# Is this 80.000km-diameter world plausible?

Characteristics:

• has close to 80.000km in diameter
• The atmosphere is composed of:

-30% hydrogen (cold gas planets have hydrogen but it makes sense it exists in a rocky planet?)

-20% oxygen

-49% nitrogen

-1% carbon dioxide and others

• The planet round these temperatures: -90ºC and - 40 ° C

• The clouds are so many that occupy between 10 km and 100 km of the atmosphere, not letting the "sun" almost illuminate the ground, only a few mountains.
• Snowing and raining a lot.
• The "normal" winds are 50 km / hour.
• There are mountains that reach the 20,000 meters.
• Can live animals with a lot of body fat and hair (such as seals and polar bears) in areas with warm water lakes (heated by dormant volcanoes)
• You can't have mountains that tall, either. At least, not made out of the kind of rocks we know about. Jan 22 '15 at 9:21
• It looks like we may have an I18N issue here with the use of commas and dots as either decimal or digit-group. So to clear it up, is your world a miniscule 8x10^2 km or is it an enormous 8x10^5 km? Dec 14 '15 at 20:48

### Atmosphere

At a diameter of 80.000km, you'll have a massive atmosphere. The exact composition will depend on distance from the star, but keep in mind that your planet is larger than Neptune. Your atmosphere will consist of lots of hydrogen and helium, which will combine to form ammonia at lower altitudes. Earth has less of these elements in its atmosphere because they can escape from its gravity well pretty easily, but your planet is closer to the size of the gas giants.

You're correct in that the clouds will block out the sun, but off in magnitude: at 80.000km you'll likely have thousands of kilometers of clouds.

### Weather

Your planet will be raining quite a bit, but keep in mind that that rain probably won't be water. With a hydrogen rich atmosphere, you'll be forming lots of ammonia, particularly at high pressures. If your planet is fairly cold, all of the water will form into ice.

Wind speeds will be far higher than 50km/h. On Jupiter, wind speeds are closer to 400km/h, and they're significantly higher than that on Neptune.

### Life

Life may be able to exist, but seals and polar bears would not. A planet of that size will have far to much gravity and far to dense of an atmosphere to support life forms like seals. Any life that evolved would likely be monocelular, and it would be tough for any immigrants to survive on this planet.

### What to change

If you want earth-like life, you'll have to significantly reduce the size of your planet. Huge planets are tough to live on. This will also reduce the amount of hydrogen in your atmosphere, since most of it will escape into space.

You may also consider increasing the average temperature of your planet, or else adding additional explanation for why the water evaporates. At below freezing, the sun won't drive the water cycle, and a few dormant volcanoes likely wouldn't be enough to do so either. It may be possible, however, to generate heat through tidal heating if your planet is in a close orbit around a brown dwarf. This could result in a warmer ocean under a cooler atmosphere, with evaporation driven by internal heat and precipitation dominated by snowfall. All land areas would probably be snow covered, with sub-glacial rivers fed by the accelerated snowmelt from the internal heating. Glacial movement would also be much quicker than on Earth.

• and if the diameter is 20.000 km and the temperatures round: - 40ºC and 5ºC? I liked that the planet was cold, the clouds to block (but perhaps not completely) the sun and that could exist as life on earth (in cold places). this theory of brown dwarf is interesting, but the red light would send to another planet in the same solar system cannot have a very different tone from the Earth :) Dec 24 '14 at 20:18
• That would be more reasonable, though you'll still have around twice the gravity of Earth. In addition, an earth that cold wouldn't form as many clouds as modern earth. You'll only have evaporation from the ocean in regions where the water temperature is above freezing, so large portions of your planet will be desert-like if it's mostly below freezing, unless you have other significant mechanisms for achieving significant evaporative rates to drive the water cycle. Dec 24 '14 at 20:49
• well, it seems I exaggerate these temperatures well :) then, if the planet were smaller (the 10,000km in diameter?), temperatures were between -40 ° C and 10 ° C (the lowest temperatures in the mountains and higher areas) would be able to "create" a cold planet, with snow and many clouds in the sky? also like to know if some live volcanoes could arise on the planet without destroying completely the snowy landscape. :) Dec 25 '14 at 13:23
• One could reduce the overall gravity using a less dense planet. Jan 22 '15 at 10:03
• @ballah Earth has a diameter of 12742 km (mean).
– user
Jan 22 '15 at 10:40

A solid-crust planet with a radius of 40,000 km? With an Oxygen-Hydrogen atmosphere? Oh my. Where do I start?

We have to start from the basics.

It might not seem like a lot, in space. We are, after all, used to millions and billions of km in the solar system and light-years in the galaxy. However, when it comes to planets, tens of thousands is a lot. Let's give your planet a good name, like Blobby.

Planet  Mean Radius (km)       Mean Radius Compared to Earth (Earth = 1)
Mars        3389                       0.52
Venus       6051                       0.95
Earth       6371                       1.00
Neptune    24622                       3.86
Uranus     25362                       3.98
Blobby     40000                       6.27
Saturn     58232                       9.14
Jupiter    69911                      10.97


So Blobby is about bigger than the ice giants, and half-way between Neptune and Saturn. Ok, so it's like 6.3 Earth radii, no biggie, right? The reason why this is a big deal will become apparent as we move on to the next sections.

