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Can a rocky planet have these qualities and exist:

  • half of earth's gravity

  • 4/5 earth radius

  • a magnetic field capable of retaining an atmosphere

  • a slightly slower rotation than earth

  • can sustain life

I am trying to create a world where a normal human will seem superhuman.

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    $\begingroup$ Welcome to Worldbuilding! Your planet is actually similar to Mars, only larger, but with lesser density. Everything you asking for is possible (however, magnetic field and dense atmosphere may be unlikely). $\endgroup$ – Alexander Jun 14 '18 at 15:57
  • $\begingroup$ Hello, Adeyemi Akintunde Oyemade, and welcome to Worldbuilding! This looks like a very interesting question. Please take our tour and visit the help center to learn more about the site. $\endgroup$ – Gryphon Jun 14 '18 at 16:10
  • $\begingroup$ Why do you need a specific radius for superhuman-seeming humans? Low gravity (and possibly higher O2 concentration while at overall lower air density for lower air resistance) would be great for athletic feats, but radius? $\endgroup$ – bukwyrm Jun 14 '18 at 16:14
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    $\begingroup$ About this part: "a magnetic field capable of retaining an atmosphere": The atmosphere is retained because of the gravity of the planet (the mass). Not because of the magnetic field. Of course a magnetic field can help to shield the atmosphere from the solar wind, but you can have a planet with a dense atmosphere and no magnetic field (for example: Venus). You can have more information about this phenomenon here: sci.esa.int/venus-express/… $\endgroup$ – Carlos Zamora Jun 14 '18 at 16:21
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    $\begingroup$ @CarlosZamora: Plus the temperature of the atmosphere; a colder planet can retain lighter elements than a hotter planet. en.wikipedia.org/wiki/Atmospheric_escape $\endgroup$ – Phil H Jun 15 '18 at 14:02
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The short answer is yes, it is possible. Your premise is quite similar to John Carter of Mars, a series of stories published in the 1910's which were then made into a movie in 2012. An American Civil War veteran is transported to Mars, where due to the lower gravity, he is essentially superhuman. I do encourage you, however, to attempt your own take on the theme. There's no reason to assume you won't come up with something new and compelling.

Long answer. For 4/5 Earth radius and half gravity, you just need a planet of appropriate density. Know, however, that being that low density means it would probably be mineral-poor. For a magnetic field, you just need a rotating planet with a molten core, and there is no reason it can't rotate at a rate slightly slower than Earth.

As for sustaining life, after the above considerations it just needs to be in the habitable zone of a star. It could be a moon of a gas giant in the appropriate zone, or a planet in its own right. At that point it's really up to you. If you would like to expand your knowledge of what makes a planet habitable, I suggest looking up articles on: The Fermi Paradox; the Drake Equation; Habitable Zones of stars; the Great Filter Theory; and technological intelligence.

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  • $\begingroup$ I am quite familiar with The Fermi Paradox; the Drake Equation; Habitable Zones of stars; the Great Filter Theory. I am also familiar with the John Carter story and from my understanding that world is very unlikely to exist due to mars weak atmosphere. $\endgroup$ – Adeyemi Akintunde Oyemade Jun 14 '18 at 16:34
  • $\begingroup$ I am also aware that the planet will be mineral poor, however, I am not sure if this fact will contradict the other requirements I want this world to have. $\endgroup$ – Adeyemi Akintunde Oyemade Jun 14 '18 at 16:41
  • $\begingroup$ It doesn't have to contradict your other requirements. A good way to keep a mineral-poor core molten would be to make it a moon of a gas giant, and have the gravitational stresses from the gas giant and others moons keep the core flexing and molten. That's why Io, one of Jupiter's moons, is so volcanically active. You can just have it experiencing less gravitational stress than Io. $\endgroup$ – N.D.A. Jun 14 '18 at 16:50
  • $\begingroup$ :o this raises a new question for me then. Would the gas giant have to exist in the star's habitable zone? and Can a gas giant exist in the habitable zone $\endgroup$ – Adeyemi Akintunde Oyemade Jun 14 '18 at 17:00
  • $\begingroup$ It would have to, and gas giants have. We've observed gas giants to do some very interesting things in solar systems, even being within 0.1 AU (1 AU being the distance from the sun to Earth) of a star. However, you could make it more simple for yourself and just make the planet a regular rocky planet, but with a large moon, relative to its own size. $\endgroup$ – N.D.A. Jun 14 '18 at 17:15
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Your two basic equations are for gravity and for density:

$$a = GM/r^2 \text{ and } \rho=3M/4\pi r^3$$

And you are given a = 5.0 (roughly half of Earth) and r = 5100000 m (roughly 0.8 of the Earth)

So solving gives a mass of about $1.9\times 10^{24}$ and a density of 3500 kg/m^3. The Earth has a density of about 5100.

Here is where your problems start, as you want a powerful magnetic field, and that needs a liquid iron core. But your planet has a density that is substantially less than that of the Earth. Iron cores are heavy. If you have a heavy iron core, but are still have a density of 3500 you need a lot of lighter stuff, like water to compensate. But you now have a "water world" with 300km deep oceans covering the surface (I haven't done the detailed calculation here)

This is your basic problem. We can turn this around: if we fix $\rho$ at 5000, then this gives a planet with a radius of 3500km

With low gravity, it is going to have a hard time holding on to its atmosphere, even with a magnetic field, but perhaps this can be handwaved away.

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Let's go through your requirements.

  • a slightly slower rotation than Earth

Surely possible, there is no constrain on this.

  • half of earth's gravity
  • 4/5 earth radius

The two above mean that the planet is poor in heavy elements such as iron. Based on our understanding of Earth science, a molten iron core is needed to have

  • a magnetic field capable of retaining an atmosphere

Without an atmosphere it's pretty hard to

  • sustain life

You can try to go around this problem by visiting the planet when it's still young, and therefore the smaller core is still molten and can sustain a magnetic field and then retain an atmosphere.

More or less like Mars in the early history of our solar system.

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Human adaptability will be your biggest obstacle for "superhuman" humans on a less demanding planet.

I think the bigger dilemma is not whether such a planet can exist, but whether humans on it would really be super human. Remember that in less gravity you will need less strength to do the day to day tasks of human life.

Since you need less strength and there is less gravity, both your bones and muscles will atrophy. So you may have a planet where humans from Earth may seem superhuman if they are just visiting, but anyone who lives on this new planet for more than a few weeks is quickly going to become weak. They will quickly become just normal humans again relative to this planet, but extremely fragile by Earth standards.

Since the purpose of the planet is to make humans seem superhuman, the qualities of humans themselves may pose a problem.

https://www.esa.int/Our_Activities/Preparing_for_the_Future/Space_for_Earth/Space_for_health/Musculo-skeletal_system_Bone_and_Muscle_loss

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    $\begingroup$ :) I am aware of this and it is one of the points addressed in this story $\endgroup$ – Adeyemi Akintunde Oyemade Jun 14 '18 at 18:28
  • $\begingroup$ That is a good tie in. $\endgroup$ – Tyler S. Loeper Jun 14 '18 at 18:32

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