 10th planet
http://www.spaceflightnow.com/news/n0602/20xena/

Posted by: SFJCody Mar 4 2006, 10:27 AM

The IAU will publish beginning of September 2006 the definition of a "Planet".
http://www.iau.org/TRANS-NEPTUNIAN_OBJECT_2003_UB.324.0.html

Posted by: BruceMoomaw Mar 4 2006, 10:48 PM

Dear God, I hope they don't decide that Pluto is a planet but this new thing isn't. I could tolerate anything but that...

In any case, now we can look forward to comparable decade-long squabbles about the precise definition of "moons" as opposed to "ring particles", and whether we should call newly discovered objects "asteroids" or "comets" when we don't know how much ice is in them. The joys will be endless!

Posted by: Bob Shaw Mar 4 2006, 10:58 PM

QUOTE (BruceMoomaw @ Mar 4 2006, 10:48 PM) *
Dear God, I hope they don't decide that Pluto is a planet but this new thing isn't. I could tolerate anything but that...

In any case, now we can look forward to comparable decade-long squabbles about the precise definition of "moons" as opposed to "ring particles", and whether we should call newly discovered objects "asteroids" or "comets" when we don't know how much ice is in them. The joys will be endless!


Bruce:

Comets are getting drier and drier, and some asteroids might be quite wet...

...sounds like tearing hair out time!

Bob Shaw

Posted by: SFJCody Mar 19 2006, 10:42 PM

No news for a while, which is kind of a shame. I heard a rumour that the satellite might have been an imaging artifact, but it's probably not true.

Posted by: Rob Pinnegar Mar 20 2006, 05:09 PM

Well, it's only been a month or so since the last news update. It would be great if there was something new every day, but, "that ain't so".

Posted by: SFJCody Apr 11 2006, 05:45 PM

UPDATE!

http://www.gps.caltech.edu/~mbrown/planetlila/

Posted by: ljk4-1 Apr 11 2006, 06:08 PM

FOR RELEASE: 1:00 pm (EDT) April 11, 2006

Erica Hupp/Dwayne Brown
Headquarters, Washington
(Phone: 202/358-1237/1726)

Ray Villard
Space Telescope Science Institute, Baltimore, Md.
(Phone: 410/338-4514; E-mail: villard@stsci.edu)

PRESS RELEASE NO.: STScI-PR06-16

HUBBLE FINDS THAT THE 'TENTH PLANET' IS SLIGHTLY LARGER THAN PLUTO

NASA's Hubble Space Telescope has resolved the "tenth planet," nicknamed
"Xena," for the first time and has found that it is only just a little
larger than Pluto. Though previous ground-based observations suggested
that Xena was about 30 percent greater in diameter than Pluto, Hubble
observations taken on Dec. 9 and 10, 2005, yield a diameter of 1,490
miles (with an uncertainty of 60 miles) for Xena. Pluto's diameter, as
measured by Hubble, is 1,422 miles.

Xena is officially catalogued as 2003 UB313. It is the large object at
the bottom of this artist's concept. A portion of its surface is lit by
the Sun, located in the upper left corner of the image. Xena's
companion, Gabrielle, is located just above and to the left of Xena.

For electronic images and additional information about the research on
the Web, visit:

http://hubblesite.org/news/2006/16

http://www.nasa.gov/hubble

For more information, contact Robert Tindol, California Institute of
Technology, Pasadena, CA, (phone) 626-395-3631, (e-mail)
tindol@caltech.edu, or Mike Brown, California Institute of Technology,
Pasadena, CA, (phone) 626-395-8423, (e-mail) mbrown@gps.caltech.edu .

The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. The Space Telescope Science
Institute in Baltimore conducts Hubble science operations. The Institute
is operated for NASA by the Association of Universities for Research in
Astronomy, Inc., Washington.

Posted by: BruceMoomaw Apr 12 2006, 12:09 AM

Robert Roy Britt, in his blog on this subject ( http://www.livescience.com/blogs/author/robbritt ), positively DEMANDS that both Pluto and 2003 UB 313 not be called "planets" yet, unless the IAU gives us permission to do so. Who died and made YOU King of the Solar System, Robert?

Posted by: Bob Shaw Apr 12 2006, 12:31 AM

QUOTE (BruceMoomaw @ Apr 12 2006, 01:09 AM) *
Robert Roy Britt, in his blog on this subject ( http://www.livescience.com/blogs/author/robbritt ), positively DEMANDS that both Pluto and 2003 UB 313 not be called "planets" yet, unless the IAU gives us permission to do so. Who died and made YOU King of the Solar System, Robert?


Bruce:

Perhaps the Touch of A Noodly Appendage has elevated him?

Let me rephrase that...

Bob Shaw

Posted by: David Apr 12 2006, 01:22 AM

QUOTE (BruceMoomaw @ Apr 12 2006, 12:09 AM) *
Robert Roy Britt, in his blog on this subject ( http://www.livescience.com/blogs/author/robbritt ), positively DEMANDS that both Pluto and 2003 UB 313 not be called "planets" yet, unless the IAU gives us permission to do so. Who died and made YOU King of the Solar System, Robert?


