A ninth planet?

[fredinno]

1 hour ago, K^2 said:

Atmospheric pressure is limited by vapor pressure of constituents. Methane is going to be frozen out, and Helium won't be sufficiently abundant. That leaves a Hydrogen-dominated atmosphere. And in plausible temperature ranges, that gives us upper limits on pressure in 1-10 bar, perhaps as high as 100 bar if there is a strong greenhouse effect, but that's a stretch already. Anything else will condense and rain down as Liquid Hydrogen.

These are very low pressures. Conditions much closer to what we see on rocky worlds than gas giants. I'll grant you, that this is very unusual for a Gas/Ice Giant, or even a large Super-Earth, but only because our study is focused on much warmer worlds. We tend to see the smaller worlds have a solid surface, and large worlds have layers. But the boundary depends on size and temperature of the world. A good example, once again, is Venus. It's an Earth-sized world, but due to its temperature, it already starts exhibiting some of the features we expect of a large Super-Earth. Likewise, Nine should be an ice giant. And had it remained in the outer system, it would certainly be one of these layered planets with no well-defined surface. But with temperatures as lows they should be on that world, I expect an environment that at the first glance looks very terrestrial. A well-defined surface, either icy with lakes of Hydrogen, or a global Hydrogen ocean, and above it, an atmosphere of a few bar with clouds of Hydrogen mist. All on a world with a surface gravity 30%-50% higher than that on Earth. If it weren't for temperatures, I'd say it'd make a good world for a colony.

But wouldn't internal heating heat the interior up to the point where it would never be solid? I mean, solid hydrogen takes place at 14K. Does anybody have any idea how much the interior alone would've at it up.. Neptune's heating is primarily internal, with the internal heating being 2.6x the external heating, leaving it at 72K at 1 bar. If it ends up being 40K (removing external heating entirely, and reducing the heat flux by 10K). That would be higher than either the points for Solid or liquid hydrogen. That means the solid/liquid slush layer would be very deep in, perhaps 100-200 bar (Iom just making a guess for this one).

[RuBisCO]

5 hours ago, cantab said:

A planet of the mass proposed is going to have a hot core from when it formed, which will be several thousand Kelvins. When the middle is that hot, whether the outside is at 20 K or 200 K doesn't make a lot of difference to how long it takes to cool.

Whether it has a solid surface then depends on how the gradients in pressure and temperature with depth compare to the melting point of whatever it is made of. There are I think two plausible structures. Either way there will be an outer atmosphere of helium and probably hydrogen. If that atmosphere is thin the pressure and temperature at its base will be such that methane is solid and there will be a solid surface. If the atmosphere is thick enough though it will grade into methane gas or supercritical fluid, with no solid surface.

Considering how much hydrogen and helium Neptune and Uranus have, at 15 Earth masses overall, I think for a planet of ten Earth masses the latter structure is *far* more likely.

I think it is best to consider what solid methane is like even at extreme cryogenic temperatures: it would have the mechanical strength of lard (lard at our temperature of course). A planets surface made of lard is not like anything we know of.

Also consider Uranus's cloud deck temperature is 120-55 k and it's black body temperature at its average distance from the sun (19.2 AU) is 90 K, Neptune is averaging 73 K, and its blackbody temperature at 30 AU is 72 K. So there is very little internal heating. This theorized planet is getting far less sunlight, enough equivalent to blackbody temperatures of 11-27 K. Using Neptune and Uranus as very poor rules of thumb as they don't have such wide temperature regimes, it is perfectly possible this planet has an average temperature below 20 K, and that hydrogen liquifies naturally. It could have a Helium atmosphere and hydrogen rain, seas of hydrogen-methane sludge. 

Edited January 22, 2016 by RuBisCO

[K^2]

8 hours ago, magnemoe said:

Uranus is 15 times heavier than earth so I guess this will be like Uranus or Neptune but smaller,

if its in the smaller end it might not keep hydrogen and helium but have an fairly dense atmosphere like titan and ice surface, not an ice giant like Uranus and Neptune but more like an super earth or water planet. 
 

This isn't a guessing game. Take a look at phase and vapor pressure diagrams for relevant substances. Given Nine's distance from the Sun, it's much too cold to be anything like Uranus or Neptune. 

