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The planet in question has a radius of 7947km, volume 1.9408 times as large as earth, & a weight about 1.55377 times that of earth.

I want the planet to be earth like, or at least earth like enough that aside from being bigger most of the aspects of it's surface & mineral deposits can be similar. Aside from the size difference it has a longer day (about 28 hours), & a significantly longer polar night at the south pole.

Is a planet like this possible or would the increased size make it infeasible for it to be earth like?

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With the given mass and volume, the density would be 80% of Earth's.

Earth's core has about 3 times the density of its mantle, but only 1/3 the total mass. Earth's overall density is only 1.2 times the density of the mantle. In short, assuming no significant effect on the ratio of densities, you'd have to basically eliminate the iron-nickel core in order to get the density that low.

This seems basically feasible, but at the very upper limit of the possible size for a planet with 1 g surface gravity. However, it would not be Earthlike, being extremely poor in iron, and inevitably with other major shifts in its composition. It might be more plausible with a carbon planet, one consisting largely of carbides rather than silicates and especially poor in heavy elements. Alternatively, perhaps it could have formed from mantle material blasted off a super-Earth in something analogous to the collision thought to have formed our moon, but again it's not something you could mistake for Earth.

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  • $\begingroup$ Would it be possible to not have major problems regarding the crusts mineral content while still having the mantle & core be poor in heavier elements? $\endgroup$
    – OT-64 SKOT
    Commented Jul 24, 2023 at 5:52
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    $\begingroup$ @OT-64SKOT Again, the core would need to be virtually nonexistent, not just a bit poorer in heavy elements. The mantle would need to consist of lighter elements to compensate for its increased compression near the center of the planet. You would then have an unstable skin of denser earthlike material on top of the mantle. If the planet's not totally geologically dead, that material will sink into the mantle and be replaced with lighter material. The whole setup would be very artificial and geologically short-lived. $\endgroup$ Commented Jul 24, 2023 at 15:34
  • $\begingroup$ No iron core means no magnetic field which probably causes a number of problems with the planet’s habitability. $\endgroup$
    – Mike Scott
    Commented Aug 9, 2023 at 17:34
  • $\begingroup$ @MikeScott the need for a magnetic field is exaggerated, and the lack of one would be at least partially compensated for by the higher escape velocity. $\endgroup$ Commented Aug 9, 2023 at 18:52
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It would seem that this is entirely plausible. Detailed calculations might or might not show up some issue or other, but it would involve a lot of work. For any fictional story I would say that this is fine.

Any detailed calculation would have to make assumptions and those assumptions might well be wrong as only you are able to specify the exact parameters...

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You write:

The planet in question has a radius of 7947km, volume 1.9408 times as large as earth, & a weight about 1.55377 times that of earth.

A radius of 7,947 km would be 1.247370899 times Earth's mean radius of 6,371 km. Thus your planet should have 1.93511 times the volume of Earth, which is close enough to your figure of 1.9408 times the volume of Earth.

The mean density of Earth is 5.514 grams per cubic centimeter.

If your planet has 1.55377 times the mass of Earth in 1.93511 times the volume of Earth it will have a mean density about 0.802936 of Earth's, or about 4.4273 grams per cubic centimeter.

A planet will have many different materials with different densities, usually stratified by density. And materials in the center of the planet will be compressed to greater densities by the weight of the material on top of them. Since your planet is more massive than Earth, it should compress the matter in its inner regions more than Earth does, so your planet must be made of a mix of materials which are less dense, on the average, than the mix of materials which Earth is made of.

According to this surface gravity calculator: https://www.omnicalculator.com/physics/acceleration-due-to-gravity a planet with a radius of 7,947 km or 1.24737 that of Earth, and 1.55377 times he mass of Earth, would have a surface gravity of 1 g, the same as Earth.

You don't specify why the planet should be Earthlike for the purposes of your story. If Earth humans are supposed to be on that planet for long periods of time, or even colonize it and have generations of humans live their whole lives there the surface gravity should be within safe limits.

Nobody knows the lower safe limit of surface gravity. But tests in centrifuges reveal limits on exposure to higher gravity.

Acceptable levels of gravity are discussed in Habitable Planets For Man, Stephen H. Dole (1964), pages 11 to 13.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf

On page 12 Dole says:

On the basis of the available date, one might conclude that few people would choose to live a planet where the surface gravity was above 1.25 to 1.50 g.

So you could decrease the radius, and/or increase the mass, of your planet to give it a higher mean density if you find that the present lower density than Earth's might cause problems. You could change your planet to increase the surface gravity up to about 1.25 g without having to mention the higher surface gravity too many times. You might even dare to increase it up to 1.5 g.