Well, actually he indicates that if the IAU does give us permission to do so, it will be the sign of the Apocalypse.

Posted by: Stephen Apr 12 2006, 03:37 AM

QUOTE (BruceMoomaw @ Apr 12 2006, 12:09 AM) *
Robert Roy Britt, in his blog on this subject ( http://www.livescience.com/blogs/author/robbritt ), positively DEMANDS that both Pluto and 2003 UB 313 not be called "planets" yet, unless the IAU gives us permission to do so. Who died and made YOU King of the Solar System, Robert?

Well, to be fair he is trying to make a legitimate point:
"Objects like Pluto and 2003 UB313 should be called minor planets or dwarf planets or something else that denotes their relative insignificance compared to the four inner terrestrial planets and the four outer giants. And therein lies the precedent: We already have terrestrials and giants. Just add dwarfs."

That sounds like the very solution astronomers used to resolve the pesky problem of Ceres and the asteroids (aka "minor planets") back in the 1800s.

Unfortunately, given we already do have a class of astronomical bodies dubbed "minor planets" you have wonder whether it would be wise to either extend that name to Kuiper Belt objects or invent a special term ("dwarf planets") for KB objects that could well end up be confused for the other one.

In particular, I notice astronomers themselves seem to be at pains to avoid using the word "asteroid" for KB objects. Yet if they did start using "minor planet" as a label for such objects, or a name which could well be confused with it (eg "dwarf planets"), then the existing association in many people's minds between "minor planets" and "asteroids" may well eventually lead to the "asteroid" label being applied to KB objects also, even if only by lay folk.

***

One further point on the more general issue of "is it/is it not a planet?"

The whole debate seems to depend not on the nature of the object but on its size. Yet in a sense it is not just size that seems to matter either. It is arguably as much about numbers. So long as there was only one body in the asteroid belt or out in the Kuiper Belt nobody--including astronomers--seemed to much mind calling both "planets". It was only when asteroids and Kuiper Belt objects started to proliferate like rabbits that astronomers started getting cold feet about the term.

Yet curiously this need to distinguish Pluto and Ceres from planets does not apply to two other general terms: stars and moons.

There are red supergiants out there and there are red dwarfs, but at the end of the day they are all termed "stars". (Only with "brown dwarf" does there seems to be a reluctance to use the word; and then it's arguably because of their nature: brown dwarfs are seen as failed stars.)

So too with "moon". While we sometimes hear the term "moonlet" bandied about, nobody seems to be (yet) suggesting we reserve that prestigious term "moon" for the larger objects circling planets and use some other label ("minor moons"? "dwarf moons"? "lunar objects"? "orbital rubble"?) for the riff-raff. (Although maybe astronomers are holding fire on that debate until Cassini or some successor probe actually images a few of the house-size "moons" in Saturn's rings.)

That raises the question of whether this present "is it/is it not a planet" debate--as well as the one back in the 1800s--doesn't involve more than modicum of--er--snobbery. Or to phrase the issue another way, I cannot help feeling the only reason this debate has arisen at all with "planet" is because Earth just so happens to bear the label "planet"; and despite all that has come and gone there is still a subconscious wish, even amongst some astronomers, for Earth to be part of a group with a certain degree of--shall we say--exclusivity.

Meaning that if Earth had been a moon rather than a planet would we now be arguing over whether to admit Janus to the hallowed ranks whilst not giving two hoots about using "planet" for Pluto and Ceres?

Thus, it is all right for Jupiter to be called a "planet". That then puts Earth among the giants. smile.gif We are even prepared to tolerate midgets like Ceres and Pluto being one--so long as there was only just one of each. Once there start to be too many of such small fry the feeling seems to be that the term "planet" is losing its currency. Hence, while nobody seems to mind labelling Janus or Miranda "moons" were they orbiting the Sun rather than Saturn & Uranus nobody would be calling them "planets". Similarly with Pluto or (say) Ida. Were Pluto or Ida in orbit around a planet astronomers would be quite happy to label both of them "moons". Only when they start circling the Sun does size suddenly become an issue.

======
Stephen

Posted by: nprev Apr 12 2006, 03:41 AM

The IAU & everyone else has to face up to one fundamental fact: the objects in the Solar System exist along a continuum of sizes, from individual hydrogen atoms to Jupiter. Defining what is and is not a planet will always be a purely arbitrary convention by any objective standard, except for the apparent distinction that a planet has to independently orbit the Sun. Maybe it's time to throw out the concept entirely...?

Well, short of that heresy, maybe we just need to distinguish between "major" and "minor" planets. If that definition were adopted, I'd say that Mercury becomes the standard minimum body, and we have eight major planets. (Let's face it: it's embarrassing that Pluto is only half the size of the Moon!)

Posted by: Stephen Apr 12 2006, 04:45 AM

QUOTE (nprev @ Apr 12 2006, 03:41 AM) *
Well, short of that heresy, maybe we just need to distinguish between "major" and "minor" planets. If that definition were adopted, I'd say that Mercury becomes the standard minimum body, and we have eight major planets. (Let's face it: it's embarrassing that Pluto is only half the size of the Moon!)

