The Question

Is a theoretical 2/3rds - 7/10ths Earth big enough to have the magnetic field, atmosphere and plate tectonics to do this, or will I wind up geologically killing the planet?

I have been toying with the idea of some kind of 31st & 1/2 Century space adventure story. Basically, at this point, humanity has managed to come together enough to develop fusion technology, and explore space in earnest for the last few hundred years. There's no magic, no replicators (but there are nanobots and asteroid mining), no warp drives (but there are relativistic speeds attainable, topping out at about .95c; and suspended animation for people and robot ship crews). It is also assumed:

Lightspeed is a "hard" barrier for the foreseeable future, and affects travel and comms. Colonies are intended to be permanent. The Motherships can continue to sustain life to get the settlement established; if things go catastrophically wrong, there is a factor of safety timewise to abort the mission and return to Earth, or stay for a few generations; but it's an either-or.

There very likely are intelligent beings in the Universe, but they exist too far away from us to visit or communicate. If there are hyperintelligent beings that have surmounted these problems, we are too primitive to be of interest to them.

Humanity has, by this point, established permanent settlements beyond the Solar System. This one would be the 5th extrasolar colony, and still within 60Ly of Earth.

The hypothetical Planet X would have the following characteristics: Edited based on contributions.

Star: FxV type; Planet X is in both the Conservative Habitable Zone and UV Habitable Zone (with a thicker ozone layer than Earth's)

Age: 1.3 - 3Gy. Not enough time to develop intelligent life, but that's not a problem, because humans are arriving from off-world.

Mass: 0.65 - 0.7 Earth-masses

Radius: 0.965 - 1.02 Earth-radii (less dense silicates, nearly pure iron core)

Gravity: 0.68-0.7 G.

Satellites: 1; with a large iron core, about 0.011 +/- 0.003 planetary masses. Appears smaller than Earth's moon due to much greater density. Slightly higher albedo than Earth's Moon. In stable orbit 105 +/- 5% of the distance the Moon is relative to Earth.

Atmosphere: ~1 earth-atmosphere at sea level; 25% oxygen, remainder inert and trace gases, more ozone, more xenon compared to Earth.

Average Surface Temperature: 278.5K, +5.35 C.

Rotation Speed: Somewhat faster than Earth's, resulting in a 17-19 hour day-night cycle, and likely strong winds and very apparent aurorae.

Axial Tilt: About the same as Earth's, resulting in distinct seasons.

Oceans of liquid water exist, as do large polar icecaps. ~30% of the surface is land, although extensive glaciation at the poles make it difficult to tell whether the poles are land or ice.

The planet is home to plant life, mostly algae and kelp-like plants in its oceans; early zooplankton may be starting to develop. On land, lichens, mosses; tall, spindly trees, but no multicellular animal life.

My biggest concern for a sub-Earth sized planet around a brighter and hotter star would be having a big enough active iron core to keep a strong enough magnetic field to resist the stronger solar wind, (despite the greater distance), and yet still have enough silicates left for plate tectonics. I figure a big, ferrous moon will help, through the tidal forces it will exert, along with the rapid rate of rotation.

Does this planet have all the necessary ingredients to maintain an active core, tectonics, magnetosphere and atmosphere, or would it be a geological time bomb?

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    $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – L.Dutch
    Commented Oct 27, 2023 at 4:17

3 Answers 3


Naively, yes, this world should be able to support human life. The temperature is low enough and the mass is high enough for it to be able to retain the necessary atmospheric components such as nitrogen, oxygen, water and carbon dioxide, without retaining hydrogen or helium.

Plate tectonics wouldn't strictly be necessary for the world to be able to support life.

However, there are innumerable other factors that may make or break this world as a potential home for humanity. Put simply, there are substances, such as cyanides, that the local life forms may produce that would poison the world for humanity, and require that humans use breathing apparatus. The low gravity may cause health issues long-term. The local life-forms may find humans to be an excellent growth medium and cause disease, or they may be incompatible with human biology to the point of uselesness or toxicity. The local plants will probably grow better there than terrestrial plants, which will make farming terrestrial plants difficult.

The low average temperature is going to mean that humans are going to need heavier clothing even in equatorial areas, which will mean that the colonists are going to have to grow or find things to make into clothes. Clothing will also be necessary in order to protect from excessive UV exposure from the F-type star.