Planetary Volume
Using a Fermi approx. and assuming it's a perfect sphere (and leaving aside the atmospheric weight), we can use $\frac4 3\pi r^3$. For Earth, with a radius of 6.371 million meters, that's roughly $1.09\times10^{21} m^3$. Your planet's radius is 6.27 times higher. 6.27 cubed is 247. So your world would have about 250 times the volume of the Earth. Are we getting worried yet?

Planetary Density The density of the materials comprising a "rocky" planet can vary roughly from $3 g/cm^3$ (pure rock) to about 8 $g/cm^3$ (pure metal). In addition to this, the force of gravity compresses the planet somewhat, making the radius smaller and the density higher. The bigger the planet the higher the mass, obviously, so Earth (5.5 $g/cm^3$) is more compressed than the Moon, Mercury or Mars. Blobby is big. Going forward, we'll assume Earth density (given compression, it'll be about 70% rock, 30% metal). Just for fun. In reality, there is a trend where planets with radii up to 1.5 Earth radii increase in density with increasing radius, but above 1.5 Earth radii the average planet density rapidly decreases with increasing radius, indicating that these planets have a large fraction of volatiles by volume overlying a rocky core (in other words, more like Neptune than Earth).

Mass
If we somehow assume Earth density of 5510 kg / m3, given Blobby's volume, of $2.68\times10^{23}m^3$, we get $1.47\times10^{27}$, which is rather close to the mass of Jupiter (with 2/3 of the radius).

Surface gravity
You can estimate surface gravity by using this formula:
$g=\frac{GM}{r^2}$
Our surface gravitational acceleration is 61.6 $m/s^2$, that's 6g. That's crushing. Not only do you not get 20,000m tall mountains, you're lucky to get little hills. Even if we reduce planetary density in half, you still get 3g. If we let a lot of the radius be gas and clouds, you get crushed by the atmospheric pressure instead. There's no way out.

So that covers the solid crust planet with the ginormous radius and the outsized mountains.

Let's move on to the part where the atmosphere is 20% oxygen when there's lots of hydrogen (30%). No, no, no. Oxygen is a reducing gas. The atmosphere would ignite given the first spark (say caused by lightning). Pure hydrogen-oxygen flames are so energetic they emit light in the UV spectrum. Your whole atmosphere would go down like the Hindenburg.

TL;DR: Hell, no. That sort of whale of a planet would either crush you to smithereens though gravity, or squish you flat as a pancake by sheer atmospheric pressure.

• While the gravity would be high that doesn't preclude high-g lifeforms. Likewise, pressure is not a killer to a creature adapted to it. Jan 23 '15 at 5:05
• "30% Hydrogen, okay... and... what's this? 20% Oxygen?" --> One spectacular firestorm later you'll have a lot less atmosphere left (~9% Oxygen, 89% Nitrogen, 2%CO2), but lots of ice. Dec 15 '15 at 10:46

With no sun and so low temperatures your planet would have hard time to develop plants to provide food for all other lifeforms.

As a result it would have no oxygen (it would oxidize other minerals), especially hydrogen. Hydrogen and oxygen do not mix (see Hindenburg).

Without food chain food pyramid would have hard time to establish. Most what you may hope for would be some extremophiles bacteria, capable of surviving in some salty ponds which do not froze because of salt.

Life requires liquid water.

Or better: biochemistry of life, as we currently understand it, requires water.

• Re "Life requires liquid water": We do not know this for certain. All we know is that the only type of life we're familiar with requires it. It's also theoretically possible for water-based life to arise around hot springs or subsurface oceans on cold planets. See e.g. Europa. Jan 22 '15 at 3:27
• I wouldn't say no oxygen--he's already postulating geothermal sources, they could also power life. There would be little oxygen, though. Jan 23 '15 at 5:06
• Where such "little oxygen" would come from? AFAIK, the only source of free oxygen is photosynthesizing bacteria. Which is life. No life, no free oxygen: oxygen is reactive and would oxidize other elements, especially in geothermal vents. Do you have other source of free oxygen? Jan 29 '16 at 23:04

I can't believe nobody's caught the biggest problem here:

30% hydrogen/20% oxygen/50% nitrogen (the traces don't matter) is a combustible mixture. The first spark and if there are by some magical means any survivors they're in a 10% oxygen/90% nitrogen atmosphere.

Oxy-hydrogen atmospheres are possible but not at these ratios.

• Actually, Serban Tanasa did in his answer above. But yes, this is the biggest problem. Jan 23 '15 at 1:52
• @HDE226868 I missed it, it was so far down below all the planetary size/gravity calculations, I didn't notice it wasn't all about getting squashed. Jan 23 '15 at 1:58
• I didn't see it either, until I saw your post. Jan 23 '15 at 1:59