# Super earth habitability

I'm trying to build a super Earth that's capable of sustaining human life for space exploration in a way that's close to reality. So far I'm lacking some understanding of planetary science and human biology.

The premise is that the planet is a young world, around 2.7 billion years old, orbiting a K star within the shorter distance from the center of the habitable zone. It has shallow seas/lakes with a rusty green color, although the greenish hue of the color is slowly disappearing due higher concentrations of oxygen as time passes by. (This was inspired by reading research articles that suggest Earth's oceans were green with iron deposits before the great oxygenation event.)

Planet mass: 2 Earths

Atmospheric composition: Nitrogen: 69%, Argon 0.82%, Oxygen 30%. Due to the higher concentration of oxygen, there are lots of negative consequences affecting the planet, such as more volcanic activity, massive forest burning, metal rusting faster, food spoiling quicker than Earth, constant rainfalls, and possible massive violent storms.

The temperature: It varies. The highest temperature I've recorded is 61°C (141°F) and the and the lowest ever measured was 10°C (50°F). The average temperature for the most part of the planet is 37°C (98°F).

Surface Gravity: I'm not sure what kind of gravity I should put. As far as I know, Earth's gravity is 9.8 m/s² (1g) but, since the planet has larger mass, the gravity characteristics differ from Earth, therefore, I'm clueless to calculate this.

Geology: Homogeneous mix of lands (smaller continents) and water that connects to different large or tiny landmasses. Instead of a large ocean, it has numerous water bodies with lots of inlets, bays, lagoons, and isthmuses. The topography is slightly flatter than on Earth.

Land coverage: 55% water and 45% land.

Orbital period and rotation: Another thing I'm at a loss for. I'm afraid I'm terrible at mathematical planetary motion but I wanted the planet to have 447 days (1.22 solar year).

Climate: Tropical.

Flora: Possibly a mega flora with wetlands, tall mangroves, and a rain forest.

My question would be, given everything that I listed, is possible human life could thrive on this planet? If so, what are the positive or negative side effects for humans to live there? Do humans suffer different gravity, atmosphere, weather effects? Would the inhabitants undergo disorder moods like seasonal affective disorder, since it has different lengths of days?

I figured since it has a higher concentration of oxygen, humans would perform better in endurance but I also read that with too much oxygen, long-term effects include oxygen toxicity and problems with metabolism and aging cells.

Correct me if I'm wrong in any those lists I've mentioned.

• Sounds great as a potential habitat. Welcome Sheilymar Torres. You can take the tour, and read up in the help center about how we work. We have a somewhat strict "one question per thread" rule here, you can edit to narrow it down, and then post another question in another thread - that's fine. – Measure of despare. Jun 28 at 5:06
• How long do you want your humans to live there? Humans can adapt to environments fairly quickly and via selective breeding. – Shadowzee Jun 28 at 5:07
• Well, it's a human colony. As long they can, I suppose. – Sheilymar Torres Jun 28 at 5:08
• i don't see how oxygen influences volcanoes, rainfalls or storms. you want your whole planet to be tropical? how many days there are in a year depends not only on the length of the year, but also on the length of a day, and you can set that however you like. – ths Jun 28 at 10:22
• I don't think you can have a whole planet that's tropical. That is, if it's tropical at high latitudes, temperatures in the equatorial regions are going to be extremely hot. Also, if you have any significant topography, you'll have desert regions. – jamesqf Jun 28 at 18:17

You really do have a lot going on in your question and splitting it into multiple threads is going to get you the best specific answers. That said taking your main question of "could human life thrive?". The short answer is yes.

The long answer is based on physics and the constraints you have listed you are going to end up with a world quite similar to earth in the grand scheme of things. All of the calculations that follow are rough estimates but in general should be close enough for use in a science based story.

Difference in gravity can be roughly calculated as mass of planet divided by radius of planet squared. Your listed mass is twice earth with a radius of 1.4 earth. 1.4 squared is 1.96. This lets us know that gravity on your planet will be about 102% of that on Earth.

