How space works
Rocky planets, super-Earths, sub-Neptunes, and gas giants
Planet labels are useful starting points, but size, mass, and composition are different measurements. A large Earth-shaped silhouette can hide a very different interior.
Updated
What makes a rocky world
Mercury, Venus, Earth, and Mars have substantial rock and metal interiors. Their surfaces and atmospheres nevertheless differ sharply. A rocky planet can be airless, covered by a dense atmosphere, volcanically active, or geologically quiet. The material class does not establish a climate.
For an exoplanet, radius can suggest possible compositions, but mass helps narrow the choices. A small radius and high density can favor a rock-and-metal interior. Different mixtures of iron, silicates, water, and gas can still fit overlapping measurements. The inference is strongest when the uncertainties are small and the observational model is well tested.
Super-Earth is a size or mass label
A super-Earth is larger or more massive than Earth within the range commonly discussed between Earth and the ice giants. Authors use slightly different boundaries. The name does not promise continents, blue seas, breathable air, or a surface one could stand on. Some objects in this broad range may have substantial volatile layers.
Sub-Neptune is also a broad population label. Many planets between Earth's and Neptune's sizes have no close Solar System counterpart. A modest envelope of hydrogen and helium can noticeably increase radius without containing most of the planet's mass. Two equally large planets can therefore have very different densities and interior structures.
Gas giant and ice giant describe different histories
Jupiter and Saturn are dominated by hydrogen and helium. Uranus and Neptune contain a larger proportion of heavier materials and are called ice giants. In planetary science, the word ice often refers to materials such as water, methane, and ammonia in formation models; it does not mean an ice giant is a frozen solid globe throughout.
Deep inside a planet, pressure and temperature create states of matter unlike ordinary surface conditions on Earth. A sharp rocky surface cannot be assumed beneath a gas giant's clouds. Radius measurements for a gas-rich exoplanet often describe the level where its atmosphere becomes opaque during a transit, which adds another reason to specify how a size was measured.
Why mass and radius belong together
Bulk density is mass divided by volume. If a spherical planet has mass M in Earth masses and radius R in Earth radii, its density relative to Earth is M/R³. Because radius is cubed, a small fractional radius uncertainty can have a large effect on density. A minimum mass from radial velocity needs extra care.
The Codex's reference profiles retain the archive's mass provenance. A value labeled M sin i is a lower bound set by the unknown orbital inclination, rather than a direct true-mass measurement. A planet with no measured radius should remain without a measured radius. Substituting a guessed radius would make a table look complete while weakening its scientific value.