How Astronomers Classify an Exoplanet

Two numbers, one density

Nobody has ever seen the surface of a planet outside the solar system in any real detail. What astronomers actually measure is usually just a handful of numbers: how much a planet's transit dims its star's light (giving radius), how much it tugs the star around via radial velocity or transit-timing variations (giving mass), and how far it orbits. From mass and radius together comes bulk density — and density is the single strongest clue to what a planet is actually made of, because rock, ice and hydrogen gas pack very differently.

The rough categories

There is no single official taxonomy, but the community has converged on broad working categories based on radius and mass, calibrated against the solar system's own planets:

Rocky / terrestrial   < ~1.5-1.7 R⊕   density near Earth's, iron+silicate
Super-Earth            ~1-2 R⊕        rocky but more massive than Earth
Sub-Neptune             ~1.7-4 R⊕     rocky/icy core + thin H/He envelope
Neptune-like            ~4-6 R⊕       ice-giant analogue, deep atmosphere
Gas giant               > ~6-10 R⊕    Jupiter-like, mostly hydrogen/helium

The boundaries are fuzzy on purpose — real planets form a continuum, not discrete boxes, and two planets with the same radius can still have quite different masses depending on what fraction of that radius is a puffy gas envelope versus a dense solid core.

The radius valley

One of the most striking discoveries from NASA's Kepler mission was a real gap in the data itself: relatively few close-in planets are found with radii between about 1.5 and 2.0 Earth radii, a feature now called the radius valley or Fulton gap (after Benjamin Fulton, who first mapped it clearly in 2017). It splits close-in small planets into two populations — bare rocky cores below the gap, and sub-Neptunes with a thin hydrogen envelope above it — and is generally explained by photoevaporation or core-powered mass loss: intense stellar radiation strips thin primordial atmospheres off close-in planets, shrinking sub-Neptunes down into bare super-Earths and leaving a depleted zone in between.

Where orbit matters

Orbital distance refines the picture further, mostly by setting equilibrium temperature. A gas giant close to its star becomes a hot Jupiter, often puffed up in radius by the intense heating; the same mass planet farther out is a cold, compact "cold Jupiter." A sub-Neptune close in is a warm Neptune, and is more vulnerable to atmospheric stripping than one farther from its star. None of this changes bulk composition directly, but it explains why planets of similar mass can look quite different, and it flags which planets are candidates for follow-up habitability studies.

Where classification breaks down

Mass and radius alone leave real ambiguity, known as the mass-radius degeneracy: the same density can be produced by a small rocky planet with a thin atmosphere, or by a larger water-world with a different internal structure, or by a rocky core wrapped in a modest gas layer. Breaking that degeneracy typically requires more information — an atmospheric spectrum from transmission spectroscopy during a transit, which can directly detect water vapor, methane or other gases and rule out some interior models outright.

Frequently asked questions

Try it live

Everything above runs in your browser — open Exoplanet Classifier and change the parameters while it is running. Nothing is installed, nothing is uploaded, the whole model lives in one tab.