# Background

In my story, HD 28185 star system harbors life. One of them is a jovian HD 28185 d (Subralis), located around 1AU of this sun-like star. In-universe, Subralis is approximately 132,000 kilometers in diameters with around six times the mass of jupiter. Its atmosphere should contains water-methane-hydrogen mixtures.

# Question

What is required to make the planet to appear white-blue-ish with white clouds, if the aforementioned settings fail to produce target coloration as posed in this question? A nice fine-tune of its atmosphere's compositon would be appreciated.

# Restrictions

1. This planet should contains water vapors at its atmosphere
2. Methane mixtures could be added at the atmosphere, as long as it doesn't affect the desired coloration
3. Hydrogen could be added to its atmosphere, as long as it doesn't affect the desired coloration

# Edits:

Replacing 420,000 kilometers to 132,000 kilometers, to make the size believeable

• You're going to have to rethink this. At 420,000 km diameter (3 times Jupiter) and with 6 times the mass, Subralis has a density of about .28 kg/m^3, or less than 1/3 that of water and 1/4 that of Jupiter. Since its mass is greater than Jupiter's, its metallic core will be larger, and density will be greater than Jupiter, not less. Jun 27, 2015 at 14:01
• A planet would not be that large, period. Planets don't grow very much once they bet Jupiter sized, as they get denser. If you put 6♃ masses of gas/ice/etc. together, it would be only slightly larger than Jupiter. What could keep it puffed up to an average of .3 $\mathrm{kg/m^3}$? Jun 27, 2015 at 14:11
• I see.., I'll reduce its size to make it more realistic. Its original idea was it expands more as it receives heat more than jupiter. Jun 27, 2015 at 17:21
• @HendrikLie it's a slow but continual process, Jun 27, 2015 at 23:00
• @WhatRoughBeast I had edited the size. I almost forget about this question due to my homeworks and assignments. Aug 29, 2015 at 10:11

Keep the sulphur content down in the atmosphere and you should get your white/blue appearance. Jupiter and Saturn are brown because of all the sulphur compounds that form because of the internal heat. Uranus and Neptune have similar composition to Jupiter but lack the internal heat for the sulphur compounds. This was a really helpful link on gas giant atmospheres.

As for atmospheric stripping, it wouldn't be difficult to give the planet a iron core to generate a sufficiently large magnetosphere.

There are two issues here.

First off, gas and ice giants are unlikely to form in the habitable regions of the solar system, since the combination of higher temperatures (we are inside the "snow line") and greater radiation from the star would tend to blow much of the light elements outwards, past the "snow line" where it would more readily condense or be gravitationally attracted to a forming planet. As the gas or ice giant forms, it can migrate across the solar system by interacting with the other bodies in the system. If it is adding energy to bodies as it passes, they will move outwards while the gas giant moves inwards (the opposite is also true). Eventually, the planet will reach some sort of equilibrium (normally after most of the interacting bodies have been cleared), and in extreme cases could end up as a "Roaster" orbiting the star far closer than Mercury in our own solar system.

The massive gravity and powerful magnetic fields on these planets seems to allow them to maintain their size and composition even at the distance of Mercury, at least so far as current observations seems to show, so a giant that migrated towards the 1AU line should remain a giant for eons.

The second issue is the composition of the world. Gas giants are mostly hydrogen and helium surrounding a large metallic or silicate core. for a blue planet, you want a lot of water, so rather than a Jovian sized world you should consider a planet more like Neptune migrating inwards. As an Ice Giant planet, it has much more water in its composition (technically a water-ammonia ocean, not very pleasant to swim in). I am not entirely sure if the extra energy of the sun will change the composition and appliance of the planet, but I suspect it will be "blue" enough for most purposes.

The issue of white clouds could be a problem. White clouds are condensed water vapour, but a giant planet has an internal heat source, so while a lot of water vapour may be in the atmosphere, it might not condense out the way it does on Earth. Very high altitude cirrus clouds may exist on the night side, but the day side atmosphere will most likely be a very hot, dense and humid mixture of gasses and water vapour.

• I thought of it as jovian planet (with hydrogen-helium major composition), but as it moves closer, its watery compounds starts to resurface. Isn't it plausible? Jun 28, 2015 at 4:54
• The formation of planets where Jupiter and Saturn are seem to favour hydrogen and helium, while farther out (Neptune and Uranus) have much higher percentages of heavier elements like water, ammonia and so on. Jun 28, 2015 at 21:20

There are a couple problems. The first is that Jovian planets atmosphere in general are made up primarily of hydrogen and helium. The second problem is moving a Jovian planet closer to a star to get it in the habitable zone of a star (where water can become liquid) causes another problem. The solar winds will strip off much of the hydrogen and helium from the planet. While this will leave your composition of the atmosphere to be much closer to what you are looking for, it most likely won't be considered a Jovian planet any more, but a large rocky body bigger than earth. Jupiter's center is considered to be 14-18 times the mass of earth, though some of that mass is metallic and liquid Hydrogen, which much of it would boil off as well as the atmospheric pressure dropped during the loss of the gasses.

Neptune has a blue appearance and Uranus is blue. Another this to be aware of is that the more massive the Jovian, the more heat it generates itself, and at 13Mj (masses of Jupiter) is an unofficial cut-off where Brown Dwarfs start.

Picture of Neptune:

Here is an excerpt from wiki about our Jovian's.

Gas giants have some of the characteristics of brown dwarfs. For example, Jupiter and Saturn are both made primarily of hydrogen and helium, like the Sun. Saturn is nearly as large as Jupiter, despite having only 30% the mass. Three of the giant planets in the Solar System (Jupiter, Saturn, and Neptune) emit much more heat than they receive from the Sun.[25] And all four giant planets have their own "planetary systems"—their moons

• Couldn't jovian strong magnetic fields protect the effect? Jun 27, 2015 at 12:49
• @HendrikLie Good question, I would have to guess it would only slow down the process a little bit. Hydrogen and Helium still escape from Earth Jun 27, 2015 at 12:53
• The cited en.m.wikipedia.org/wiki/HD_28185 indicates that itnreally does have a planet of 5.7 ♃ masses 1AU from a star slightly cooler than ours. At 7.5 billion years old, it would be losing gas, hot, etc. Being a real planet there might be more elaborate speculation published in scientific papers. Jun 27, 2015 at 14:26
• I see, bowlturner. They'd be losing gases and heat after certain time. I wonder what would be the condition of the planet at our current time. @JDługosz Jun 27, 2015 at 18:02
• Modeling has been done regarding hot jupiters. This is less extreme. You can find information on that to figure temperature and loss rates. Jun 28, 2015 at 5:23