So I am making a planet for people to live there, but I have trouble making the conditions that could sustain life.

So I had an idea of having a planet bigger than Earth, rotating aroung a star that is bigger than our Sun. It would also contain 1 moon, also bigger or 2 moons, depends on conditions and planet's gravity.

The conditions has to be, if possible, smaller gravity (probably impossible due to the planet size, but I don't know if the sun could somehow effect planet's gravity), breathable air (oxygen level would be 25-30%). Could have more Lunar effect on the planet, like even higher tides. The planet could also contain places dangerous for people, like places on the planet (not every place on the planet, but areas on the planet, this is not necessarily)

Also I was thinking if there another celestial body visible from the planet, like a distant moon, or a planet.

I was wondering what kind of density would that planet have, how could I make it for people to be able to live normally there?

  • $\begingroup$ 30% O2 is quite a lot. What's the atmospheric pressure at the surface? $\endgroup$ – a CVn Jul 1 '19 at 14:05
  • $\begingroup$ Huh, no idea, but in the way that 30% of oxygen would not be too toxic in long term. $\endgroup$ – Sebastjan Kovač Jul 1 '19 at 14:23
  • $\begingroup$ You may want to play around with Universe Sandbox 2. It is a really awesome tool for solar system scale worldbuilding and can even test out the long term stability, effects on your design. This is not a paid ad! $\endgroup$ – Lupus Jul 1 '19 at 14:42
  • $\begingroup$ Welcome to Worldbuilding SE. You can help improve the quality of answers you receive by making sure your goals and limitations are clear. Formatting them in bullet points, or in individual paragraphs will help make your requirements clearer. $\endgroup$ – EDL Jul 1 '19 at 14:45
  • $\begingroup$ Hello Sebastjan! Thanks for joining us. It shouldn't surprise you that asking about larger-than-Earth habitable planets is popular on this site (e.g. this Q). Sometimes it's easier for you to simply write your story, because the physics we know doesn't permit a lot of variability. We can calculate the density needed to have a 2X diameter planet with 1G - but rocks and metal don't accommodate that number and you need the rocks and metal for the planet to work. (*continued*) $\endgroup$ – JBH Jul 1 '19 at 16:46

Having a larger planet with a smaller gravity field does sound hard, but it might be possible. The secret ingredient is aluminum. Gravity is a universal force that draws all two objects with mass together. In order to have less gravity than Earth, yet a bigger planet, you just need the core to be made out of a less dense metal, hence less mass. Earth's core is made up of nickel-iron, which have a normal density of 8908 kg/m^3 and 7850 kg/m^3 respectively in pure forms. Aluminum, by contrast, has a density of 2712 kg/m^3. So, if you want to make the planet like this, swap the nickel-iron core for a nice new aluminum-iron one.

A carbon-cored planet is a bad idea, because there'd be no way for water to form.

The original suggested answer was a titanium core, but that wouldn't generate a magnetic field and keep in the atmosphere. Thanks to Morris the Cat for pointing this out.

Oxygen levels are a bit trickier. 25-30% oxygen is high, our atmosphere only has 20% oxygen. Not sure why you'd want it that high, but I don't think there'd be that many consequences. (Except if you used tanked air to scuba dive - you'd need to be more careful.) More tidal effects just needs a heavier moon. Earth already has enough dangerous spots. Another celestial body is easy if you want a moon, hard if you want a planet, because planet orbit separately. I mean, you can see them (Venus is the easiest to spot if you know what you're doing) but they look small.

| improve this answer | |
  • $\begingroup$ Actually the distances between planets in the known exrasolar planetary systems vary very greatly. In the system of TRAPPIST-1 the planets in the habitable zone orbit so close together that sometimes a planet seen from another planet will appear larger than the Moon seen from Earth. $\endgroup$ – M. A. Golding Jul 1 '19 at 14:35
  • $\begingroup$ Is that temporary, like only a few days a years, or for extended periods of time? $\endgroup$ – Halfthawed Jul 1 '19 at 14:39
  • $\begingroup$ Like the other answer, you're missing a big problem here. With a core like that, you've got no magnetic field, and that ultimately means no atmosphere either. $\endgroup$ – Morris The Cat Jul 1 '19 at 15:22
  • $\begingroup$ @MorrisTheCat Would an aluminum core be better? $\endgroup$ – Halfthawed Jul 1 '19 at 15:30
  • $\begingroup$ @Halfthawed I don't think so. Aluminum isn't conductive either. Off the top of my head I can't think of any way to meet the density requirements the OP asked for AND generate a magnetic field sufficient to allow habitability. $\endgroup$ – Morris The Cat Jul 1 '19 at 15:34

The surface gravity of a planet is determined by the formula g = 4/3 π G ρ r, where ρ is the mean density and r is the radius.

To obtain lower surface gravity with a higher radius, the density must thus be more than proportionally lower. If the radius e.g. is 1.5 that of Earth, the density would have to be less than 2/3 that of Earth; say half, for significantly lower gravity (75% Earth gravity).

Earth's mean density is 5.514 g/cm^3, so your planet's mean density would havce to be around 2.26 g/cm^3. This is slightly lower than the density of silicon, aluminum, or concrete. Your planet would probably have a composition like carbonaceous chondrites, consisting mainly of the minerals olivine and serpentine, with some water, coal, and other nitrogen compunds. Metals will likely be in short supply, except as the result of meteorite impacts, and hence be quite valuable.

As for your other requirements, I see little problems. Danger zones could be tidal zones, seeing as you want stronger tides, or polar regions or mountains.

| improve this answer | |
  • $\begingroup$ There's a big, big problem with a planet like that. With that kind of composition, you've got no iron core. Without a sizable iron core, you have no magnetic field. This means no protection from radiation and, over time, no atmosphere. This is exactly what happened to Mars. $\endgroup$ – Morris The Cat Jul 1 '19 at 14:58
  • $\begingroup$ @MorrisTheCat: The planet is far bigger than Mars, and in fact has a mass two-thirds again as great as the Earth. Though the surface gravity is smaller than the Earth's, the gravity well is deeper and hence better able to hold onto the atmosphere. Venus has no magnetic field to speak of, yet holds a very think atmosphere in spite of the close proximity to the Sun. $\endgroup$ – Klaus Æ. Mogensen Jul 2 '19 at 7:56
  • $\begingroup$ Klaus, Venus has some additional factors in play keeping its atmosphere thick, but without a magnetic field you STILL have no protection from solar radiation, which means it's not habitable. $\endgroup$ – Morris The Cat Jul 2 '19 at 13:15

Not the answer you're looking for? Browse other questions tagged or ask your own question.