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Ilya
2010-Nov-15, 06:07 PM
A recent occulation shows that dwarf planrt Eris is smaller than astronomers had thought -- same size as Pluto, perhaps even slightly smaller. That does not change Eris' known mass -- more than a quarter more than Pluto. Which means Eris is significantly denser. And this just does not make sense...

Mike Brown's Livejournal blog:

http://syndicated.livejournal.com/browns_planets/20695.html

baric
2010-Nov-15, 06:28 PM
I've never understood the primacy that volume is given over mass in the scientific community. Mass just seems like a more reliable measure of size than volume.

Eris is bigger than Pluto. Neptune is bigger than Uranus. Mercury is bigger than Titan.

Just my two cents.

Ara Pacis
2010-Nov-15, 06:50 PM
Size is based on seeing, mass is based on weighing. (I know the difference between mass and weight, but the former can be sense remotely fairly accurately, while the latter needs to be calculated and requires more input and context.

baric
2010-Nov-16, 03:06 AM
Size is based on seeing, mass is based on weighing. (I know the difference between mass and weight, but the former can be sense remotely fairly accurately, while the latter needs to be calculated and requires more input and context.

I'm not so sure it's that straightforward. Unless you find a satellite or an occultation, you are left with wildly variable assumptions about albedo.

Fortunately for Eris, we have both :D

Bynaus
2010-Nov-16, 09:57 AM
I've never understood the primacy that volume is given over mass in the scientific community.

I don't think this is the case. On the contrary, I think scientists are much more likely to consider the "size" of an object to depend on its mass, rather than its diameter/volume.

The interesting thing, in the case of Eris, is the fact that a small diameter, together with a high mass, means a high density. Which is interesting, because it is unexpected.

AndreasJ
2010-Nov-16, 10:14 AM
I think "size" is a woolly concept that should be avoided in scientific discourse. (And as I suggested in another thread, science journalists should be forbidden from using the word at all.)

I'm not sure why Brown thinks giant* collisions couldn't account for the variability of the larger bodies? Why couldn't a big impact blast off a lot of the ice of a less crazy proto-Eris?


* It feels a bit funny to talk about "giant collisions" when the total mass of the Kuiper belt is unlikely to exceed that of Theia, the body that slammed into early Earth in the giant impact.

Jerry
2010-Nov-16, 10:47 PM
We get a mass for Pluto from the moons...where/how is the mass of Eris calculated?

...never mind, I didn't know about Dysnomia, Eris's moon.

So we have Dwarf planets in the same general orbit with very different density...

...why am I not surprised?

slang
2010-Nov-16, 11:33 PM
We get a mass for Pluto from the moons...where/how is the mass of Eris calculated?

...never mind, I didn't know about Dysnomia, Eris's moon.

Really! Well, remember that Mike Brown chose the name (http://en.wikipedia.org/wiki/Dysnomia_%28moon%29#Name) for several reasons, one of them being that the name means "lawlessness", a seemingly not so subtle reference to the controversy about Eris' initial name of Xena, which was (IIRC) denied because Xena wasn't a 'real' goddess (irony a-plenty). My head tells me that "dysnomia" resonates with "wrongly named", but I don't trust my memory of Latin and Greek classes of 30 years ago. Anyway, the naming of both this dwarf planet and its moon is a great story all by itself.

grant hutchison
2010-Nov-16, 11:56 PM
Really! Well, remember that Mike Brown chose the name (http://en.wikipedia.org/wiki/Dysnomia_%28moon%29#Name) for several reasons, one of them being that the name means "lawlessness", a seemingly not so subtle reference to the controversy about Eris' initial name of Xena, which was (IIRC) denied because Xena wasn't a 'real' goddess (irony a-plenty).Mike Brown tells it (http://www.gps.caltech.edu/~mbrown/planetlila/) a little differently:
... the name Xena (and satellite Gabrielle) were simply placeholders while awaiting the IAU's decision on how an official name was to be proposed.
We officially suggested the name [Eris] on 6 September 2006, and it was accepted and announced on 13 September 2006.Eris is the goddess of discord, and:
In the astronomical world, Eris stirred up a great deal of trouble among the international astronomical community when the question of its proper designation led to a raucous meeting of the IAU in Prague. At the end of the conference, IAU members voted to demote Pluto and Eris to dwarf-planet status, leaving the solar system with only eight planets.
Grant Hutchison

slang
2010-Nov-17, 12:46 AM
Thanks for the correction, I should have checked. Still, the process and background of naming of both bodies makes for an interesting story (and might help remembering that Eris has a satellite).

danscope
2010-Nov-17, 04:34 AM
Just think: Eris could be made of ..... half GOLD and half iron . But it isn't gonna do you any good.

AndreasJ
2010-Nov-17, 07:53 AM
Just think: Eris could be made of ..... half GOLD and half iron . But it isn't gonna do you any good.
Nah. The infered density, while higher than expected, is way too low for a body with a very high content of such elements.

Dysnomia contains Gk nomos "law, custom", not Lat. nomen "name".

Cobra1597
2010-Nov-17, 02:37 PM
Nah. The infered density, while higher than expected, is way too low for a body with a very high content of such elements

True, those elements would be exceedingly unlikely. Still, the difference in density with respect to Pluto is interesting for reasons other than "there be gold in that dwarf!" While their probably isn't gold, the difference in density could still suggest a different composition than Pluto's, which would tell us that Kuiper Belt Objects may have a more varied compositional makeup than we thought before this discovery.

That isn't the only possible explanation for the difference in density, but it is a plausible and interesting one. We should all be even more excited and expectant for the New Horizons mission to get out there! Granted, it will not be able to study Eris, but hopefully it can study enough Kuiper Belt Objects to give us an idea of the range of compositions, densities, and other physical and chemical characteristics. We've more interesting things to look for now than we did before.

AndreasJ
2010-Nov-17, 06:53 PM
True, those elements would be exceedingly unlikely.
danscope suggested Eris is all gold and iron, which isn't unlikely: it's flatly impossible.

Still, the difference in density with respect to Pluto is interesting for reasons other than "there be gold in that dwarf!" While their probably isn't gold, the difference in density could still suggest a different composition than Pluto's, which would tell us that Kuiper Belt Objects may have a more varied compositional makeup than we thought before this discovery.

That isn't the only possible explanation for the difference in density, but it is a plausible and interesting one.
Er? What other possible explanations do you have in mind? Eris is too small for gravitational compression to be significant, and Pluto is too big for porosity to be.


We should all be even more excited and expectant for the New Horizons mission to get out there! Granted, it will not be able to study Eris, but hopefully it can study enough Kuiper Belt Objects to give us an idea of the range of compositions, densities, and other physical and chemical characteristics. We've more interesting things to look for now than we did before.

For getting a handle of the distribution of properties I'd expect more from Earth-based observation. NH's chief significance will be giving us a close look at a (not necessarily representative) sample.

Ilya
2010-Nov-19, 09:22 PM
So we have Dwarf planets in the same general orbit with very different density...

...why am I not surprised?
You tell us. Why aren't you surprised?

Mike Brown obviously was surprised.