The next thing to talk about is orbital period/distance. A quick and dirty calculation for this is that the square root of the orbital distance cubed gives you orbital period in comparison to earth. Obviously there is some flexibility to this, but it gets us close. Now you list the orbital period as 1.22 earth years so we can work this backwards. 1.22 squared is 1.488. the cubic root of that is 1.13 which means your planet orbits at roughly 1.13 AU, or about 13% further out than earth.

The habitable zone around a K type star ranges from 0.1 to 1.3 depending on the size of the star itself. So your planet checks out as being plausible. That said your temperature listings are a bit higher than earth so you are going to need a rather large K star or alternatively to shorten your orbital period so your planet can be closer to it's star, or assume a higher mix of heat trapping gases in the atmosphere.

You specifically call out the oxygen levels as being higher than earth currently, but they are not higher than what the earth has seen. Findings have show that between roughly 350 and 300 million years ago the oxygen level in the atmosphere was roughly 30-35%. So again your levels are reasonable on an earth like planet. Oxygen toxicity doesn't really become an issue until you start looking at high concentrations of oxygen at high atmospheric pressures. At a normal atmospheric pressure oxygen toxicity doesn't become an issue for humans until about 50% oxygen levels. On average even at 95% oxygen at 1 atmosphere a human averages 14 hours before showing signs of oxygen toxicity, which can be easily reversed by a low oxygen environment at this point. So clearly, you are a ways off from the atmosphere causing immediate harm. That said there could be some long term cell degradation at a 30% level but it isn't anything that is likely to keep a sustainable human population from being viable.

As far as rotational period of the planet that has far more to do with random chance, which direction did big objects impact the planet, than anything that is consistent across planets in general. So you can pretty much pick any rotational period, and thus day length you desire for your story.

Overall you have created an extremely earth like planet, with some room to play with the edges to make it a little more or less so. The way it appears now though the Flora and fauna, and it's aggressiveness/hostility towards humans is likely to have a far more meaningful impact on a colony surviving than the actual physical set up of the planet system.

• It seems like gravity should be more than just 102% for a planet double earths mass. Also O2 toxicity needs to consider concentration and pressure. 30% at 1 atm is ok. 30% at 2 atm would be tough on people. – Willk Jun 28 at 12:05
• Yeah, gravity works a bit counterintuitively sometimes because the distance aspect is squared. To find gravity(g) at the surface of a planet you multiple the gravitational constant (G) by the mass of the planet (m) and divide that figure by the radius of the planet (r) squared. When comparing against earth though you can just use m/r^2. Where those figures are the comparison to earth. With twice the mass and 1.4 the radius you get 2/(1.4^2) which gives you 2/1.96 or 102% that of earth, with an earth like gravity and atmosphere the atmospheric pressure will also be about 1 atm at sea level. – Crouse Jun 28 at 14:02
• a super earth will also have a more dense core, due to its own weight. so to have this ratio would require a slightly different material make up, Id assume ore water than what the OP had suggested. – Sonvar Jul 18 at 5:33

As Crouse mentioned, the planetary diameter you listed means this world is not very dense at all. This would mean that it's severely lacking in heavier elements and will be very metal poor, significantly curtailing the speed with which a technological civilization can grow (unless they've moved on from steel to bulk carbon alotrope manufacture and can make large quantities of graphene, carbon nanotubes, synthetic diamondoids, etc.). This planet would have like that evolved under basically the same constraints as on Earth.

The problem is that volume grows with the cube of the radius. Assuming the same density as Earth, planet with two times the mass would have a radius of 6370 km (assuming a perfectly spherical planet, which won't be the case, by way of comparison, Earth radius ranges from 6378 km at the equator to 6357 at the poles, so this planet would be about the same size as Earth). This planet would however have a gravity that of 2G's: 19.62 m/s^2. (By comparison, a roller-coaster probably won't go much above 1.6 Gs, imagine living under that strain all the time)

I'm going to go forwards assuming that 2G surface gravity variant, and point out a few things about high gravity worlds:

1. Due to the dense atmosphere, your flying creatures can actually be bigger than on Earth. While the gravity is higher, meaning they need more force to keep them aloft, lift increases with atmospheric density faster than it drops off with the increase in gravity. So counter-intuitively, bigger bird analogues.

