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The Question Could this hypothetical Super-Earth (Planet Y) support the lives of human settlers arriving from off world, or have I created any reasons for the planet to be inhospitable?

Planet Y

Star: Early KxV; likely between K0V and K3V. Orbit is stable, regular, within the Conservative Habitable Zone and outside the tidal locking zone.

Size: Edited based on answers. Note also that the following values are approximations (calculator-assisted bar napkin math)

Mass: 1.85 MEarth

Radius: 1.25 REarth

Gravity: 1.2 G

Density: 5.7 g/cm^2

Escape Velocity: 13.75 km/s

Core: Active, molten iron-nickel-cobalt core, rocky mantle, tectonically active crust. Around 80% of planet's surface is covered in water. Poles have no solid land mass, and small ice caps.

Mantle: Molten Silicate

Crust: Tectonically active, volcanic activity is present. Approximately 80% of the surface is water, small ice cap at south pole.

Average Surface Temperature: 22 C.

Atmosphere: 1.2 ATM at sea level, ~17% oxygen, remainder buffer/inert/trace gases. Less ozone, more water vapour than Earth's atmosphere.

Satellites: 3

Y i: Large, (0.03 +/- 0.02) planetary mass regular satellite, in stable orbit. Primarily silicate composition, small metallic core.

Y ii: Small, irregular, solid moon.

Y iii: Small, irregular moon; rubble pile with thin crust. Possibly re-accreted after disruption.

Day Length: 32 Earth-hours.

Axial Tilt: Approximately 20 degrees, distinct seasons.

Life

Plant life on land and in oceans; oceans also host early zooplankton, but nothing higher ordered.

With Planet X (lower-G sub-Earth orbiting a F6V-F9V star), I had a nearly inverted set of problems- packing enough of a core into a lighter planet in a much higher UV environment to keep it alive geologically and give it a protective magnetosphere.

From my research, my problems are making sure the planet doesn't have a runaway greenhouse effect, and that the higher gravity and higher pressure atmosphere don't cause long term ailments.

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I see you have omitted to give the density or the escape velocity of the planet.

If the planet has 1.4 the radius of Earth, it will have 2.744 times the volume. With three times the mass in 2.744 times the volume, the planet will have an average density of 1.093 Earth's average density, and thus about 6.028 grams per cubic centimeter. Since the planet is more massive than Earth, its gravity will compress its internal material more than Earth does, so possibly it is made out of a similar mix of materials.

According to this online escape velocity calculator:

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

The escape velocity a planet with 3 times the mass of Earth and 1.4 times the radius of Earth will be 16.375 kilometers per second, 1.4639 times that of Earth.

And according to this surface gravity calculator

https://philip-p-ide.uk/doku.php/blog/articles/software/surface_gravity_calc

The surface gravity of your planet should be 1.53 g.

You write:

Could this hypothetical Super-Earth (Planet Y) support the lives of human settlers arriving from off world, or have I created any reasons for the planet to be inhospitable?

I think that humans who arrive directly from Earth may find a surface gravity of 1.5 or 1.53 g to be both uncomfortable and unhealthy. They might die sooner and enjoy life less.

The limit of surface gravity tolerable for humans was discussed by Stephen H. Dole in Habitable Planets for Man (1964).

https://apps.dtic.mil/sti/tr/pdf/ADA473471.pdf

On pages 11 to 13 and on page 12 he decided that probably few people might want to settle and live permanently on a planet with a surface gravity higher than 1.25 or 1.5 g.

And I agree that most people wouldn't want to settle on a planet with a surface gravity of 1.5 g.

Possibly the ancestors of the human settlers settled on a planet with a higher surface gravity than Earth, and became adjusted to that surface gravity over many generations, so that the ones who settle on your planet might find it only slightly higher than they like.

Possibly a society advanced enough for interstellar travel might have invented gravity control. If so the settlers on our planet might use gravity control to reduce the gravity within their buildings and settlements to tolerable levels, and might even wear anti gravity belts when outside their communities.

On page 53 Dole gives the characteristics of an Earthlike planet with a surface gravity of 1.5 g. Dole calculates that it should probably have a mass of 2.35 Earth mass, a radius of 1.25 Earth radii, and an escape velocity of 15.3 kilometers per second [1.36778 times that of Earth]. Such a world would have a volume 1.953 that of Earth, and thus an average density 1.203 times that of Earth, and thus grams per cubic centimeter.

Dole says that assumes that other processes won't make the limit of habitabity even lower. For example, a world might be covered entirely in ocean, or might have an atmosphere so dense that it is toxic, or an atmosphere so opaque that plants on the surface can't get enough light to produce oxygen, etc.

I note there are many lifeforms on Earth which thrive in conditions which would be swiftly lethal to unprotected humans. Most scientific discussions of the habitability of other worlds discuss habitability for liquid water using lifeforms in general and not for humans - or beings with same requirements as humans - in particular. Planets habitable for humans should be a subset for planets habitable for liquid water using life.

Here is a link to an article from 2013 on the potential habitability of exomoons.

https://arxiv.org/ftp/arxiv/papers/1209/1209.5323.pdf

On pages 3 to 4 the mass range for habitable planets and moons is discussed.

