Journey to Planet 9: Part III

If Planet 9 is a mini Gas Giant, rather than an Ice Giant or Super Earth, then it’ll be similar to Jupiter and Saturn in size, even if it’s much lighter. Jupiter’s gravity compresses hydrogen into its dense metallic phase, thus causing that planet to be much smaller than it would be if it was just a gas ball.

With a mass of 10 Earths and a radius of 8, the ‘surface’ gravity will be just 10/64 times Earth’s, or about Lunar gravity. Because I’m assuming a silicate core of just 1 Earth mass, it means the heat-flow from radioactive decay is diluted by all that extra area to radiate it from. Instead of ~38 K effective temperature, for an Earth mass of silicate, it’ll be ~13 K, below the Triple point of hydrogen, which is 13.84 K at 0.0704 bar pressure. At the Triple point the gas, liquid and solid phases co-exist. Colder than that and only ice and gas co-exist. Light from the Sun will achieve an equilibrium temperature of 9 K, so there’s no warmth from that source. Only radioactivity and residual formation heat are likely to stir the atmosphere.

Could Planet 9 be a Hydrogen Ice Planet, wrapped in thick gaseous helium? No. Too hot, even at -260 K. It’s possible that hydrogen has condensed into ice clouds in the outer fringes, but lower down condensed oceans of hydrogen resting on compressed molecular hydrogen might be possible. Really depends on the efficiency with which heat is convected through the H2/He mix. The outer reaches, if hydrogen is confined to the depths, will be enriched in helium, which makes the task of atmosphere mining much easier.

Just how easy? Presently we have no data on its rotation rate. If it was rigidly solid, then the maximum spin rate is 10 hours – at that point its surface gravity would equal the centrifugal force created by its spin, thus it would fly apart. Long before it reached that point, because on planetary scales all matter behaves like a fluid object, it would distort into an oblate spheroid, which would decrease the effective gravity at the equator significantly.


The relationship between the spin and the flattening of the planet gets complicated because it depends heavily on how the mass of the planet is distributed – mostly in a dense core or spread evenly? Gas planets, because gases compress significantly as the pressure rises, are especially centrally condensed. For our hypothetical planet the flattening becomes significant even if the day is 20 hours, but not enough to disrupt the planet. Modelling the planet as a Maclaurin Spheroid, this would mean an ellipticity of 0.362, an eccentricity of 0.77, and a difference between the rotational and orbital speeds of just over 3 km/s. This would make the planet *incredibly* easy to mine via gas scoop.

But it would also mean aerobraking a madly careening e-sail flying at ~143 km/s would be significantly easier, due to the lower density gradient in the lower gravity atmosphere. The surface gravity is about 1/8th Earth, and 1/20th Jupiter, so even though it’s significantly colder – 10 times colder than Jupiter – the slower rate at which it gets denser with depth means more room to brake in.