My alien planet is a super Earth. It revolves around a K-type star in the habitable zone and has a lower gravity. The mass is that of 2 Earths(or how ever superearths mass normally are).The atmosphere is similar to present Earth's and the Carboniferous Period (I used these for inspiration when coming up with the atmosphere).

The atmosphere of the planet has 25% oxygen to avoid global fires. Argon is 0.92% and nitrogen is around 65% and there's like 0.1 percent of other gases like carbon dioxide,methane and water vapor. The average temperature is between 59 and 49 degrees Fahrenheit. The average humidity is around 57%(this leads to a warm wet climate right?)while the highest could be around 60° on a hot season, but it's not 60° the whole planet.

Here's an image of the continents : the giant one in the middle is in the equator and the one above is higher above for snow.

Drawing of the continents

Does this mess up my plan for mega fauna and mega flora? Because that's what I plan to have on my planet. For those wondering what the environments are on the planet I will put the link to my question about my planet biomes here : Is my environmental map consistent?

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    $\begingroup$ Questions should be self contained. I don't know what "does this mess up my plan for my hexapodal mega fauna and my mega flora?" is supposed to mean. $\endgroup$
    – Daron
    May 13, 2022 at 22:19
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    $\begingroup$ What is a "Super Earth"? I presume it is larger than plain ol' Earth. But why would that mean lower gravity? $\endgroup$
    – Daron
    May 13, 2022 at 22:20
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    $\begingroup$ A maximum humidity of 60% means it never rains... anywhere... ever... $\endgroup$
    – addaon
    May 13, 2022 at 22:20
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    $\begingroup$ Average temperature and humidity doesn't mean much. Earth is very different from place to place and the animals are different too. $\endgroup$
    – Daron
    May 13, 2022 at 22:21
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    $\begingroup$ Ok if 60 is exceptional I don't see real issues with your description, it resembles Earth, the gravity being smaller on a large planet means you have something like Vance's Tschai which had an Earth-like climate as well. Its nature did not sound implausible to me, when I read the book. Large forests, oceans.. and BIG animals. It was mainly chalcon (lighter than granites) without an iron core. $\endgroup$
    – Goodies
    May 13, 2022 at 23:05

2 Answers 2


There are a number of issues.

It is possible to have a planet with twice the size of Earth and a lower gravity, but it requires the radius of the planet to be at least 1.45 the radius of the Earth. You didn't specify how much lower the gravity would be.

With twice the mass of Earth and a radius of 1.45 that of Earth, the gravity would 9.34 m/s2 compared to 9.8 m/s2 for Earth, which is just lower. The escape velocity for such a planet would be 13.135 km/s, compared to 11.184 km/s for Earth. This means the planet would most likely hold the composition of the atmosphere that you are proposing. The one problem would be the average density of the planet. It would be 3.618 g/cm3, compared to 5.514 g/cm3 for Earth. Mars has a density of 3.93 g/cm3. Compared to Earth, the planet would be depleted in heavy metals, particularly in its core, which then raises the question, would life be able to form on such a planet?

If the both the planet mass and radius are twice that of Earth, the escape velocity is the same as Earth, so your proposed atmosphere is still okay. However, the gravity drops to 4.91 m/s2, which is more than the gravity on Mars at 3.7 m/s2. However, the average density drops to 1.379 g/cm3 which is very similar to that of Umbriel. Such a density would imply it is composed mostly of water ice, so your planet would not have life on it.


This has been added after comments from the OP.

If the planet has twice the mass of Earth and had the same gravity as Earth, then I think it could work.

  • The radius of the planet would be 1.41 times the radius of Earth
  • The gravity would be 9.82 m/s2, effectively the same as Earth's
  • The escape velocity would be 13.32 km/s, which is higher than Earth's, so the atmosphere would be okay.
  • The average density of the planet would be 3.935 g/cm3, which is the same as for Mars, a rocky planet some people would like to colonize.
  • $\begingroup$ ah ok....ok thats a lot...it could still work the way i want if i just have it the same gravity of earth right?becauase earth had dinosuars and they where pretty big now that i think about.im not great at math so i kinda hoped for these awnsers to clear things up. $\endgroup$ May 14, 2022 at 6:11
  • $\begingroup$ @ErikSanchez: Check my edit, at the end. Looking good :-) $\endgroup$
    – user81881
    May 14, 2022 at 6:57
  • $\begingroup$ ah ok thank u and i hoped it was fun for u having to figuer these things out $\endgroup$ May 14, 2022 at 20:35
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    $\begingroup$ The planet would also have to have lower metalicity and a smaller core (in comparison to its diameter) than does Earth. That's going to have a big impact on plate tectonics, mountain building and volcanology as well as its magnetic field. All of which would have a big impact on its habitability. $\endgroup$
    – Mon
    May 15, 2022 at 5:46

Should be fine

There was a time in the Earth's history where we had a lot of large animals. Look:

enter image description here

Your world is like the prehistoric Earth. It is believable you have big animals too.

There is no need to worry about the exact atmospheric content. Just say "It's like the Carboniferous but different" and get on with writing it.

Low gravity only makes large animals easier.

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    $\begingroup$ Problem is, you can't really have a planet with twice the mass and a lower gravity ^^'. $\endgroup$ May 14, 2022 at 0:18
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    $\begingroup$ @Tortliena not if you want it to have a protective magnetic field, yes $\endgroup$
    – Trish
    May 14, 2022 at 8:17
  • $\begingroup$ @Tortliena Make the planet less dense than Earth. Then the surface is further from the core and gravity is lighter there. $\endgroup$
    – Daron
    May 14, 2022 at 12:15

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