Hot Jupiters, Super-Earths & Mini-Neptunes: Observation, Measurement & Evidence

Hot Jupiters are gas giants on very short-period orbits. Super-Earths are larger or more massive than Earth but smaller than Neptune, while mini-Neptunes occupy a similar size range but retain substantial volatile envelopes. Radius alone does not uniquely determine composition.

Transit surveys measure radii and periods, radial velocities measure masses, and combined density measurements reveal a wide range of compositions. The observed radius distribution includes features interpreted as evidence for atmospheric loss and formation differences.

Close-in giant planets likely form farther from their stars and migrate inward or undergo dynamical rearrangement. Intermediate-size planets can acquire varying mixtures of rock, water, hydrogen, and helium, and intense stellar irradiation can strip atmospheric envelopes from some worlds.

Transit radii plus radial-velocity or TTV masses yield density distributions. Population features such as the radius valley show that atmospheric evolution has altered the observed planet census.