The problem is that it's easy to say that now with just one solar system and nine (or eight or ten, depending on who's counting) planets. What happens when we start finding solar systems where the equivalent of Mercury is the size of Europa or Triton? Is the response of astronomers to be: Sorry, you have to be "this big" to qualify as a planet. smile.gif

======
Stephen

Posted by: BruceMoomaw Apr 12 2006, 04:51 AM

I remain convinced of what I said in 1999: if you are going to call some things "planets" at all, it must be on the basis of their being above some size -- and since that size is ultimately entirely arbitrary, let's set it at some figure that (just barely) lets Pluto retain its historical status that everyone's used to, while not letting a lot of smaller riffraff join the club. 2000 km seems the perfect figure for this purpose. Then -- for the sake of everyone's sanity -- let's DROP THE SUBJECT.

Posted by: Betelgeuze Apr 12 2006, 05:15 AM

QUOTE
The problem is that it's easy to say that now with just one solar system and nine (or eight or ten, depending on who's counting) planets. What happens when we start finding solar systems where the equivalent of Mercury is the size of Europa or Triton? Is the response of astronomers to be: Sorry, you have to be "this big" to qualify as a planet.

For some reason the exoplanets in the PSR 1257+12 system never get a lot of attention, yet they are as small or even smaller as Pluto. Did we discover small planets or are they the first asteroids/minor planets we found around another star?

Posted by: Jyril Apr 12 2006, 05:45 AM

PSR 1257+12 B and C are http://vo.obspm.fr/exoplanetes/encyclo/star.php?st=PSR+1257%2B12 more massive than the Earth, A is about the size of the Moon. There's probably yet another body, but not even the discoverers call it a planet as it is has only one fifth the mass of Pluto.

Posted by: Stephen Apr 12 2006, 06:05 AM

QUOTE (Jyril @ Apr 12 2006, 05:45 AM) *
PSR 1257+12 B and C are http://vo.obspm.fr/exoplanetes/encyclo/star.php?st=PSR+1257%2B12 more massive than the Earth, A is about the size of the Moon. There's probably yet another body, but not even the discoverers call it a planet as it is has only one fifth the mass of Pluto.

Interesting.

Yet again it seems to be only the term "planet" which attracts this debate. Consider neutron stars. They are not even as big as many asteroids or Kuiper belt objects, much less Pluto, yet no astronomer seems to be suggesting that their diminutive size means we should stop calling them "neutron stars" and dub them (say) "neutron objects" or "neutron dwarfs" instead.

======
Stephen

Posted by: Jyril Apr 12 2006, 08:16 AM

It's the mass that dominates. Even the least massive neutron stars are more massive than the Sun, and the upper limit of neutron stars is somewhere three times solar mass. So calling them "dwarfs" would be misleading despite their tiny size. But of course calling them stars is also wrong because they are not real stars anymore.

Posted by: David Apr 12 2006, 08:28 AM

QUOTE (nprev @ Apr 12 2006, 03:41 AM) *
The IAU & everyone else has to face up to one fundamental fact: the objects in the Solar System exist along a continuum of sizes, from individual hydrogen atoms to Jupiter. Defining what is and is not a planet will always be a purely arbitrary convention by any objective standard, except for the apparent distinction that a planet has to independently orbit the Sun. Maybe it's time to throw out the concept entirely...?

Well, short of that heresy, maybe we just need to distinguish between "major" and "minor" planets. If that definition were adopted, I'd say that Mercury becomes the standard minimum body, and we have eight major planets. (Let's face it: it's embarrassing that Pluto is only half the size of the Moon!)

Earth people are known for their curious definitions, but this one is quite astounding; every schoolchild knows that planets, other than the minor planets, have an atmospheric depth of at least 5000 km, are massive enough to attract a large system of satellites, and have rings.

Some silly proposals have been made to include the rocky, almost atmosphere-less inner bodies as "planets", but they have perished from their self-evident absurdity. After all, our schoolchildren have only four tentacles; why should we expect them to memorize more than four planets? Jupiter, Saturn, Uranus, and our own beloved Neptune: it was good enough for Zaljyx, and it's good enough for me!

Posted by: edstrick Apr 12 2006, 09:31 AM

....it was good enough for Zaljyx, and it's good enough for me!

If you think these verses floor us
Then go write another chorus
Just as long as you don't bore us
Then it's good enough for me!

http://www.whitetreeaz.com/vintage/realotr.htm

For 880 verses to "REAL OLD TIME Religion" and enough not yet entered to put the count over 1000.

(Yes... they've got verses to offend everybody who isn't laughing so hard their glasses fog up)

Posted by: Decepticon Apr 12 2006, 01:24 PM

Here is a image comparison... http://www.nasa.gov/images/content/146570main_xena_size.jpg

Posted by: JamesFox Apr 12 2006, 01:37 PM

When I read that blog by Robert Roy Britt, I noticed the he acts as if it's a proven scientific fact that Pluto and 2003 UB313 are not planets, but the only reason he gives is that the 8 other planets have low inclination, as opposed to high ones. He also ignores (like most others), the fact that the term minor planet is already in use.

