# Could a planet larger than earth with half the gravity have a denser atmosphere?

I want to create a world that abides completely by our understanding of reality. The whole premise is that "If the Universe is large enough, this would all be non-fiction", assuming the laws of physics are constant everywhere. It absolutely must be 100% scrutiny-proof.

Firstly, the planet is designed to be a sort of superficial "Parallel Earth". However, the actual workings of the planet and its species can be adapted as needed. The planet will have large amounts of gigantism (pun intended). The planet will include human-like natives; ray-like whales that are 60-80m long; 'Sauropods' on par with the largest titanosaurs. But the most limiting factor is having theropod-like dragons (technically wyverns), preferably upwards of 10m in length.

I've plugged some numbers into various calculations and I've settled on the following properties:

Surface Area: 602,000,000km2 (18% larger than Earth)
Radius: 6.923.5km (8.5% larger than Earth)
Mass: 3.524*10^24kg (41% less than Earth I *think*)
Volume: 1,390,160,000,000km3 (28% larger than Earth)
Gravity: 4.9m/s/s (Half of Earth)
Density: 2.83g/cm3 (54% less than Earth)


With the atmospheric density 6x higher than Earth (around 7.2kg/m3) this gives an 850kg limit of flight if the 70kg Argentavis is used as a maximum, or a 2400kg if the Quetzalcoatlus is used as a limit. I originally had it at 3x Earth's.

Would these figures yield a stable planet?

Would the mass of the planet have a large enough magnetic field to hold such an atmosphere? Could I increase the atmospheric density to 12x Earths or higher? What sort of atmospheric composition would it need? How would I achieve a planet with the right conditions and parameters if this isn't stable?

The larger surface area allows more room for life (and thus giant life), whilst having larger oceans to support a denser atmosphere. A higher level of volcanic activity would also be included.

I did intend for it to be a binary planet as well, but all these kinds of things can be changed.

• You might consider putting some of the heavier noble gases in the atmosphere. Or maybe some sulfur hexafluoride. – David Elm May 16 '17 at 5:00
• Also, if your planet was actually a moon of a gas giant, then I think that could solve a lot of your problems right away. – David Elm May 16 '17 at 5:31

If you're hoping for the answer to be "definitely" you're out of luck, but I might be able to give you "possibly."

The planet you propose is going to have some issues, so I'll outline those immediately.

• First of all, out current model for planetary formation indicates that the closer a planet lies to its parent star, the denser that planet will become. Based on local extrapolation, this would put your planet outside the orbit of Mars, which is outside the habitable zone (for our sun). That's not a good sign for the development of complex life.
• Your planet also requires more atmosphere and less gravity. This is somewhat counterintuitive at first glance. Both Mars and the Earth demonstrate that a small planetary body will likely have a small atmosphere, while many other bodies of smaller size have no atmosphere.
• Lastly, you'll need a magnetic field around the planet to protect from ionizing radiation, and to reduce atmospheric loss from solar winds. At the proposed density, your planet will be slightly denser than Pluto. This is too light to expect there to be a metallic core with which to generate this magnetic field. That's a problem.

So is there a way to overcome these issues? Possibly.

• Addressing the habitability issue, either your planet's orbit could have moved closer inward toward the sun, placing it in the habitable zone, the star is big enough that it warms the planet even at its increased radius, or the planet's atmosphere is dense enough that a paltry amount of sunlight could keep things warm. Probably some combination of the first and third solutions would make this planet more habitable and be fairly reasonable.
• As for the atmosphere/gravity issue, there are examples of small gravity wells retaining massive atmospheres. Venus is a good example, though it is somewhat strange in its own right, and the moon Titan is another. Both of these bodies demonstrate that a large atmosphere can exist around a small planet, so this is plausible for your planet, and perhaps even likely depending on who you ask (though be aware that in both examples the atmosphere is "super-rotating," blowing around the entire planet at hundreds of miles per hour, like a planet sized hurricane. Definitely cool. Definitely dangerous.).
• Lastly, the magnetic field. This is tough. Venus herself has a small magnetic field generated by interactions between the high atmosphere plasma and the solar winds, but this would not be enough to protect most life forms we know. On the other hand, this would be enough to protect against atmospheric loss from solar winds, so that's exciting. There is also the possibility of a lightweight metal core, composed of something like magnesium perhaps, that could provide a magnetic dynamo while still retaining a low density. This is pure theory, and current models of the universe suggest it is highly unlikely, but that is not the same as impossible.

So, could this planet exist? Maybe. Does it exist? Probably not. On the other hand, the chances of Earth developing just like it did were also quite low, yet here we are wasting our time on Stack Exchange when we should be working. I'd call that a win.

• As for the atmosphere/gravity issue, there are examples of small gravity wells retaining massive atmospheres. - I suggest to add information, why it's possible for them. – MolbOrg Jul 6 '16 at 22:46
• Both Venus and Titan demonstrate that planets with lower gravity than Earth - 0.904 and 0.138 g - can have denser atmospheres - 92 and 1.45 times Earth pressure - and without having significant magnetic fields. – jamesqf May 16 '17 at 4:38

This link is a good primer on how atmospheric density relates to other planetary factors. In particular, you propose a planet that has less gravity than Earth and greater air density.

Given that less gravity will reduce air density, all else held equal, because the gas will be held less closely to the surface, you have to have a compensating factor: either the gas must be much heavier (and thus easier for the gravity to act on), or the atmosphere must be a lot hotter with a lot more gas (akin to Venus). Venus has an atmospheric density around 80x Earths, so this doesn't seem entirely outside the realm of reason - but the gasses would be heavier, and therefore probably toxic as far as life-on-Earth would be concerned.

