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Saturn - the biggest floater under the sun?

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Ian Dunn
390646.  Thu Aug 07, 2008 1:17 pm Reply with quote

It is reported in the "Dictionaries" episode of Series D of QI that the "Biggest floater under the Sun" is Saturn, because it's density is less than water. However, it looks like QI is wrong again, if this source is to be believed.

If there was a bathtub big enough to hold it, Saturn would float

Is it even worth bothering to consider what would happen if there was a bathtub big enough to contain the planet Saturn? It's probably about as much use as considering whether you would be justified in killing Jack the Ripper if you could go back in time before he started his murder spree (and if you knew who he was). Still, since it's clearly one of the greatest scientific questions of our time, and since the UK's National Space Centre in Leicester has been bold enough to exhibit a model of Saturn floating in a bath let's contemplate the details for a moment.

You might instinctively think of a large planet like Saturn - at something like 760 times the volume of the Earth - as being "heavy", and therefore the idea of it floating in a big bathtub full of water seems unlikely. But weight, of course, is the result of gravity so for any floating to be going on you'd probably want to position your enormous bathtub on an even more enormous planet, which could act as a stable surface while exerting gravitational pull on your water and your Saturn. If your planet - let's call it Balneum, the Latin word for "bath" - wasn't more massive than Saturn then everything would just end up sticking to Saturn since it would exert the stronger gravitational attraction. Or something.

You'd also need to be careful about what sort of planet Balneum is: a planet like Saturn itself wouldn't be such a great idea, because in common with Jupiter, Neptune and Uranus, Saturn does not have a solid surface. Although Saturn has a small core of rock and ice, it is mostly made up of hydrogen, in its metallic form around the core, and then in its gaseous form for the rest, with a bit fo helium and some other stuff thrown in for good measure. You could then argue the case as to whether the bulk of what we call Saturn is actually the "atmosphere" around the real planet itself - in which case Saturn, as a solid core, certainly wouldn't float in your bathwater.

However, if you take the "visible" extent of Saturn - ignoring the rings for a moment - as being the planet, it is certainly the only planet in our solar system to be less dense than water, having an average density of 0.6873 g/cm3 [cubed]. In this ridiculous bath-time scenario, though, Saturn still would not float since its rock-and-ice core would be so much heavier than its gaseous outerlayers, so the core would immediately sink through the hydrogen-helium gases and down through the water to the bottom of the bath while the gaseous outer layers would settle above the surface of the water. (Or, if Balneum is a planet with an atmosphere similar to Earth's, rather than no atmosphere at all, Saturn's gases would start to drift upwards and away from the bath, since hydrogen and helium are less dense than the Earth's air.)

In other words, this nonsense about Saturn floating in the bath is based purely on the idea that its average density is less than that of water - but it doesn't take account of the fact that the planet is not an approximately consistent density all the way through being part gas and part solid. While Saturn is in its current position in the solar system, the outer layers of gas are attracted by the gravitational pull of its core; but the minute you stick it in a bath on a more massive planet, the centre of gravity will be at the centre of Balneum, meaning that both the core and the outer gaseous layers are all pulled towards Balneum, at which point the difference in the relative densities and states of Saturn's core (a solid much denser than water and Saturn's outer layers) and its outer layers (gases much less dense than water) would come into effect. Do you really care, though?


Returning finally to Saturn's rings, these would melt if your bath water was hot, since they are mostly ice particles.

Source: Emus Can't Walk Backwards: Another Round of Dubious Pub Facts by Robert Anwood. Pages 92-95.

Last edited by Ian Dunn on Wed Dec 10, 2008 4:28 pm; edited 3 times in total

390688.  Thu Aug 07, 2008 3:11 pm Reply with quote

The only thing mentioned in the quote is that Saturn is not a solid object capable of maintaining it's structure when placed in a solar system sized bath.

Although not stated explicitly, I'd say that this was implicit in the answer. Perhaps Stephen should have given a quick disclaimer to this effect to be clear... or maybe he did? I can't remember.

Ian Dunn
390790.  Fri Aug 08, 2008 1:56 am Reply with quote

As far as I can recall, all he said was that Saturn would float if you found a big enough body of water.

The problem with such a body of water is that you would have to rest it on a body bigger than Saturn, therefore there would be a problem with the gravity.

The core of Saturn is denser than water so that would sink. The gases would just settle above the surface, depending on the atmosphere of the larger body. If the larger body at a nitrogen-rich atmosphere like Earth, then the hydrogen and helium-rich atmosphere of Saturn would float away from the bath because it is lighter.

So the problem is that Saturn could not float because you have to take into account where the water is placed.

EDIT: Added guide to the episode here and YouTube clip.

391409.  Sat Aug 09, 2008 9:16 am Reply with quote

Fair enough, and an interesting and worthwhile expansion on the simple point that
(Saturn) is certainly the only planet in our solar system to be less dense than water

which is what caught our eye.

However, these two comments:
Is it even worth bothering to consider what would happen if there was a bathtub big enough to contain the planet Saturn? It's probably about as much use as ... (etc)

Do you really care, though?

are fatuous, IMO.

Nice Retractions Special / additional material, in that I don't think it invalidates the point being made, but it certainly indicates that there's more to be said.

Ian Dunn
391413.  Sat Aug 09, 2008 9:47 am Reply with quote

Thanks very much Flash. I understand that what some of what is said is just the opinion of the author, but it does go into much more detail than what was given in the episode.

391415.  Sat Aug 09, 2008 10:15 am Reply with quote

If you could find a plastic bag big enough to fit Saturn and then place the sealed plastic bag in a ginormous bathtub, it would float.

Ian Dunn
391432.  Sat Aug 09, 2008 12:11 pm Reply with quote

Efros wrote:
If you could find a plastic bag big enough to fit Saturn and then place the sealed plastic bag in a ginormous bathtub, it would float.

How would that work exactly?

391434.  Sat Aug 09, 2008 12:16 pm Reply with quote

Overall density would be less than that of water, providing of course you had a humungous planet on which to site this ginormous bathtub with probably several solar systems supply of water and a huge plastic bag containing Saturn.

All in a days work...

391459.  Sat Aug 09, 2008 3:08 pm Reply with quote

what i don't get is that even with gas planets you often here an atmosophere mentioned. i mean, if it's all gas, how do they determine where the planet ends and the atmosphere begins?



Ian Dunn
391530.  Sun Aug 10, 2008 2:42 am Reply with quote

Well Almond, the think you have to note is that Saturn does have a core. All the gas planets have one. This core is denser than water and would sink.


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