A minimum mass of an exomoon is required to drive a magnetic shield on a billion-year timescale (Ms ≳ 0.1M⊕, Tachinami et al. 2011); to sustain a substantial, long-lived atmosphere (Ms ≳ 0.12M⊕, Williams et al. 1997; Kaltenegger 2000); and to drive tectonic activity (Ms ≳ 0.23M⊕, Williams et al. 1997), which is necessary to maintain plate tectonics and to support the carbon-silicate cycle. Weak internal dynamos have been detected in Mercury and Ganymede (Kivelson et al. 1996; Gurnett et al. 1996), suggesting that satellite masses > 0.25M⊕ will be adequate for considerations of exomoon habitability. This lower limit, however, is not a fixed number. Further sources of energy – such as radiogenic and tidal heating, and the effect of a moon’s composition and structure – can alter our limit in either direction. An upper mass limit is given by the fact that increasing mass leads to high pressures in the moon’s interior, which will increase the mantle viscosity and depress heat transfer throughout the mantle as well as in the core. Above a critical mass, the dynamo is strongly suppressed and becomes too weak to generate a magnetic field or sustain plate tectonics. This maximum mass can be placed around 2M⊕ (Gaidos et al. 2010; Noack & Breuer 2011; Stamenković et al. 2011). Summing up these conditions, we expect approximately Earth-mass moons to be habitable, and these objects could be detectable with the newly started Hunt for Exomoons with Kepler (HEK) project (Kipping et al. 2012).

The papers which claim that the upper mass limit for a world to habitable is about 2 times the mass of the Earth include:

https://iopscience.iop.org/article/10.1088/0004-637X/718/2/596/pdf

https://meetingorganizer.copernicus.org/EPSC-DPS2011/EPSC-DPS2011-890.pdf

There has been much discussion about the hypothetical habitability of planets of the type called Super-Earths.

In general, super-Earths are defined by their masses. The term does not imply temperatures, compositions, orbital properties, habitability, or environments. While sources generally agree on an upper bound of 10 Earth masses14 (~69% of the mass of Uranus, which is the Solar System's giant planet with the least mass), the lower bound varies from 11 or 1.94 to 5,3 with various other definitions appearing in the popular media.57 The term "super-Earth" is also used by astronomers to refer to planets bigger than Earth-like planets (from 0.8 to 1.2 Earth-radius), but smaller than mini-Neptunes (from 2 to 4 Earth-radii).[8][9] This definition was made by the Kepler space telescope personnel.[10] Some authors further suggest that the term Super-Earth might be limited to rocky planets without a significant atmosphere, or planets that have not just atmospheres but also solid surfaces or oceans with a sharp boundary between liquid and atmosphere, which the four giant planets in the Solar System do not have.[11] Planets above 10 Earth masses are termed massive solid planets,[12] mega-Earths,[13][14] or gas giant planets,[15] depending on whether they are mostly rock and ice or mostly gas.

https://en.wikipedia.org/wiki/Super-Earth

Further theoretical work by Valencia and others suggests that super-Earths would be more geologically active than Earth, with more vigorous plate tectonics due to thinner plates under more stress. In fact, their models suggested that Earth was itself a "borderline" case, just barely large enough to sustain plate tectonics.[82] However, other studies determined that strong convection currents in the mantle acting on strong gravity would make the crust stronger and thus inhibit plate tectonics. The planet's surface would be too strong for the forces of magma to break the crust into plates.[83]

Earth's magnetic field results from its flowing liquid metallic core, but in super-Earths the mass can produce high pressures with large viscosities and high melting temperatures, which could prevent the interiors from separating into different layers and so result in undifferentiated coreless mantles. Magnesium oxide, which is rocky on Earth, can be a liquid metal at the pressures and temperatures found in super-Earths and could generate a magnetic field in the mantles of super-Earths.[93] That said, super-Earth magnetic fields are yet to be detected observationally.

https://en.wikipedia.org/wiki/Super-Earth

A writer who doesn't care that their story might have a very low score on the scale of science fiction hardness:

https://tvtropes.org/pmwiki/pmwiki.php/SlidingScale/MohsScaleOfScienceFictionHardness

Like Star Wars, for example, can just ignore the possible problems of a world like yours being habitable for humans. It would not seem obviously uninhabitable to the majority of readers.

But a writer who wants a higher score and cares more for scientific plausibility might want to do some more research in order to decide whether they consider a habitable planet with your attributes to be plausible enough for their story.

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    $\begingroup$ Thank you for a very thorough answer! To answer your last question, this planet will require some re-thinking. I wanted something that's a bit extreme, but such a high-G environment would be too extreme. 1.25G would be a better upper bound. Given how dense it is, this planet would likely compress itself further, and become even denser, with an even more intolerable G level. Bringing the mass down would also make any explanation as to why it has active plate tectonics more plausible. $\endgroup$ Commented Oct 30, 2023 at 5:40
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    $\begingroup$ This is a good answer. Want a challenge? We get a lot of "here's my planet, can it support life?" questions. An official "Guideline" question with a community wiki answer that walked people through everything you just did with some generalized, "here's what you need to do to make your planet conform with known science" (e.g., making things like density, etc. line up) would seriously help the Stack. $\endgroup$
    – JBH
    Commented Oct 30, 2023 at 6:56

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