Many of those objecting to Pluto's status also seem to act as if the whole purpose of defining a planet is to provide a list for schoolchildren. In doing so, they tend to automatically ignore extrasolar planets - some even go so far as to declare that planets must orbit our sun (or even worse, insist that they must orbit at Neptune's distance or closer!) . They then compile lists of attributes that separate Xena and Pluto from the other 8, and use them to make anti-Kuiper belt planet definitions.

The inclination argument is one I've seen before. For example, I remember seeing a nice chart of inclination versus mass that seems to boost the anti-pluto case. However, a number of large extrasolar planets have very high eccentricities, even crossing the orbits of other planets in thier systems. Although their inclinations are unknown, it seems likely that some of them have high inclinations. In addition, would there be some sort of rule against having Pluto in a Quaoar-like orbit? Perhaps something like this exists in some extrasolar system.

I suspect that many people simply don't like Pluto or Xena, and feel that they are too small and wierd to be included alonside Mars or the Earth. So definitions are invented to exclude them, in an attempt to pretend that they are not being arbitary. Ignoring extrasolar planets makes this easier. When you look at these definitions more closely, however, hard boundaries and classifications are revealed as just as arbitrary as a lower size limit.

In the end, planets are largeish, round, and do not orbit planets seems to be the only general (if vague) definition in common use, before people started to try to kick Pluto out of the club. Since there is, in my opinion, no accepted scientific definition, a formal planet definiton should take the previous into account.

Also, we have to deal with the minor planet issue.

Posted by: AndyG Apr 12 2006, 01:46 PM

QUOTE (nprev @ Apr 12 2006, 04:41 AM) *
Defining what is and is not a planet will always be a purely arbitrary convention by any objective standard, except for the apparent distinction that a planet has to independently orbit the Sun.

Independently? Well, that rules out the Earth-Moon binary system then. ;-)

Andy

Posted by: angel1801 Apr 12 2006, 02:01 PM

QUOTE (TritonAntares @ Feb 2 2006, 09:57 PM) *
...as Triton did.

As child I read in an old astronomy library book from the '50s - I think it was from Otto Struve - Triton's diameter should be ~6000 km:
'Wow, a moon nearly as large as Mars...' ohmy.gif
I'm not quite sure what it was before Voyager II - maybe about 3500 km,
but at the the end it came down to poorly 2720 km, probably the same size as UB313 now.

If we get more cases like Pluto, Triton, UB313, we'll find a 'shrinking law' from 'detection-diameter' to real diameter at the end... tongue.gif

Bye.


I do remember that someone used the "speckle infrarometry" method to measure Triton's diameter before the Voyager 2 flyby. They got a figure of 2,500 km. Could they use this method on UB313?

Posted by: Planet X Apr 12 2006, 04:48 PM

QUOTE (angel1801 @ Apr 12 2006, 09:01 AM) *
I do remember that someone used the "speckle infrarometry" method to measure Triton's diameter before the Voyager 2 flyby. They got a figure of 2,500 km. Could they use this method on UB313?


Right now, I wouldn't be surprised to see 2003 UB313 end up being smaller than Pluto. Later!

J P

Posted by: ljk4-1 Apr 12 2006, 06:08 PM

Astrophysics, abstract
astro-ph/0604245

From: Michael Brown [view email]

Date: Tue, 11 Apr 2006 17:18:35 GMT (43kb)

Direct measurement of the size of 2003 UB313 from the Hubble Space Telescope

Authors: M.E. Brown, E.L. Schaller, H.G. Roe, D.L. Rabinowitz, C.A. Trujillo

We have used the Hubble Space Telescope to directly measure the angular size of the large Kuiper belt object 2003 UB313. By carefully calibrating the point spread function of a nearby field star, we measure the size of 2003 UB313 to be 34.3$\pm$1.4 milliarcseconds, corresponding to a diameter of 2400$\pm$100 km or a size $\sim5$% larger than Pluto. The V band geometric albedo of 2003 UB313 is $86\pm7$%. The extremely high albedo is consistent with the frosty methane spectrum, the lack of red coloring, and the lack of observed photometric variation on the surface of 2003 UB313. Methane photolysis should quickly darken the surface of 2003 UB313, but continuous evaporation and redeposition of surface ices appears capable of maintaining the extreme alebdo of this body.

http://arxiv.org/abs/astro-ph/0604245

Posted by: ljk4-1 Apr 12 2006, 09:37 PM

Tenth planet as bright as fresh snow

It is only slightly larger than Pluto, new images prove - but this
means the distant world must be incredibly reflective, suggesting it
is constantly being resurfaced.

http://www.newscientistspace.com/article/dn8985?DCMP=NLC-nletter&nsref=dn8985

Posted by: David Apr 13 2006, 07:46 PM

QUOTE (edstrick @ Apr 12 2006, 09:31 AM) *
If you think these verses floor us
Then go write another chorus
Just as long as you don't bore us
Then it's good enough for me!


Gimme that old time solar system
Extra planets? Never missed 'em!
If with one hand you can list 'em
Then it's good enough for me!

Jove and Mars, war's bloody fury,
Saturn, Venus and MerCUry,
I decide, as judge and jury,
That they're good enough for me!

