rating: 0+x

Basic Information

A mini-neptune or sub-neptune is a much smaller version of the gas giant or ice giant planet. A sub-neptune is likely to be larger than the Earth, but not a lot larger, and much smaller than our Neptune. We're talking about a radius less than 4 times that of the Earth. While there are no mini-neptunes in our solar system, data from the Kepler space telescope suggests that mini-neptunes are the most common planet type in our galaxy. Why other systems are full of them and our system doesn't have one at all remains something of a mystery.

Mini-neptunes have a thick atmosphere, beneath which may be layers of ice, rock or liquid. One popular theory is that the surface of such planets is molten magma, super-saturated with compressed hydrogen. Or, if that theory is wrong, they may be more like a classic ice planet concept. We just don't know for sure with the data we have at the moment.



Game and Story Use

  • Since these seem to be exceedingly common galactically, it may help establish verisimilitude to liberally pepper your planetary system maps with a handful of these.
  • The super-compressed hydrogenated magma core may make these planets a valuable source of mineral wealth such as metallic hydrogen fuel, but with corresponding difficulty of extracting the wealth under such titanic pressure and harsh conditions.
  • Why doesn't our solar system have one?
    • Is it a sign of precursor meddling?
      • Did some ancient force artificially inflate our Neptune? What may be hidden there, waiting for us? Is there a stellar megastructure serving as infrastructure or framework inside our Neptune.
      • Or is it the other way around, with some ancient force stripping the atmospheres and shrinking gas giants in other systems for some nefarious reason? (We are talking about a huge amount of gas, though, so whatever your explanation, it's going to need to epic.)
    • Or is it indicative of some fairly unique set of local circumstances that also helps explain why life is here and space-faring civilizations appear to be rare or hidden? Did the existence of Jupiter warp the foundational development of our system in some way that made life more likely to survive on earth, and also allowed our Neptune to grow beyond the norm?
      • This could theoretically mean that we should expect to find mini-neptunes all over the place, but not in any system where advanced life is likely to have evolved. The logic behind that is pretty spurious and unlikely to hold true, but it's not automatically impossible and it sounds cool, so let's assume for a moment that this crazy idea is right. The implications would be:
        • The homeworld of any given intelligent species is likely to be in a system with no mini-neptunes. So that would mean SETI would be smart to use the lack of mini-neptunes as criteria for which star systems to investigate. Which means it also might help the author or GM decide on a good origin world for their aliens based on Kepler data.
        • You might decide on some other more sinister explanation for this hypothetical correlation, like the existence of a ravenous intergalactic lifeform that uses mini-neptunes as a spawning ground, and kills all competition in systems with such worlds in them. In this scenario, we only exist because there's no local breeding ground for The Great Devourers.
        • Any unique resources that can only be found on a mini-neptune would automatically be out-of-reach for new star-going civilizations until they'd progressed enough to establish a space colony several light years from home. This suggests a tech tree with prizes locked away for the endgame. If you go with the metallic hydrogen fuel notion (or something similar), then you get sort of an ironic relationship where only after you've sent a generation ship or von neumann probe to reach out to another system, you discover the destination has an abundance of resources that would have been really helpful to have access to before you sent that ship out in the first place.
  • If you want an alien that's really alien, there are probably some hypothetical types of biochemistry ideas for extremophile life that might evolve in the intense pressure and heat of the hydrogen-magma. Perhaps Silicon-Based Life would be reasonable, much as it might on a Hot Jupiter?
    • Combine that with the meddling hypothesis - there's a really advanced species out there that has "terraformed" a lot of planets into their favoured type … we're just not noticing them due to the differences in tech and biology.
Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License