According to this escape velocity calculator:

https://www.omnicalculator.com/physics/escape-velocity

A planet with 1.55377 the mass of Earth and 1.24737 the radius of Earth would have an escape velocity of 12.484 kilometers per second, 1.116 times that of Earth. Which means that the planet should he able to hold onto a dense atmosphere for geological eras of time.

Robert Quattlebaum's answer suggests quite reasonably that your planet, less dense than Earth, would probably have a much smaller iron core than Earth does. It then suggests that might produce a weaker magnetic field than Earth's. And it correctly suggests that a weaker magnetic field would offer less protection to the atmosphere from the stellar wind of the star. And it goes on to suggest that the planet would thus tend to have a thinner atmosphere than Earth does.

If only there was a well studied and otherwise Earth like planet without a planetary magnetic field to test that theory. A planet like Venus, which has no detectable magnetic field. Thus nothing protects atmospheric gases on Venus from being knocked off into space by the solar wind.

The atmospheric pressure at the surface of Venus is about 92 times that of the Earth, similar to the pressure found 900 m (3,000 ft) below the surface of the ocean. The atmosphere has a mass of 4.8×1020 kg, about 93 times the mass of the Earth's total atmosphere.[28]

https://en.wikipedia.org/wiki/Atmosphere_of_Venus#Troposphere

Obviously the loss rate of the Venusian atmosphere due to the solar wind has been fairly slow, and your planet would have a somewhat higher escape velocity to retain atmosphere.

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  • $\begingroup$ the main reason i want to keep it very close to earth aside from size is so it can seem reasonable that a society similar to earth with similar biology on the planet could exist. my world is meant to be an alternate early-mid 20th century instead of being colonised by humans from elsewhere. $\endgroup$
    – OT-64 SKOT
    Commented Jul 24, 2023 at 5:38
  • $\begingroup$ Excellent answer, I've added a header to my answer that refers to yours instead. $\endgroup$ Commented Jul 25, 2023 at 20:49
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I recommend following up with this answer, which I think gives this question a more thorough treatment. Leaving my original answer below for posterity.


If (as you say in this comment) that you've tweaked the numbers to keep the surface gravity at 1G, then that means that this planet is significantly less dense than earth.

That lower density might indicate a smaller iron core, which might make for a less powerful magnetic field. A less powerful magnetic field would not do as good of a job at shielding the planet from solar wind, which would eventually reduce the surface atmospheric pressure.

So it seems plausible that the planet could remain Earth-like (breathable atmosphere, surface water, terrestrial vegetation, etc), but perhaps with some significant differences (thinner atmosphere, more UV damage, etc).

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  • $\begingroup$ It would have a surface escape velocity nearly 3 km/s higher than Earth, which would greatly reduce atmospheric losses. Atmospheric density is mostly due to other factors...see Mercury and Mars, where Mercury has a magnetic field and higher surface gravity than Mars but Mars is the one with a substantial atmosphere, or Venus and Earth, where Venus has around 90 times as much atmosphere as Earth despite having no intrinsic magnetic field. $\endgroup$ Commented Jul 22, 2023 at 20:48
  • $\begingroup$ Actually if the planet is larger but its gravity is the same, the escape velocity should increase, since the gravity field of the larger planet is more powerful in space. So the atmospheric losses would be significantly lower than on Earth, tweaking the numbers on the star can even allow the planet to accumulate or balance helium, meaning some alpha source hits the planet yet helium escapes the gravity well via stellar radiation more probably than stays. This would result in increased atmospheric density tho, as Earth apparently has lost quite a lot of atmosphere over its history. $\endgroup$
    – Vesper
    Commented Jul 23, 2023 at 20:23
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    $\begingroup$ You might want to look at my answer. $\endgroup$ Commented Jul 24, 2023 at 2:55
  • $\begingroup$ "nearly 3 km/s higher": Correction: 1.3 km/s higher, as in M. A. Golding's answer (I'd typo'd the mass). Still significant. $\endgroup$ Commented Jul 24, 2023 at 3:58
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Absolutely. "Super-earths", terrestrial planets that would be at least somewhat hospitable to human life (liquid surface water, oxygen atmosphere, nitrogen soils) are thought to be possible in a range of between 3/4 and just over twice Earth's mass. A planet 1.5 Earth-masses with the basic necessities is well within the plausible.

