Did You Know: Gas Giants, All Gas?
We’re often told that the solar system is a simple dichotomy between terrestrial (rock) planets and gas/ice giants, or inner solar system vs. outer solar system. But are all of the gas giants purely gas?
Moment of Inertia
Inertial measurements of a planet can actually suggest what constitutes its core; such is the case with Saturn (Helled 2013). In measuring a spacecraft’s acceleration about a planet, as well as other qualities like precession, angular momentum, and rotation rate, scientists can estimate a planet’s moment of inertia. But there are a variety of other factors that are needed to understand the internal structure of a planet, leading to far more complicated mathematics for this post.
Moment of inertia is suggestive of density and the distribution of mass in a planet. With known measurements and models, the composition of planetary interiors, and thus cores, can be determined to a certain point.
So what’s in there, and why?
While science is generally up in the air about the composition of Jupiter’s core (i.e. whether it is solid), Saturn likely has a rocky core of 12 Earth masses (1 Earth mass = 5.9 x 10^24 kg); Neptune and Uranus have similar-sized, although differently composed, cores. So why is this?
During the early years of the solar system, planets formed through the process of planetary accretion. The system was originally a proto-planetary disc of dust that rotated around the young Sun; over time, particles collided and grew bigger, and larger masses accreted, or gathered, more material due to the basic idea of gravity as a function of mass. Therefore, larger bodies increased in size at an exponential level.
The terrestrial planets formed close to the sun from planetesimals (proto-planets) and never reached the proper mass to capture gases, while the future gas giants formed further from the sun, where there was more material, and were able to gain enough mass to capture gas.
So while we consider the gas giants to be enormously gaseous, they still likely have rock (silicates), metal, liquids, and/or solid ice at their cores. Saturn’s core even has an effect on its shape; it is far more oblate (stretched out/deformed from a sphere) than Jupiter because of a larger core. Meanwhile, Neptune and Uranus’s cores also consist of methane, ammonia, and water as both solids and liquids (they are indeed the ice giants!).
(Image sources: 1: x | 2: x | 3: University of Rochester; additional info source: x)