Astronomers can rarely see an exoplanet directly. Instead, missions like Kepler and TESS measure how much a star dims when a planet transits it (giving the planet's radius) and ground telescopes measure the star's tiny wobble from radial velocity (giving the planet's mass). Combining the two yields bulk density — the single number that tells you whether a world is a ball of rock and iron, or a puffy envelope of hydrogen and helium.
More than 5,000 confirmed exoplanets have been found so far, and the single most common size in the galaxy — one with almost no equivalent in our own solar system — falls right in that 1.5–2 Earth-radii "radius valley," split between rocky super-Earths and gas-wrapped sub-Neptunes by nothing more than how much atmosphere survived their star's early, intense radiation.
Set a simulated exoplanet's mass, radius and orbital distance and watch it slot live into a real classification category — hot Jupiter, super-Earth, ice giant or potentially habitable rocky world — exactly as astronomers reason from transit and radial-velocity data.
Mass and radius combine into bulk density, which separates rocky from gas-wrapped worlds; orbital distance relative to the host star's habitable zone then determines whether a rocky planet could host liquid water.
Drag the mass, radius and orbital-distance sliders, switch the host star type, and watch the classification badge, density, equilibrium temperature and habitable-zone status update instantly as the planet orbits.
Planets between about 1.5 and 2 Earth radii are surprisingly rare — a "radius valley" splits them into dense rocky super-Earths and puffy, gas-wrapped sub-Neptunes, likely because early stellar radiation strips thin atmospheres away.