Rock or gas? which measured parameters reveal a rocky exoplanet in the NASA exoplanet archive

More than six thousand exoplanets are known, but for most of them it is not known whether they are made of rock or of gas. This study uses the 1,363 planets in the NASA Exoplanet Archive whose mass and radius are both well measured to ask which parameters best separate rocky planets from the rest. A planet was called rocky if its radius is no larger than that of a planet of the same mass with an Earth-like composition; 135 planets met this condition. Because this label is calculated from mass and radius, density separates the two groups almost perfectly (96% balanced accuracy) by construction. Among the parameters not used to define the label, the mass of the host star separates them best (69%), followed by the star's temperature and the orbital period, while the planet's temperature and the light it receives are close to chance. For giant planets above one Jupiter mass the radius hardly changes with mass, so density rises steeply with it (Spearman ρ = 0.78), and 27 of the 38 giants of 6-13 Jupiter masses are denser than the Earth. The author's expectation that density shows the type best holds only as a consequence of the definition, and his explanation of dense gas giants is consistent with the data.