("What's the answer to the riddle,
"'Where is Terra?'" you may quibble --
She is fixed right in the middle,
And that's good enough for me!)

Little Pluto? Let 'em bomb it!
Thinking of it makes me vomit,
'Cause it's nothin' but a comet --
It's not good enough for me!

Neptune, "god of all the ocean" --
Just a tiny star in motion!
Why should it cause a commotion?
It's not good enough for me!

Uranus, or else UrAnus
Who knows what its bleedin' name is?
No, don't tell us, you can't train us --
It's not good enough for me!

Don't care what the latest rage is
Gimme the System of the sages
Living in the Middle Ages
It's good enough for me!

Posted by: tedstryk Apr 14 2006, 04:28 AM

QUOTE (Stephen @ Apr 12 2006, 06:05 AM) *
Interesting.

Yet again it seems to be only the term "planet" which attracts this debate. Consider neutron stars. They are not even as big as many asteroids or Kuiper belt objects, much less Pluto, yet no astronomer seems to be suggesting that their diminutive size means we should stop calling them "neutron stars" and dub them (say) "neutron objects" or "neutron dwarfs" instead.

======
Stephen


Stars have a clear definition, in that they are capable or were capable of sustained nuclear fusion. So there are clear criteria here. The problem is that, despite what Britt thinks, the term planet is still largely pre-scientific, without clear criteria behind it.

QUOTE (nprev @ Apr 12 2006, 03:41 AM) *
The IAU & everyone else has to face up to one fundamental fact: the objects in the Solar System exist along a continuum of sizes, from individual hydrogen atoms to Jupiter. Defining what is and is not a planet will always be a purely arbitrary convention by any objective standard, except for the apparent distinction that a planet has to independently orbit the Sun. Maybe it's time to throw out the concept entirely...?

Well, short of that heresy, maybe we just need to distinguish between "major" and "minor" planets. If that definition were adopted, I'd say that Mercury becomes the standard minimum body, and we have eight major planets. (Let's face it: it's embarrassing that Pluto is only half the size of the Moon!)


That works until we find a KBO larger than Mercury.

Posted by: edstrick Apr 14 2006, 09:50 AM

"Don't care what the latest rage is...Gimme the System of the sages...Living in the Middle Ages...It's good enough for me!

Yeehaw!

It may be rather hard to find, I haven't see in reprinted in ages, but look for Lester Del Rey's "The Sky is Falling", which I think was originally published in John. W. Campbell's "Unknown Worlds", the hard-fantasy companion to Astounding Science Fiction, sadly killed by WW-II paper shortages. There was a soft cover digest-mag sized reprint around 1960 in the US.

One classic scene has the resurrected engineer hero looking at a piece of the crystal sky-sphere that had fallen, with a glowing dot of star inside the crystal.

Posted by: Bob Shaw Apr 14 2006, 11:51 AM

QUOTE (edstrick @ Apr 14 2006, 10:50 AM) *
"Don't care what the latest rage is...Gimme the System of the sages...Living in the Middle Ages...It's good enough for me!

Yeehaw!

It may be rather hard to find, I haven't see in reprinted in ages, but look for Lester Del Rey's "The Sky is Falling", which I think was originally published in John. W. Campbell's "Unknown Worlds", the hard-fantasy companion to Astounding Science Fiction, sadly killed by WW-II paper shortages. There was a soft cover digest-mag sized reprint around 1960 in the US.

One classic scene has the resurrected engineer hero looking at a piece of the crystal sky-sphere that had fallen, with a glowing dot of star inside the crystal.


And he gets the girl!

Bob Shaw

Posted by: BruceMoomaw Apr 14 2006, 07:21 PM

QUOTE (tedstryk @ Apr 14 2006, 04:28 AM) *
That works until we find a KBO larger than Mercury.


Actually, it doesn't work right now. The trouble is that some of those super-Mercurian iceballs may be in the Oort Cloud, and if so we'll NEVER know whether they exist there or not -- they're too far away for any conceivable type of direct observation. (The remarkable orbit of Sedna, which is pretty big itself, is suggestive.) Alan Stern suggested long ago that there's a real chance of some icy planetesimals out there bigger than Earth (indeed, we can't quite rule them out in the Kuiper Belt at this point).

Alan HAS suggested an alternative "scientific" definition for the minimum size of a planet -- namely, objects big enough to round themselves gravitationally. The trouble is that

(1) This definition, too, has seriously fuzzy edges -- consider Iapetus' distinct but not overwhelming departure from the spherical, and the fact that Proteus looks like a marshamallow. The last straw may have been the discovery of the remarkable rapid spin rate and resulting high elongation of 2003 EL61, which approaches Pluto's diameter on its long axis -- but is only half that on its short axis. Unlike the upper-size scientific definition of a planet -- an object too small to ignite deuterium fusion in its interior, as brown dwarfs do -- the "roundness" definition seems to me just too seriously shaky.

(2) If we do accept it, then there's also a veritable swarm of new objects which will have to be called planets, including at least four asteroids and a very big collection of KBOs. This may be acceptable to scientists, but the public will raise hell.