It is of note that even in this situation, your very massive creatures would probably tend towards lower elevations, where the density is likely to be higher.

The most unlikely thing is a planet less dense than Earth by a massive amount. I'm not entirely sure how to run the numbers, but such a planet is liable to be almost hollow if it has a larger solid surface, or be made up of elements that are a lot lighter than Earth's crust and core are composed of. Whether those elements could support complex life is unclear (you need a lot of heavier elements, such as Iron, for life).

As a side note, I don't believe that, for purposes of air density, the electromagnetic field has a great deal of import. It probably shields some gas leakage, but it won't act as an impermeable membrane.

• or the atmosphere must be a lot hotter - temperature have effects on density, but it makes it less dense in case hotter. But it does not change pressure, significantly, in cases earth like and OP's like planets. – MolbOrg Jul 6 '16 at 21:11
• Compare Earth and Venus: same gravity and size (volume), but Venus has 80x pressure and a much higher temperature. Higher temperature means heavier particles are in a gaseous state. This is the Pressure-Volume-Temperature relationship in chemistry. – Nathaniel Ford Jul 6 '16 at 21:21
• You talking about good pieces of information, but you assembly them wrong. U may wish to read Atmosphere of Venus specially direct your attention to composition(obviously), and more interesting part is graph for pressure height of atmosphere. This one very interesting and important part, more information here Scale_height. It's tricky, even I knew about, but as example I was thinking that Jupiter atmosphere is deeper (because it's mostly gas) and it was surprise for me, when I calculated deepness – MolbOrg Jul 6 '16 at 22:39
• Since Venus is smaller, hotter and has a denser atmosphere, wouldn't this indicate that I could simply pump more heavier particles into the atmosphere? The solar system is essentially designed around life on this planet and doesn't need to be suitable for Earth life. I envisioned animal life evolving after photosynthesis so (esp. larger) species can photosynthesize C02 though grass-like hairs. Presumably native life would be adapted to the different atmospheric composition? – Zac Walton Jul 6 '16 at 23:11
• You don't need heavier particles. You can just increase tbe amount of whatever particles you have to get a thicker atmosphere. – The Square-Cube Law Jul 7 '16 at 4:05

Planet magnetic field is the result of the internal structure of planet and rotation of that internal structure or planet.
As you concerned with numbers, there is no way prove or disprove planet magnetic field existence in your case, with provided numbers and description. We may only speculate that planet is not dense enough and probably there will be not much iron and so on, but that is not necessarily true.

Magnetic field prevents heating upper layers of atmosphere with charged ions, and that prevents particles from upper layers have escape velocity, and that leads to less atmosphere leaking.

Density, as pressure, depends more on a total mass of the atmosphere. Earth had a more dense atmosphere (as science believe), Venus has 92 times bigger atmosphere pressure and $5.243 kg/m^3$ density. The same mass atmosphere and earth's recipe (N2+O2 mix) and earth temperature and venus pressure and the resulting atmosphere will be 92 times denser ($\approx 127 kg/m^3$). (not CO2, because it will be partially liquid and atmosphere will have less pressure)

If you do not take H2 as the main part of the atmosphere, a composition is irrelevant, kinda, only mass is important. Although there not so much gasses to choose from.

This way you may set your pressure as you like, and density according to your preferences of composition and temperature for that pressure.

Composition any, but if you will take earth like it will be a safe choice, but keep in mind that CO2 will condense at 53 bar pressure at 293K temperature (room temperature). So 53 bar is sort of upper limit for that temperature, because of plants etc. But if the temperature is higher than 304K, pressure is unlimited. If you wish to have winters then something like 30bar and below. Take look at CO2 vapor pressure

As stability is the concern, Venus has weak magnetic field and atmosphere do not fly away. The main problem is hydrogen because it is the lightest and evaporation of other gasses is slower, significantly, but even in not stable cases "evaporation" is constant for a planet, so more you have initially for longer it lasts. And for our current understanding (at least my) as life was born in water, more water is better, there are some problems, especially where oxygen will go(when hydrogen will fly away) and bunch of other problems, but even if magnetic field isn't strong enough, more water initially may help, and even not strong field protects to some extent, that means lucky combination is possible. It may also explain higher pressure, but pure oxygen isn't safe, so assume there was a lot of water and lot of NH3 this will explain earth mix, and also it correlates with fact that planet is light, so probably it was formed from lighter fraction, especially in earlier stages of the universe.

Also one of the possible sources of water for earth maybe was Asteroids or Origin of water on Earth

So, how much water in each particular case planet gets, depends on processes of forming this particular system and some luck, this means you may scientifically accurate, as our today's knowledge goes, set arbitrary initial amounts of water on a particular planet.

The larger surface area allows more room for life

Earth is large enough, to support any biological possible large creature, and it was a time when she supported it.

Short speaking 6bar pressure is not a problem, for your planet.

You ask a lot of separate questions, I'm just answering on the magnetic field.

The Earth has a large magnetic field because it's rotating molten iron core acts as a dynamo generating electric and magnetic fields due to the rotation.

The overall density of your planet means it would most likely not have significant amounts of iron or other heavier elements, so it would most likely lack a magnetic field unless it had some other mechanism to generate one.

I may point out that Titan, the big moon of Saturn, has less than have the surface gravity or escape velocity of Earth, and yet it has a denser atmosphere.

Of course the factors that enabled Titan to acquire and keep its atmosphere would probably not work if that atmosphere needs to have a chemical composition and temperature similar to Earth's atmosphere.

Similarly Venus has a slightly lesser surface gravity and escape velocity than Earth and has a many times denser atmosphere - again with a highly different composition and temperature.