[pfdietz]

Rare earths are neither particularly rare, nor particularly likely to be unusually abundant on a Planet 9.

However!

A Mars or Earth-sized planet sufficiently far out in the solar system might be sufficiently cool to be able to retain helium in its atmosphere, without being a gas giant planet. If so, it might conceivably be the best place in the solar system to obtain 3He, an isotope that has considerable attractiveness for use in fusion reactors. A D-3He fusion rocket might even provide a nice way of getting out there and back in a reasonable time.

[jhgb]

I don't see how the "attractiveness" of helium-3 is anything non-zero. If you can't even fuse deuterium with tritium, which is the easiest thing there is, helium-3 fusion goes out of the window.

[pfdietz]

DT is certainly easier to fuse, but it presents very difficult, IMO likely intractable engineering problems. D3He would finesse those problems (neutrons, material damage, tritium breeding), and potentially enable direct conversion of fusion energy to electrical energy (or, perhaps, more volumetrically efficient transfer of thermal energy to coolant). So IF the physics can be made to work, D3He could end up being more practical.

The company to watch on this is Helion, and perhaps Princeton Satellite Systems.

https://www.helionenergy.com/

https://vimeo.com/553784697

[caymanjim]

The moon already has a whole lot of helium 3 that'd be economically viable right now, if there were a use for it yet.

[pfdietz]

The moon has 3He, at a concentration of maybe 10ppb, in regolith the heating of which would use more energy than the released 3He would yield in fusion.

Helium at cosmic isotope ratios is 120ppm 3He. So if there were a planet out there with an atmosphere of this helium, extracting the 3He would be much easier (just compress, liquefy and separate, which you'd have to do with lunar helium anyway.)