2. There seems to be a truism that flora and fauna on high G worlds is short and stocky, but people usually think this is for structural support reasons, which isn't the case. Stocky creatures have more mass that has to be held up, which means stronger support structures which means more mass, and thus a vicious cycle is born. You can keep lower to he ground, but you give up the advantages of a higher vantage point. It seems likelier that evolution would favor creatures that are lithe - with weight saving adaptations like hollow bones being common, and high density muscle that can apply a lot of leverage for minimal mass. That and creatures are likely to be smaller than the Earth average, to take advantage of the square cube law. Because objects fall faster, animals might also have faster reaction times. The bigger issue is fluids - an organism that is oriented flat to the ground has less of a pressure gradient that the heart has to struggle with, that's the read driver for shortness in high gravity environments.

3. High atmospheric pressures and oxygen content mean that simpler lung structures can be used, which means you can have giant insects and arthropods. Novel gas exchange mechanisms such as frog like skin breathing might also be more common.

4. Higher atmospheric density means that the planet will probably be warmer than Earth, with winds exerting more force for a given windspeed. There is also likely less temperature variation from the poles to the equator. As the strength of storms is driven by temperature difference, this means that to overcome the greater uniformity of the thick atmosphere and get large temperature differences, the planet will have to rotate slower, with longer day and night cycles creating hotter noons and colder midnights.

5. Flora will likely be more limited in height than on Earth both due to wind loading and the greater energy expenditure of transporting nutrients up the trunk. It will likely grow faster due to having an easier time absorbing CO2 and O2 from the denser atmosphere, much like how plants placed in high pressure environments seem to grow faster on Earth. Trunks might be surrounded by scaffold like branches that add rigidity while reducing wind loading.

How will this affect human colonists? Well, you talked about them being more disease prone due to higher O2 concentration, so that's a thing. The different day/night cycle will also have adverse health effects over the long run, reducing lifespan much like how night shift have been proven to be terrible for one's health. Arthritis would likely be endemic due to the higher gravity if nothing was done, so people will likely use some sort of load bearing wearing exoskeleton - through not necessarily a powered one, that that would be very useful indeed. These conditions will also retard growth, as seen in experiments where mice were placed in a 4G environment, the extra energy demands means slower growth of all organs. Heart issues will also be endemic even with technological aid, and people will likely be forced to wear compression suits similar to those worn by fighter pilots. Motorized scooters that let people lie back would likely be very common to help reduce the issue. Pregnancy would also be very risky, so exo-woumbs might be popular.

2Gs is really at the edge of what un-augmented humans can deal with.

• Thank you for extrapolating it. I don't plan to have fauna on that planet but it's an awesome notion. I've planning to use augmented and non-augmented human and the side effects you've mentioned is not surprising but the methods how to reduce the 2Gs symptoms was interesting and practical. – Sheilymar Torres Jun 28 at 20:40
• Your math is off, there won't be 2Gs on the planet. Planetary gravity is calculated by M/D^2, so 2/(1.4^2) giving me 1.02Gs. – TheDyingOfLight Jun 29 at 1:34
• I was calculating the diameter for a second version of the planet that doesn't have a extremely low density. – Algebraist Jun 29 at 19:41

There are already some good answers here, but I do want to pick up on one thing about your atmospheric composition; lack of density figures.

When it comes to things like oxygen deprivation (not enough) and oxygen toxicity (too much) the difference is not in the percentage of oxygen, but rather in the partial pressure. That means, that in order to know whether that percentage of oxygen is dangerous or not, we need to know what the pressure is in the first instance.

For example; the Apollo astronauts flew to the moon in a pure oxygen environment, because their capsule was only pressurised to below 0.3 ATM, meaning that the partial pressure of oxygen was still only 0.3 atmospheres (ATM), which is a bit more than the volume of oxygen in normal air at sea level. On the other hand, deep sea divers often dive with argon-thinned air mixes, so that at pressure when they breathe in more air, the volume of oxygen is the same as it would be at sea level.

It is also important to note that you can't just say that double the mass of the Earth will result in double the pressure of atmosphere; the difference between Venus and Earth is a prime example of this. The pressure is also impacted by heat, meaning that Venus and its runaway greenhouse effect creates a massively hot, high pressure environment.

Put simply, depending on what your starting atmospheric pressure is, your air mixture could already be too high for humans to breathe safely for any length of time.