Combine these two factors, and I think that in the end we have to accept that the definition of "planet" will be unavoidably highly arbitrary. (2003 EL61 produces enough problems by itself; its long diameter is about 2000 km, which would fit my own proposal for defining a "planet" -- but its short axes don't.)

Posted by: David Apr 14 2006, 08:18 PM

QUOTE (BruceMoomaw @ Apr 14 2006, 07:21 PM) *
Alan HAS suggested an alternative "scientific" definition for the minimum size of a planet -- namely, objects big enough to round themselves gravitationally. The trouble is that

(1) This definition, too, has seriously fuzzy edges -- consider Iapetus' distinct but not overwhelming departure from the spherical, and the fact that Proteus looks like a marshamallow. The last straw may have been the discovery of the remarkable rapid spin rate and resulting high elongation of 2003 EL61, which approaches Pluto's diameter on its long axis -- but is only half that on its short axis. Unlike the upper-size scientific definition of a planet -- an object too small to ignite deuterium fusion in its interior, as brown dwarfs do -- the "roundness" definition seems to me just too seriously shaky.


But if you turn the definition on its head, and say that a minor planet is something that is small enough that it can stably retain a non-round shape, then you almost have a workable boundary, one that for "major" planets would exclude objects smaller than Iapetus or 2003 EL61 (and possibly even larger objects). And instead of adding in "a swarm" of small objects to the "major" planet category, the only marginal additions (at this date) would be Pluto, 2003 UB313, and possibly 2003 FY9. It would also place the boundary just around the 2000 km diameter measurement that has been proposed as well.

Of course, we might settle all this, and then when New Horizons arrives at Pluto, discover that it's actually shaped like a top hat with the crown pointing toward us. laugh.gif

Posted by: Planet X Apr 14 2006, 09:08 PM

I came up with a unique definition according to mass awhile back. Basically, I set the lower size limit for a major planet at the lowest possible diameter for a body with the density of water to have a mass at 0.001 Earth mass. Incredibly, the result came out to be a diameter of 2250 km! In this scheme, the only known objects in the Kuiper Belt that would qualify as major planets would be 2003 UB313 and Pluto. 2005 FY9 and 2003 EL61 are out! Later!

J P

Posted by: SigurRosFan Apr 17 2006, 04:52 PM

Interesting comment:

- http://www.astro.uni-bonn.de/~bertoldi/ub313/index.html

Posted by: SFJCody Apr 17 2006, 06:17 PM

Brown et al make more measurements with Keck.
http://www2.keck.hawaii.edu/schedule/index.php?sched=Both+Tels&year=2006&month=-2&go=GO#

Wed Mar-15 0 1 M. Brown Schaller, Barkume HQ NIRC CIT JR MK SJ C196N

Thu Mar-16 7 1 M. Brown Schaller, Barkume HQ NIRC CIT JR MKoc SJ C196N


Thu Apr-20 61 2 M. Brown M. Brown, Schaller HQ OSIRIS-LGS(5) CIT CW/JR JL SJ C213OL


Fri Apr-21 69 2 M. Brown M. Brown, Schaller HQ OSIRIS-LGS(5) CIT JR JL SJ C213OL

Posted by: BruceMoomaw Apr 17 2006, 06:45 PM

QUOTE (David @ Apr 14 2006, 08:18 PM) *
But if you turn the definition on its head, and say that a minor planet is something that is small enough that it can stably retain a non-round shape, then you almost have a workable boundary...


No, you've still got a seriously fuzzy one, given the question of what constitutes "round". (Iapetus? Proteus?) Ultimately, you HAVE to set some kind of arbitrary figures for the different dimensions of such worlds. (2003 EL61 presents a problem already, since it's almost as wide as Pluto on its long axis but only half as wide on its short axes. I suppose you'll have to make my "2000 km diameter" definition an object's minimum diameter along any axis to call it a planet.)

Posted by: David Apr 17 2006, 07:17 PM

QUOTE (BruceMoomaw @ Apr 17 2006, 06:45 PM) *
No, you've still got a seriously fuzzy one, given the question of what constitutes "round". (Iapetus? Proteus?)


Nope and nope. In any case, 2003 EL61 is considerably larger than both, and is clearly "unround", which would put both objects well below the seriously fuzzy boundary.

QUOTE
Ultimately, you HAVE to set some kind of arbitrary figures for the different dimensions of such worlds.


Don't expect any argument from me on that point. biggrin.gif

Posted by: JRehling Apr 17 2006, 07:59 PM

As if this issue hadn't already been faced on Earth with respect to what is a Mountain/Hill/Knoll or what is a River/Stream/Creek or what is a City/Town/Village.

In fact, the usages are quite arbitrary on Earth, depending upon geography, comparison class, and the particular language. There are mountains in Maryland that no one in western Colorado would even be able to perceive. I laughed when a New Yorker speaking Italian used "paese" to refer to Brooklyn. The natives of North America thought that the Allegheny/Ohio was a single river that was joined by the Monongahela where we now have Pittsburgh. The French agreed with them, while the English gave the Allegheny and the Monongahela equal status as sub-entities that joined to make the Ohio. (There, identity as well as category becomes an issue.)