As for the star, F-type stars have a lifespan of 2-4 billion years before moving into their giant phase, so this 1.3-3 billion-year-old star could quite easily be on the verge of turning into a red giant. It wouldn't happen overnight, but solar expansion might become an issue in a million years or less... not significant over a human lifespan or even the lifespan of a human society, but it ought to be an issue of concern for the colonists if they're thinking long-term.

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    $\begingroup$ How long-term should the colonists think to worry about an issue that is a million years away? It took about half as much since the first Homo Sapiens walked the Earth, and the whole human history spans much less than 10,000 years (1% of a million years). 31st-century humans arriving there as an evolved species would have plenty of time to evolve ten times more. $\endgroup$
    – Simone
    Commented Oct 26, 2023 at 7:27
  • $\begingroup$ It's definitely going to be a tough haul for the colonists. A northern sky, with aurorae from twice to an order of magnitude more brilliant than Earth's, with giant snowflakes falling in slow motion in the foreground... are harder to appreciate when it's -65°C and feels like -100. Low gravity, while not low enough to cause atrophy of the heart and muscles, will definitely impact lymphatic drainage and especially digestion. $\endgroup$ Commented Oct 27, 2023 at 4:02

Plate tectonics seems pretty much necessary.

  1. In absence of plate tectonics there is no orogeny. Just erosion. In the 3 Gy timespan pretty much all the landmass would be washed into the oceans. That includes sulphur and phosphorus, which are essential to life.

  2. In absence of plate tectonics there is no volcanic activity. The heavy elements percolated down below long ago. No lead, copper, iron within easy reach.

  3. No fossil fuels obviously (no uranium within easy reach as well). Looks like a problem with heating for your colony.

  • $\begingroup$ I went with the assumption that plate tectonics are a requirement for habitability, so I wanted to make sure that I didn't create any conditions that would make tectonic activity impossible based on what we know now. $\endgroup$ Commented Oct 26, 2023 at 18:47
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    $\begingroup$ It's all necessary. Pick 'one' to not try and see if you can get away with it. Reading the list I kept waiting to see the core mentioned, which is now the entire question. "big enough to have the magnetic field" $\endgroup$
    – Mazura
    Commented Oct 27, 2023 at 2:18


Not having plate tectonics is a bit of a problem because this would probably reduce the number of continents and islands. Without plate tectonics, the planet would not really be able to separate a supercontinent into several other continents or develop volcanoes. You would need volcanoes so that landforms could exist separate from the ocean. Maybe you could say that pressure built up and the volcanoes violently ripped through the crust, but that would probably take significantly longer because of the fewer eruptions. Or maybe you could have an asteroid collision release lava onto the planet, but this would be a one time event, and unlikely, too.

CO2 Cycle

Another problem with not having volcanoes is that CO2 might be in short demand. Plants would not be able to exist without frequent eruptions, because they would consume all the CO2 and then die. Maybe you could make “nocturnal” plants in addition to “diurnal” plants, because it is a known phenomenon that (“diurnal”) plants can consume oxygen and release carbon during the night.


Also, you are a world builder, you can really do whatever you want and many people probably wouldn’t even take the time to try to figure out whether your world is realistic. Anyone who might want to would need to program a complicated simulation with all the parameters listed in your question. The only people I can think of who would do that are the game theorists, and they closed off the opportunity for a ‘book theory’ when they made the style theorists.

Lastly, I wanted to mention that due to time dilation, you do not even need sleep chambers because at those speeds they will only experience seconds before they reach their destination. This also means that they can come and go without needing extra rations or something. The only disadvantage is that time outside the ship will pass normally, and relatives would pass with it.

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    $\begingroup$ "Basalt weathering removes carbon dioxide from the atmosphere and sequesters it for extremely long timescales." "The geologic carbon cycle is responsible for controlling Earth’s natural climate. In short, volcanoes emit CO2 into the atmosphere, chemical weathering sequesters it into rivers, and carbonate precipitation in oceans traps it as solid rock." carbonclaire.com/the-carbon-cycle - No volcanoes, no basalt. No basalt, no living. - No plate tectonics, no volcanoes? $\endgroup$
    – Mazura
    Commented Oct 27, 2023 at 2:03
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    $\begingroup$ I kind of forgot about this stuff so I was just going on my intuition. I hope a majority is right. I also might’ve contradicted myself around the end. $\endgroup$ Commented Oct 27, 2023 at 14:29

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