[Nuke]

relevant xkcd is relevant

[Findthepin1]

28 minutes ago, Azivegu said:

Its an ice planet, like Eeloo. Thats why it should be named so and why I created a petition to have it named so here: http://wh.gov/iwz9K

We actually don't know what it is.

[Plusck]

6 hours ago, A35K said:

Why is everyone talking like it is a fact? There is nothing yet that 100% proves its existence, let alone what it is made of. Don't get me wrong, it would be amazing if there was a new planet, but we're not at the stage to be absolutely certain that it's there...

A lot of the media totally ignored the "possibly" part and ran with "there is a ninth planet!".

Of course, it would be a neat explanation for all those oddly bunched orbits, so it's a good fit there. It's also a good fit for our desparate need to fill Pluto's still-vacant position.

[wumpus]

Thor save us all if it is an ice giant...  Then again, I don't think Thor favored those who didn't trust in their own might as well, better do it ourselves.

[K^2]

28 minutes ago, fredinno said:

But wouldn't internal heating heat the interior up to the point where it would never be solid? I mean, solid hydrogen takes place at 14K. Does anybody have any idea how much the interior alone would've at it up.. Neptune's heating is primarily internal, with the internal heating being 2.6x the external heating, leaving it at 72K at 1 bar. If it ends up being 40K (removing external heating entirely, and reducing the heat flux by 10K). That would be higher than either the points for Solid or liquid hydrogen. That means the solid/liquid slush layer would be very deep in, perhaps 100-200 bar (Iom just making a guess for this one).

It can depend on internal composition, of course, and these things do vary planet to planet, but if we assume that it's a Fifth Giant, I would not expect its initial composition to vary wildly in composition from the Ice Giants. And internal heating ramps up really quickly with size. You don't expect a 10M_⊕ to be anywhere close to 60% heat production of 15M_⊕.

I don't get where you are getting the 40K from. Equilibrium temperature at Neptune's orbit is about 50K. Equilibrium temperature at 200AU, the closest Nine gets to Sun, is about 20K. Triple point of Hydrogen is at 32K and 12 bar.

[Bill Phil]

1 hour ago, Just Jim said:

Understood.... I'm just hoping for something really new and weird...

It might have some really weird colors.

[Just Jim]

1 minute ago, Bill Phil said:

It might have some really weird colors.

Hmmmm.... if it's a big ice giant, you would think it would be even colder than Neptune or Uranus. 
And who knows for sure what mix of frozen gasses it would consist of. 
And given the universe is turning out to be a really weird and wonderfully beautiful place (thanks Hubble...).
Yeah.... I think some cool colored clouds is a really good possibility. 

[Nothalogh]

13 hours ago, KerikBalm said:

there goes all the progress done by the test ban treaties

The test ban treaties killed Orion, you call that progress? 

[Bill Phil]

1 hour ago, Nothalogh said:

The test ban treaties killed Orion, you call that progress? 

Even before then, JFK was horrified by the battleship version.

[Bill Phil]

Planet 9 From Outer Space

[Just Jim]

4 minutes ago, Bill Phil said:

Planet 9 From Outer Space

Oh no............ 

[fredinno]

1 hour ago, K^2 said:

It can depend on internal composition, of course, and these things do vary planet to planet, but if we assume that it's a Fifth Giant, I would not expect its initial composition to vary wildly in composition from the Ice Giants. And internal heating ramps up really quickly with size. You don't expect a 10M_⊕ to be anywhere close to 60% heat production of 15M_⊕.

I don't get where you are getting the 40K from. Equilibrium temperature at Neptune's orbit is about 50K. Equilibrium temperature at 200AU, the closest Nine gets to Sun, is about 20K. Triple point of Hydrogen is at 32K and 12 bar.

Neptune is 72 K at one bar, meaning it produces 22K of heat. Ok, that makes more sense. I shouldn't expect Nine to get any more than 25K then. Which would mean the planet would have hydrogen-helium slush after maybe 30-50 bar. (liquid hydrogen is liquid at 20K.) Which is actually lower than Venus' surface. Since the temperature difference between liquid and solid H2 is so low, it doubt it would be a differentiated solid and liquid.

 

This would really make for one weird planet...

[fredinno]

3 hours ago, daniel l. said:

What if planet nine is like Mann's planet in that it has no solid surface but is dominated by frozen clouds?

