Of Gas Dwarfs and Waterworlds

[Apollo7]

Ok quick science question here, i'm finally developing a new solar system, using a G4 V type star and an adjusted Bode's Law to place planets, it's not an amazing or flashy system but I've added in two intriguing planet types, a Gas Dwarf and a Waterworld.

I wont go into the particulars of the entire system, as it is far too large, but for the two planets in question the Vital Statistics are:

1856-C (Gas Dwarf)
Mass 3.92182E+25 Kg
Radius 17393.602359 Km
Density 1.779224 gm/cm^3
SMajorAxis: 0.582687 AU
Temp (ambient): 344.78 K

1856-F (Waterworld)
Mass 5.51074E+24 Kg
Radius 7145.104957
Density 3.606582 gm/cm^3
SMajorAxis 2.330748 AU
Temp (ambient): 172.39 K

Now, one question is related to density, should the Gas Dwarf have a density more inline with a rocky planet? Keeping in mind that the said world is about 6.5 times more massive than earth? Also though the Waterworld is far beyond the temperate CZ it's temperature at the waterlevel is 27.01 C with a GreenHouse Ratio of 12.144 (by comparison Earth's is 0.65, and Venus is at 90). So while it might be logical to assume the waterworld has a lower density due to a composition of ices and rock, would the elevated temperature necessitate a fully rocky composition and thus a higher density?

Also the Gas Dwarf orbits close to the parent star and passes within .15 AU of 1856-B, a small mercurian type planet. I have the Dwarf currently with 7 moons, with the outermost orbiting at a distance of about 800,000Km, would it be reasonable to assume that the Gas Dwarf could retain its retinue of moons, despite its closeness to the star and proximity to its nearest neighbor? Might I need to make the inner planet resonate with 1856-C so that it might remain on the otherside of the sun when at Perihelion?

Anyway thats all for now, its good I have a place to actually report this stuff and get feedback, cheers!

[granthutchison]

What's a gas dwarf?

Grant

[Evil Dr Ganymede]

What's a gas dwarf?

Grant

I think he means what I think of as a "Failed Core" - a terrestrial world that's big enough to hold onto hydrogen and helium, but didn't snowball into a fully-blown gas giant for some reason (migration?). So you basically end up with a terrestrial world with a few Earth masses, and a deep H2/He atmosphere.

Or he may mean something like Uranus and Neptune, but AFAIK you can't form those in the inner system - they should only form in the outer system where the accretion times are longer.

[Apollo7]

Ok a Gas Dwaf is as Ganymede has said, it is a terrestrial world, with a large silicate/iron center and a few masses of H/He envelope, you might call it an overblown venus. Mine is less than half as massive as Neptune, and as I recall Neptune is often refered to as an Ice Giant or Sub-Jovian, or am I wrong here?

Also my Green House ratios are not random they came from the site called The Solar System Collabratory Home Page which is hosted by the University of Colorado (persumambly at Boulder), home page is http://solarsystem.colorado.edu/home/lowRes.html.

simply my question revolves around the fact that I haven't created or used these types of planets before and I'm trying to remain at least somewhat true to reality. Stargen creates them and it got me interested in their formation/creation.

[eburacum45]

Waterworlds don't have to be cold to have an ice mantle; it is pressure that keeps the ice mantle solid at the bottom of the deep ocean.

A Gas Dwarf, eh? also known as a failed core, or as John Dollan's PCL had it, a subjovian... would these worlds also not contain a lot of water? or has it all been destroyed by photodissociation because the planet is close to the sun?

Also, if the gas dwarf is close to the star, it might not be able to keep moons in stable orbits-
there is a formula somewhere for this, I'll see if I can find it.

[granthutchison]

OK. Is there any literature on "failed cores/gas dwarfs", or are they completely speculative? On first sight they seem like exceedingly unlikely things to find in the inner system.