That said, the higher mass, 1.55x Earth's, will mean that gravity is elevated compared to Earth. The radius of 7947km pencils out to 1.173x Earth's radius, which when we combine with the mass term in Newton's inverse-square gravity formula gives us $\dfrac{(1)1.55}{1.173^2} \approx 1.13x$ Earth's gravity at the surface of the planet.

This poses a few practical problems, but nothing drastic. First, and obviously, everything weighs 13% more, including the astronauts themselves. So a strapping young 185lb male astronaut now tips the scales at nearly 210lb, into overweight territory for the same height and frame size. Similarly, the 80lb of survival ruck you expected him to carry while exploring this new planet is now about 90lb. For astronaut plus kit to be the same total weight as he'd have likely trained with on Earth, you're talking a pack limit of just 55lb. Beyond that, you're going to see markedly diminished physical performance and endurance during the initial stages of setting up camp and exploring his surroundings, until the astronaut's muscles acclimate to the higher gravity. Building muscle mass will increase weight (but burn fat), so your astronaut's going to get denser as he acclimates, and will likely end up looking pretty shredded just to do the same things with the same pack on that he could do with much less muscle mass on Earth.

That's just a matter of muscle, though. Much harder to retrain is intuition. The higher gravitational force, for the same mass, produces a higher acceleration of gravity than 1g. Physicists and engineers planetwide use 9.8m/s as the shorthand for the acceleration of objects in free-fall toward Earth's surface, and this doesn't change much even out to LEO (weightlessness while in orbit isn't because Earth isn't pulling on us, it's because we're moving so fast to the side that we keep missing).

On your new planet, this is no longer the constant; the new planet's 13% higher gravity now means objects fall toward it at 11.07m/s. While this may again seem a small difference, for a human astronaut that's spent his entire life in 9.8m/s gravity with that behavior of gravity baked into every nerve cell in his body, conscious or otherwise, an extra meter per second squared is plenty to seriously screw with the astronaut's judgment of a gap he can jump or a fall he can absorb. Misjudging your capabilities is what causes injury, and even something as innocuous as a scrape can be 100% guaranteed fatal in a situation where you're billions of miles from the nearest emergency room, in an environment potentially containing single-celled nasties no Earth life form has ever had to contend with in 4.6 billion years of evolution. So, any truly human explorers of this planet will need to exercise extreme care when moving around; the relatively sterile environments of the Moon and Mars are more tolerant of minor superficial injury, the only problem there is such an injury likely comes with a suit compromise as well, which presents more acute problems.

The higher gravity is also going to produce higher atmospheric pressures at surface level; roughly double, depending on how thick the atmosphere is. As a writer, you can tweak this; less atmospheric mass means lower surface pressure, but if you go too thin the atmosphere is more easily blown away by the solar wind (and then no more air). More plausibly, humans can easily tolerate higher atmospheric pressures; in the world of SCUBA, your lungs are under double the pressure at just 10m as they would be at sea level, and divers do this all the time. Higher oxygen content than Earth's may complicate this; oxygen toxicity, caused by spending too much time on high-concentration O2 and/or in a hyperbaric chamber (our super-Earth could well be both), can produce a range of effects from coughing and mild throat irritation up to nausea, confusion, paranoia, convulsions, and it can be fatal. Again, just wave your wand and your planet's partial pressure of oxygen can be adjusted to human-safe levels at the higher pressure, allowing indefinite stays without major side effects.

All this said, there are some inconsistencies in your stated dimensions. You're spec'ing this super-Earth's radius at 1.173x Earth, but a volume 1.9408x Earth's. Any planet big enough to be Earth is going to be basically a sphere, and the volume of a sphere is $\dfrac{4}{3}\pi r^3$. If Earth's radius is 1, your super-Earth's radius is 1.173, and all else being equal that means the volume would be 1.614x Earth's; your planet is about 20% bigger on the inside than the outside. If you want a planet twice the volume of Earth, you find it at a radius about 26% larger than Earth, not 17%.

Your volume of nearly double Earth's would also, at similar Earth density, would also produce approximately double the mass. The actual mass increase you've stated to 1.553x means this super-Earth is only 81.5% the density of Earth's. That's not impossible, but to do that, iron (34% of Earth's mass) would have to be a trace element on this planet. We'd also have to trade quite a bit of silicon for magnesium, not so much because of any big mass savings there, but because magnesium is oxidized with half the oxygen as silicon, allowing us to save more mass there without taking too much out of the atmosphere.