The real heart of the "planet" problem is that scientists who often find the need for precise, hierarchical, mutually-exclusive categories come from the same species that found the "river" issue ultimately unimportant, or highly contextual and which yearns to do the same with the planet issue.

It's pretty useful to distinguish between viruses and bacteria, but I can't figure a reason why it is so important to distinguish between the category Pluto belongs to and the category Triton belongs to. Suppose we found such a body in another solar system that spent several orbits looping around a gas giant but would regularly be ejected into solo orbits until being periodically recaptured. Would we really want to say that Zelph is a planet and will remain so until Apr 22, 2078, when it will cease to be a planet for a span lasting until May 3, 2198?

Suppose an object were adjudicated to be 0.9999 the mass/diameter of an arbitrary cutoff until a new measurement added the needed quantity (to 1.0001) to cross the magic threshold. Would we want to say that it is a planet, defying our previous belief? The motivation for such a category eludes me.

Categories are a form of "data reduction"... a swirl of complex properties can be communicated with one handy label. They make sense when the world is chopped up that way. We've learned that nothing along the line of "planet" is so handy as we thought it was before (choose your date: 1801, 1978, 2003, 2005). What would it take to make a learned scientist realize that "planet" isn't a category like "bacterium" (scientifically useful), but is more like "river" (folksy and contextual)? A committee of longbeards is the last group who deserves to get their hands on this.

Posted by: The Messenger Apr 17 2006, 08:45 PM

QUOTE (JRehling @ Apr 17 2006, 01:59 PM) *
A committee of longbeards is the last group who deserves to get their hands on this.
::applause::

I vote that we use time-of-discovery as the cutoff, and anything discovered after 1950 is something other than a planet. Nine is hard enough on grade school kids, a few of whom think science should be something other than a bunch of mythological name tags to hang on orbiting objects.

While I am at it, I want one time zone PERIOD: UTC; and we all adjust our schedules accordingly. I reset my alarm three times trying to figure out when Venus Express was being placed into orbit, and still overslept rolleyes.gif

Posted by: dvandorn Apr 17 2006, 08:47 PM

Well -- for scientific purposes, it would be far more useful to catelogue Solar System bodies using several different criteria, such as size, mass, primary/secondary composition, current location and presumed location of origin. That's a six-criteria statement, which isn't all that hard to represent in some form of classification system.

But that's categorization for scientific purposes. As has been recognized here, there are also cultural purposes for such categorizations. And those criteria are far different. That's a point I've been trying to make for quite some time.

-the other Doug

Posted by: BruceMoomaw Apr 17 2006, 09:59 PM

Size makes more sense than discovery date, by any standard (scientific or popular). After all, two planets were discovered after we started discovering asteroids.

Posted by: JamesFox Apr 18 2006, 01:56 AM

When it comes to 2003 EL61, it's odd shape is due to a super-high rotation rate. Since many planets in the Solar System are oblate spheroids, one could perhaps say that objects count, if they were spherical if not rotating.

Irregardless of what is decided, I'll state that I'll be happy if at least small, spherical worldlets (like Ceres, Quaoar, and Sedna), are given a status distinct from regular small asteroids, KBO's, and comets. Even if they are not as impressive as the like of Mercury or Mars, they are still much more than 'lumps of rock', and deserve names, symbols, and sailor senshi biggrin.gif .

Posted by: Stephen Apr 18 2006, 04:15 AM

QUOTE (BruceMoomaw @ Apr 17 2006, 09:59 PM) *
Size makes more sense than discovery date, by any standard (scientific or popular). After all, two planets were discovered after we started discovering asteroids.

IMHO neither make much sense.

But let's suppose for argument's sake some kind of line did get drawn on the basis of size. What happens if KBOs bigger than the agreed line start being found? One or two might well be accommodated, but suppose a dozen or more were found? Are they all to be honoured with the label "planet" or do the goalposts get moved again? (And if the newly found KBOs are allowed the honour why not Pluto?)

This line of argument can be taken further. What happens if KBOs start being found out there which are bigger than Mercury? Do we start calling them planets or does Mercury also get demoted from the "planet" club when the line gets moved yet again to weed out the unwanted?

If there has to be a line of some kind which defines what is a planet and what is not then no matter where you draw it, be it to exclude Pluto or to include Pluto, there surely ought to be some kind of rational scientific justification for putting that line in a particular place. If the position is simply some arbitrary value then what exactly has been achieved by having a line at all, let alone at that particular point?

======
Stephen

Posted by: BruceMoomaw Apr 18 2006, 06:40 AM

Well, you've got to set the goalposts at SOME point. And the case for setting them at just a bit smaller than Pluto is pretty good, precisely because Pluto has been thought of as a planet for so long -- given the rate at which KBOs are being discovered now, it's unlikely that there will ever again be a gap so long between the discovery of KBOs more than 2000 km wide.

Of course, we should also very definitely be teaching kids that at this point the word "planet" doesn't have any scientific significance, that all it means is "object bigger than such-and-such a diameter", and that at this point we have no idea how many "planets" the Sun has even by that arbitrary criterion (and, for that matter, we never WILL know, thanks to the possibility of big Oort Cloud objects). If we're not going to admit that "planet" is strictly an arbitrary standard, we should stop using the word at all -- but I very much doubt that's going to happen. All we can do is make sure that people know what it actually signifies at this point.

