A newly discovered exoplanet has a measured mean density of 1.1 g/cm³ and a radius 11 times Earth's. What does this density most strongly suggest about its internal composition?
AIt is a rocky super-Earth with an unusually thin crust
BIt is primarily composed of hydrogen and helium, similar to Jupiter
CIt is an ice giant dominated by water, methane, and ammonia
DIts density is too low to have any solid core
Mean density near 1 g/cm³ is the signature of a gas giant — Jupiter is 1.3 g/cm³, Saturn is 0.7 g/cm³, both overwhelmingly composed of hydrogen and helium. Rocky planets have densities well above 3 g/cm³ (Earth is 5.5 g/cm³). Ice giants like Neptune are around 1.6 g/cm³. A low-density planet of 11× Earth's radius is unambiguously a gas giant. Mean density is how we fingerprint bulk composition without direct observation.
Question 2 Multiple Choice
Why do gas giants like Jupiter exist far from their host star, while rocky terrestrial planets form closer in?
AGas giants are older and had more time to migrate outward through the disk
BBeyond the frost line, water ice and other volatiles could condense, dramatically increasing available solid material and allowing cores to grow massive enough to gravitationally capture hydrogen and helium envelopes
CStronger gravity far from the star attracts more gas
DGas giants form when rocky planets collide and merge at large distances
The frost line (roughly 3–5 AU) is the key. Inside it, temperatures are too high for water ice and other volatiles to condense — only metals and silicates survive, producing smaller rocky planets. Beyond it, volatiles condense into solids, providing far more material for planet building. Cores that reached ~10 Earth masses then had enough gravity to capture the vast hydrogen-helium envelopes that define gas giants. This compositional gradient explains the entire inner-vs-outer solar system architecture.
Question 3 True / False
Earth's iron core was confirmed by drilling samples taken from deep in the mantle.
TTrue
FFalse
Answer: False
We have never drilled more than about 12 km into Earth — a tiny fraction of the ~6,371 km radius. Knowledge of Earth's core comes from indirect evidence: mean density (5.5 g/cm³ far exceeds surface rocks at ~2.7 g/cm³), seismic waves (which refract and reflect at density boundaries, revealing the core's size and liquid/solid state), and the magnetic field (generated by convection in a liquid conducting outer core). All planetary interior knowledge is inferential, not from direct sampling.
Question 4 True / False
Saturn's mean density is lower than liquid water, which proves it cannot have a rocky or icy core.
TTrue
FFalse
Answer: False
Saturn's mean density of ~0.7 g/cm³ reflects its enormous hydrogen-helium envelope — the average includes the vast outer layers of extremely low-density gas. Saturn almost certainly has a rocky/icy core estimated at 10–20 Earth masses at its center; that core contributes only a small fraction of Saturn's total volume. Mean density is an average over the whole planet, and a small dense core can be completely masked by a large low-density envelope. Density alone does not rule out solid cores in gas giants.
Question 5 Short Answer
How do scientists determine the internal structure of planets they cannot physically sample, and what is the most fundamental observational constraint?
Think about your answer, then reveal below.
Model answer: Planetary interior structure is inferred from multiple indirect lines of evidence: mean density (calculated from mass and radius) constrains bulk composition; seismic waves (for Earth) reveal density and phase boundaries; the moment of inertia (from rotational dynamics) shows how mass is distributed radially; magnetic field geometry constrains core size and dynamics; and gravitational harmonics measured by orbiting spacecraft reveal interior mass distribution. Mean density is the most fundamental constraint because it is derivable from just two observable quantities (mass and radius) and immediately indicates whether a planet is rocky, icy, or gaseous.
The key skill is reasoning from observable quantities back to internal structure. No direct observation is possible, so every conclusion is a model constrained by multiple independent measurements. When those measurements converge, confidence in the model grows.