That, believe it or not, is going to be what really stretches the boundaries of what we'd call habitable here. With no iron in the planet's core, there's no magnetic field to turn aside the ionizing radiation of the solar wind (and there isn't a star in the galaxy you could put this planet in orbit of that wouldn't be bathing it in ionizing radiation). Humans' radiation tolerance is pretty good, but average radiation exposure just living your life on Earth is about 2.4mSv; outside LEO and the Earth's protective magnetic field, your astronauts are exposed to about 300mSv/yr, over 100x normal. Can native life on this super-Earth have adapted a more robust cellular and DNA makeup? Sure, but our astronaut's looking at a yearly dose about 6 times NASA's target limit for astronauts, which puts our astronaut at significantly higher cancer risks. The increase in solar wind bombardment also contributes to atmospheric loss; the solar wind literally blows away the thin upper layers of our atmosphere, and without a magnetic field that loss would be accelerated (think of Mars; there's ample evidence it originally had an atmosphere, but it's too small, so the low gravity and the solidifying core combined to allow the solar wind to strip that atmosphere). The higher mass would help keep more, but this is a fine balancing act that a magnetic field makes much easier, as long as your planet has a molten iron dynamo in the middle.

On the flip side, the higher rate of bombardment of the atmosphere with ionizing particles would increase cloud formation; this reduces surface radiation, potentially mitigating the cancer risks to our astronaut (and his need to slather with sunscreen), but also reduces surface reflection (cloud layers actually keep the earth's surface warmer underneath given the same amount of solar gain), and coupled with the higher density it might get uncomfortably warm during the day. Venus, about Earth's size, has a surface temp of about 400*F largely due to its thick methane/sulfuric acid cloud layers insulating the surface.

That's an extreme, but your worldbuilding might emphasize an exceptionally hot and muggy environment, toeing the limit of what a human can survive unsheltered, during the hot hours of the afternoon. That's especially true when you have two more hours of daylight; while there are definitely other factors, a very large component of the difference between summer and winter temperatures is simply how much time the Sun spends above the horizon. However the variables mix up to produce an ambient daytime peak, an average of 14 hours of daylight matches the longest length of an Earth day on the summer solstice in June, so you can expect daytime surface temperatures to be scorchers; luckily, you also get two more hours of darkness for the ground to cool, so it won't stay as hot as it does in Earth's summer solstice, where those 2 extra daylight hours also mean 2 fewer nighttime hours to cool back down.

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A planet around 1.2 times the radius of Earth is certainly plausible and could potentially remain Earth-like. However think of that:

First up - gravity! On this planetary beefcake with 1.6 times the mass of good ol' Earth, you can expect some supercharged gravity giving everything extra weight. We're talking waterfalls with oomph, hailstones that hit like a sack of bricks, and fierce dunk contests. Maybe not ideal for high-flying slam dunks, but hey - check out these thunderous jams!

With higher gravity compressing the atmosphere, you'll also get some crazy air pressure cooking up exotic weather. Storm clouds will loom huge and imposing on the horizon before unleashing fury, and the very air itself would feel more err, airy. Each breath would fill your lungs to capacity...at least until the oxygen leaves you totally breathless.

And all that internal churning and burning deep below the surface? We're talking volcanoes gone wild! Expect fireworks galore with massive eruptions visible from space. The whole planet might glow red hot at times - now that would be a great show!

Of course, that also means the rocks themselves would get their atoms all rearranged under the pressure and heat into weird mineral mashups rarely seen on Earth. A geologist would have a field day! Or just get overwhelmed by the sheer variety. Better pack extra notebooks.

So while this beefy Earth could still harbor life, it would be one heck of a spicy meatball. A little crazy, a lot more intense, but just as fascinating to explore! We may end up with some bona fide superheroes...or supervillains! Only one way to find out.

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    $\begingroup$ the weight number i got using a website & due to the increased size the weight increase is just enough to keep the surface gravity at 1g $\endgroup$
    – OT-64 SKOT
    Commented Jul 22, 2023 at 11:16
  • $\begingroup$ The increased mass would still allow for higher atmospheric pressure and more vigorous internal geophysics as I described $\endgroup$ Commented Jul 22, 2023 at 11:18
  • $\begingroup$ Even planets smaller than Earth can have very high atmospheric pressure...look at Venus or Titan. The larger, deeper gravity well makes a dense atmosphere more likely, but isn't required for it and doesn't require it. And a small-core low density planet with Earthlike surface gravity would have lower internal gravity to drive mantle convection and less primordial heat from a slowly solidifying iron core, so it'd probably be less tectonically active. $\endgroup$ Commented Jul 23, 2023 at 15:24

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