Apparently, yes, it might work. It would be nothing like it when you step on though, being slushy (due to hydrogen boiling/melting temperatures being so close to each other) and painfully obvious it isn't habitable (it's in the Oort cloud, and a composition of an ice giant!)

In other words, nobody would ever come to save Dr. Mann on Mann's planet.

[AccidentsHappen]

1 hour ago, Bill Phil said:

It might have some really weird colors.

I hope so, I mean look at Pluto. It's colour was unexpected, and so was pretty much everything about it. It's so far away from the sun, yet places like Sputnik Planum (part of Pluto's "heart") is very young, indicating fairly recent cryovolcanic activity. I've always wondered if we could find a planet that's atmosphere is coloured like a soapy bubble. You know, when it goes rainbow and it gets Jupiter storm patterned all over it? That'd be so cool.

[Just Jim]

11 minutes ago, AccidentsHappen said:

I hope so, I mean look at Pluto. It's colour was unexpected, and so was pretty much everything about it. It's so far away from the sun, yet places like Sputnik Planum (part of Pluto's "heart") is very young, indicating fairly recent cryovolcanic activity. I've always wondered if we could find a planet that's atmosphere is coloured like a soapy bubble. You know, when it goes rainbow and it gets Jupiter storm patterned all over it? That'd be so cool.

This is what I keep thinking... Pluto, Charon, and the other moons all turned out to be such a great surprise!  If there is another planet we haven't found yet, I can't begin to imagine what it's going to be, or look, like. 

And as far as I'm concerned, the weirder the better!!! 

[K^2]

3 minutes ago, fredinno said:

I shouldn't expect Nine to get any more than 25K then. Which would mean the planet would have hydrogen-helium slush after maybe 30-50 bar. (liquid hydrogen is liquid at 20K.) Which is actually lower than Venus' surface. Since the temperature difference between liquid and solid H2 is so low, it doubt it would be a differentiated solid and liquid.

This isn't quite how phases work. Relevant values are the melting point, which varies with pressure, but not much. We can just take it at 14K. The triple point, which is about the same 14K, but at 7kPa, so we don't care about it. And the critical point, which is 12 bar and 32K. Between these 14K and 32K, actual boiling point is going to vary with pressure. It's only 20K at 1 bar. At this point, it becomes very important that Nine's atmosphere is almost pure Hydrogen. It's going to have some Helium in it, but not too much.

So lets picture what happens to 30 bar atmosphere at 25K. We look at the phase diagram, and at 25K, the vapor pressure of Hydrogen is 3.5 bar. That means, at anything above 3.5 bar, Hydrogen from atmosphere is going to condense into liquid. It's going to keep condensing, until pressure drops to 3.5 bar. There is absolutely no mechanism to stop it. Which means that at 25K, atmospheric pressure on Nine would be 3.5 bar. It can't get any higher. It can't really be lower, either, at least, until all Hydrogen from all the lakes boils off. That's a possibility, but I think there is enough H₂ there to maintain pressure at 3.5 bar and have enough left over for lakes or oceans.

Helium in atmosphere would provide a touch of buffering. Technically, it's partial pressure of H₂ that's going to be at 3.5 bar, and total pressure would be slightly higher, because partial pressure of He would add to it. That would prevent things from being constantly at the threshold of boiling. It would also allow for things like cloud formation and weather. Though, I suspect, not particularly exciting weather, since the Solar power input is pretty low.

There are essentially two  possibilities here. First, Nine maintained most of the Hydrogen an Ice Giant would start out with. Given a thin atmosphere of 3.5 bar, most of that Hydrogen would end up on the surface, certainly flooding it all. That would result in an ocean world. Constant overcast, perpetual drizzle. Liquid Hydrogen from horizon to horizon. Not very exciting at all.

Second possibility is much more fun. A catastrophic event, such as impact with one of the moons, could have knocked out a huge chunk of that atmosphere when Nine got ejected from the System. This could lead to a world with very little Hydrogen on its surface. The surface of such a world would be dominated by solid Methane, which forms Plastic Crystal at the above temperature and pressure. Which means surface will be solid, but soft. Hard to say just how much plasticity it will have at 25K, though. In either case, significant elevations are not possible, even if there is some tectonics. This means that entire planet would be lowlands, either resembling Titan with its lakes, or an endless marsh. Weather on a world like this could have more familiar patterns with partial cloud cover and actual weather systems.