Posted by: AndyG Apr 18 2006, 09:08 AM

QUOTE (BruceMoomaw @ Apr 18 2006, 07:40 AM) *
Well, you've got to set the goalposts at SOME point.

Why not set goalposts using the highest surface escape velocity? This neatly captures elements of diameter and mass, and avoids issues of how to define 'rotundity'.

Set limits something like:

asteroid < 750m/s
> 750m/s small planet < 2500m/s
> 2500 m/s medium planet <5000m/s
> 5000m/s large planet

Ceres has an escape velocity of ~500m/s. It's an asteroid, albeit a large one.

Pluto is ~1070m/s. Small planet.

Mercury & Mars are medium planets. Earth & Venus are large.

If the body orbits a planet, it's a moon. Change "planet" to "moon" and set the size qualifier one higher. "Asteroid" changes to "moonlet". So Titan and Ganymede (~2770m/s) become "large moons" and Europa (2040m/s), as with our Moon, become "medium moons". Miranda is a moonlet. And quite right too.

Andy G

Posted by: ugordan Apr 18 2006, 09:21 AM

QUOTE (AndyG @ Apr 18 2006, 10:08 AM) *
"Asteroid" changes to "moonlet". So Titan and Ganymede (~2770m/s) become "large moons" and Europa (2040m/s), as with our Moon, become "medium moons". Miranda is a moonlet. And quite right too.

Somehow calling a 500 km diameter object (such as Enceladus) a "moonlet" is a bit of a stretch for me. These moons are indeed small by standards of our own Moon, but they still merit being called regular moons. After all, they are round, not irregular. Are you also suggesting we put Enceladus in the same category as, for example, Daphnis, a recently discovered moonlet around Saturn, which is a couple of km, tops?

Posted by: edstrick Apr 18 2006, 10:23 AM

Also... at all 4 giant planets, there is a clear division of moons with relatively few objects that are really transitional.

There is "outer gravel", of which Phoebe is a large example.

There are large and small "major moons". Large are Triton size and bigger, small are Iapetus/Rhea/Titania/Oberon size and smaller down to the smallest round moons

There are "inner gravel" moons and "ice chips", all clearly substantially smaller than the smallest round moons with no as yet proven overlap.

And there are Intra-ring moonlets and ring rubble.

All 4 systems except Neptune's have these features fully developed, though scaling between gas giant and ice giant systems confuses things a bit.

At Neptune, the capture of Triton into an elliptical retrograde orbit apparently took out all the pre-existing major moons, but the outer and inner gravel and rings are still there.

Clearly there is a pattern shared with variations between all 4 outer planet systems and we need to pick "natural" divisions to define a nomenclature.

Posted by: David Apr 19 2006, 08:31 PM

QUOTE (AndyG @ Apr 18 2006, 09:08 AM) *
Why not set goalposts using the highest surface escape velocity? This neatly captures elements of diameter and mass, and avoids issues of how to define 'rotundity'.


Yes, it does capture "elements of diameter", but it does so in a distinctly non-intuitive way -- keep the mass of an object the same, but increase its diameter, and its surface escape velocity will decrease. It's only because larger objects tend to have considerably more mass than smaller ones that a list of objects by escape velocities looks anything at all like a list of objects by diameter or mass. (Surface gravity is even worse; going by gs, Neptune outranks Saturn, Earth outranks Uranus, and Mars and Mercury are almost the same). The information you get from a ranking by escape velocity you might get just as well, and less ambiguously, using a ranking by mass.

The question of where to draw the line remains in any case. Let's say we use escape velocity. Should we put the division at one quarter the Ve of Earth's, admitting only Mercury-size and above? Or, let's say, one-tenth the Ve of Earth, letting in Pluto? And we're going to have to wait a while to make a judgment on 2003 UB313, because we're still unsure about both its mass and diameter.

In short, it's no worse a proposal than others, but doesn't seem to be any better, either.

Posted by: edstrick Apr 20 2006, 09:36 AM

The arm-waving idea I've posted here before is that a planetesimal becomes a planet when, during accretion, its gravity field is great enough so that it's accretional capture diameter is significantly, perhaps 2 times, greater than it's physical diameter.

During accretion, at any given time and any given distance from the sun, there is a mean encounter velocity between planetisimals. It's probably bigger near resonances and near accreting giant planets. The mass of an accreting body and the average encounter velocity between it and candidate stuff to be accreted determines the "capture diameter".

Something 1 km across will have a capture diameter of something like 1.0001 km. Something 10,000 km across might have a capture diameter of 25,000 km. That's what lets accreting planets start to grow faster than their smaller competitors and end up eating the competition. That really is a "physics of origins" related distinction between planet and planetesimal

Posted by: ljk4-1 Jul 21 2006, 02:40 PM

Though it is not online, the July 24, 2006 issue of The New Yorker magazine
has an article on the tenth "planet" and its discoverer, Michael E. Brown.

Powered by Invision Power Board (http://www.invisionboard.com)
© Invision Power Services (http://www.invisionpower.com)