Glossary

The words behind the worlds. Plain-language definitions of the terms this site uses to describe a planet: what it means, and a real world or tool that shows it. No jargon left unexplained.

• Habitable zoneThe band of orbits around a star where a rocky planet could hold liquid water on its surface, given the right atmosphere.
• Earth Similarity IndexA score from 0 to 1 for how much a world resembles Earth in size and temperature, with Earth itself defined as 1.0.
• Tidal lockingWhen a world's spin matches its orbit so the same face always points at its partner, the way the Moon always shows Earth one side.
• Super-EarthA rocky planet larger than Earth but smaller than the ice giants, up to roughly twice Earth's radius.
• Sub-NeptuneA planet between Earth and Neptune in size, usually wrapped in a thick gas envelope with no true surface to stand on.
• Gas giantA huge planet made mostly of hydrogen and helium, with no solid surface, like Jupiter or Saturn.
• Equilibrium temperatureThe temperature a planet settles at from its star's light alone, before any greenhouse warming from an atmosphere.
• Minimum mass (M sin i)A lower bound on a planet's mass from the wobble method: the true mass could be higher, depending on the unknown tilt of its orbit.
• Transit methodFinding a planet by the tiny, repeating dip in its star's light each time the planet crosses in front of it.
• Radial velocityFinding a planet by the small gravitational wobble it tugs in its star, seen as a rhythmic shift in the star's light.
• Light-yearThe distance light travels in one year, about 9.5 trillion kilometers, used to measure the gulf between the stars.
• Surface gravityThe pull you would feel standing on a world, set by its mass and its size, and what decides your weight there.