With regard to your other questions, I'm not sure I understand what you're after with "waterworld" - I can't see any reason atmospheric greenhouse would influence the overall composition of the planet during formation, but maybe I'm misunderstanding you.
For the "gas dwarf", I'm sure you've already calculated that the satellites are well within the Hill radius for the two perturbers, but beyond that I guess we'd need to know how old your system is, and for how long the gas dwarf has been in synchronous rotation (if it is) before we could say more about the likely survival of the satellites.

Grant

[Guest]

Fialed cores are really a concept that has emerged from the various interstellar RPG'S ;
from here
http://members.tripod.com/Gaspode100/mekton/worlds.htm
we find a definition of sorts
Failed Core - A planet which accumulated an atmosphere but which never accumulated enough mass to become a gas giant

then gives as an example Triton;

which is nothing like the large inner system rock/gas objects we are discussing.

These worlds are often envisaged as having a rocky core overlain by ice and a thick atmosphere; the hot ones lose the ice. But I haven't seen any serious speculation about them from Exoplanetary specialists yet.

[eburacum45]

logs in to claim last post

[Evil Dr Ganymede]

The "Triton" assessment is incorrect - I bet I know where he got that from too (the world generation rules from the 2300 AD roleplaying game use that definition).

There isn't an example of a Failed Core world in the solar system. They are conjectural, but they appear to be possible - you can have large silicate worlds that are massive enough to retain helium but not hydrogen, for example.

I'd imagine they'd be rather rare, since you have to have the right combination of events to form them - large enough silicate body starts to retain H2 and/or He but then has most of its envelope blown away before it can become a full gas giant. I wonder if you'd be more likely to find them around more massive stars (A and F) which form more quickly? I'd imagine they have quite a lot of volatile content on the surface, so they're probably water worlds underneath the atmosphere too.

[Apollo7]

Yes I know the Hill Radii well and in the case of the inner planet the nearby worlds 1856-B and 1856-D are well outside of its Hill Radius. However, we've all heard the stories of how Jupiter (at least in theory) prevented the formation of a planet in the Asteroid Belt, and since asteroids are well beyond Jupiter's Hill Radius, I have to wonder how it realsticaly prevented a planet from forming there.

What I might do is switch the position of the two planets, moving the waterworld within 1 AU and the "failed core" out to 2.5 AU or so. I was also thinking there are some new theories on how the Solar System formed, if I recall reading correctly. They involve the sun being formed in the outskirts of a vast nebula where other, hot and massive stars were forming and then being ejected from that region. I had speculated perhaps the mass of planets is influenced not just by the mass of the solar disc, but the available H/He envelope in the region it forms. Stargen generally places Gas Dwarfs on G type stars or above, though it seems predisposed not to place "waterworlds" in orbit around stars of F class or greater.

When I think of a Waterworld I suppose I think of a quasi-massive, terrestrial planet that has alot of water, and since greater gavity would reduce the height of mountains and landmasses they all sorta reside under a veil of water. Or do I have my mechanics wrong here? In any event I'm still working on this system so thanks for the replies.

[eburacum45]

A large waterworld here, made with a lot of help from Grant;
http://www.orionsarm.com/worlds/Panthalassa.html

available in this collection of add-ons
http://www.orionsarm.com/downloads/OA_Planets_1.zip

[granthutchison]

Yes I know the Hill Radii well and in the case of the inner planet the nearby worlds 1856-B and 1856-D are well outside of its Hill Radius. However, we've all heard the stories of how Jupiter (at least in theory) prevented the formation of a planet in the Asteroid Belt, and since asteroids are well beyond Jupiter's Hill Radius, I have to wonder how it realsticaly prevented a planet from forming there.

I'm not sure what you mean, here. The Hill radius is an estimate of the region in which your planet's gravity dominates over the gravitational effects of its star - within that region, and in the absence of other perturbing masses, satellites will stay within the gravitational influence of their parent planet. But your two neighbouring planets can certainly disrupt that satellite system from outside that Hill sphere, if they're massive enough or move close enough; and the fact that the asteroids are outside Jupiter's Hill sphere simply means that they can't stay in any sort of orbit around Jupiter - it doesn't mean that Jupiter has no significant gravitational influence on their orbits around the Sun.

Grant