Astronomers sort the thousands of confirmed exoplanets into broad categories using just two measurable properties: mass and radius. This simulation lets you dial in both (plus orbital distance) and instantly classifies your hypothetical world while plotting it on a real mass-radius diagram.
Dividing mass by the cube of radius gives density, the single best clue to bulk composition. Small, dense worlds are rocky terrestrials; slightly larger but still dense ones are super-Earths; low-density worlds of similar size are mini-Neptunes wrapped in thick volatile envelopes; and large, light worlds are ice giants or gas giants.
Drag Mass and Radius to reshape the planet and watch its point move across the mass-radius diagram and its classification update. Adjust Orbital distance to see the illustrative colour tint shift from a hot, close-in world toward a cool, distant one — and watch a close-in massive planet flip into "Hot Jupiter."
Mini-Neptunes — planets between Earth and Neptune in size — are the single most common type of planet found by the Kepler mission, yet there is no equivalent world anywhere in our own Solar System.
This exoplanet classification simulation computes bulk density from mass and radius (density = mass ÷ radius³, scaled to Earth = 5.51 g/cm³) and applies threshold rules broadly consistent with how astronomers group real discoveries into terrestrial planets, super-Earths, mini-Neptunes, ice giants, gas giants and hot Jupiters. Your planet is plotted live on a logarithmic mass-radius diagram next to the approximate zones each category occupies.
Two planets can share a radius yet be completely different worlds if their masses (and therefore densities) differ — a dense small planet is rock and iron, while a low-density planet of the same size must carry a deep atmosphere or ice layer. This mass-radius relationship is the primary tool real astronomers use to guess bulk composition from a handful of measurements.
Move Mass and Radius across their full ranges (from Mars-sized rocky bodies to planets bigger than Jupiter) and watch the classification label and mass-radius plot update instantly. Move Orbital distance inward to see the planet's colour warm up and, for large enough planets, flip the classification to Hot Jupiter.
The most common exoplanet size found by the Kepler space telescope — between 1.5 and 2 times Earth's radius — has no counterpart in our own Solar System, sitting in the gap between super-Earths and mini-Neptunes known informally as the "radius valley."
Radius usually comes from the transit method, measuring how much a planet dims its star's light as it passes in front; mass usually comes from the radial-velocity method, measuring the tiny wobble the planet's gravity induces in its star. Combining both gives density and a window into composition.
Both can have similar radii, roughly 1.5 to 3.5 times Earth's, but a super-Earth is dense and rocky like Earth scaled up, while a mini-Neptune (or sub-Neptune) is far less dense because a thick hydrogen-helium or water-vapour envelope inflates its radius without adding much mass.
A Hot Jupiter is a gas giant, comparable in mass and size to Jupiter, that orbits extremely close to its star — often closer than Mercury is to the Sun — completing an orbit in just days. Their existence was a major surprise, since standard models predicted gas giants should only form far from their star.
Ice giants formed with less of the surrounding hydrogen and helium gas available, or formed later when the protoplanetary disc's gas was already dissipating, leaving them relatively enriched in heavier "ices" like water, ammonia and methane compared to true gas giants.
The categories mirror real ones used in exoplanet science, but the exact thresholds here are simplified for teaching purposes. Real classification also draws on detailed interior models, atmospheric spectra, and comparison to Solar System benchmarks, not mass and radius alone.