Second version being more exciting also happens to be less likely, unfortunately. An ocean world seems like the most likely possibility.

[adsii1970]

I believe that NASA is theorizing it is the size of Saturn and is probably another gas giant.

[fredinno]

20 minutes ago, K^2 said:

This isn't quite how phases work. Relevant values are the melting point, which varies with pressure, but not much. We can just take it at 14K. The triple point, which is about the same 14K, but at 7kPa, so we don't care about it. And the critical point, which is 12 bar and 32K. Between these 14K and 32K, actual boiling point is going to vary with pressure. It's only 20K at 1 bar. At this point, it becomes very important that Nine's atmosphere is almost pure Hydrogen. It's going to have some Helium in it, but not too much.

So lets picture what happens to 30 bar atmosphere at 25K. We look at the phase diagram, and at 25K, the vapor pressure of Hydrogen is 3.5 bar. That means, at anything above 3.5 bar, Hydrogen from atmosphere is going to condense into liquid. It's going to keep condensing, until pressure drops to 3.5 bar. There is absolutely no mechanism to stop it. Which means that at 25K, atmospheric pressure on Nine would be 3.5 bar. It can't get any higher. It can't really be lower, either, at least, until all Hydrogen from all the lakes boils off. That's a possibility, but I think there is enough H₂ there to maintain pressure at 3.5 bar and have enough left over for lakes or oceans.

Helium in atmosphere would provide a touch of buffering. Technically, it's partial pressure of H₂ that's going to be at 3.5 bar, and total pressure would be slightly higher, because partial pressure of He would add to it. That would prevent things from being constantly at the threshold of boiling. It would also allow for things like cloud formation and weather. Though, I suspect, not particularly exciting weather, since the Solar power input is pretty low.

There are essentially two  possibilities here. First, Nine maintained most of the Hydrogen an Ice Giant would start out with. Given a thin atmosphere of 3.5 bar, most of that Hydrogen would end up on the surface, certainly flooding it all. That would result in an ocean world. Constant overcast, perpetual drizzle. Liquid Hydrogen from horizon to horizon. Not very exciting at all.

Second possibility is much more fun. A catastrophic event, such as impact with one of the moons, could have knocked out a huge chunk of that atmosphere when Nine got ejected from the System. This could lead to a world with very little Hydrogen on its surface. The surface of such a world would be dominated by solid Methane, which forms Plastic Crystal at the above temperature and pressure. Which means surface will be solid, but soft. Hard to say just how much plasticity it will have at 25K, though. In either case, significant elevations are not possible, even if there is some tectonics. This means that entire planet would be lowlands, either resembling Titan with its lakes, or an endless marsh. Weather on a world like this could have more familiar patterns with partial cloud cover and actual weather systems.

Second version being more exciting also happens to be less likely, unfortunately. An ocean world seems like the most likely possibility.

SO basically you're saying temperature differences for phase changes don't matter, and it would be a relatively shallow ocean of Hydrogen, with solid hydrogen at the bottom?, with sea level at 3-4 bar?

Edited January 23, 2016 by fredinno

[fredinno]

16 minutes ago, adsii1970 said:

I believe that NASA is theorizing it is the size of Saturn and is probably another gas giant.

WISE already deleted any chances of that.

[Vanamonde]

Numerous threads on the same subject have been merged into this master thread. 

[AccidentsHappen]

I'm sad that my thread got merged... 

They got rid of the vote too! Aw well, as long as the theories stay alive, I'm happy!

Edited January 23, 2016 by AccidentsHappen

[K^2]

2 hours ago, fredinno said:

SO basically you're saying temperature differences for phase changes don't matter, and it would be a relatively shallow ocean of Hydrogen, with solid hydrogen at the bottom?, with sea level at 3-4 bar?

I very much doubt that pressure will ever get high enough for solid Hydrogen, and temperature will only increase as you go deeper. Even if it has all of its original Hydrogen, you'll run out of that, before you're deep enough for it to solidify. Next layer bellow liquid Hydrogen will be solid Methane. Likely in Methane I state, which is a Plastic Crystal.

Temperature matters, but a large planet works like a pressure cooker. As you increase the temperature, instead of liquids starting to boil, pressure rises.