Exoplanet Glossary

The astronomical unit (AU) is a standard measure of distance in astronomy, defined as exactly 149,597,870,700 meters—the average distance from Earth to the Sun. In the Galactic Codex, it helps describe how close an exoplanet orbits its star (e.g., “0.14 AU” means about one-seventh the distance from Earth to our Sun). Smaller numbers mean tighter, hotter orbits; larger numbers mean wider, cooler ones. This handy unit lets us compare alien worlds to our own Solar System in a simple, familiar way!

A biosignature is a sign(s) in a planet’s atmosphere (like oxygen or methane) that could indicate life. Not proof, but a big “maybe aliens!” clue scientists hunt for. 

An exoplanet is a planet orbiting a star beyond our Solar System. Over 5,700 confirmed—ranging from rocky Earth-twins to massive gas giants.

The Goldilocks zone is the “just right” distance from a star where liquid water could exist on a planet’s surface—not too hot, not too cold.

Hot Jupiters are massive gas giants orbiting extremely close to their stars—scorching hot with wild winds and sometimes molten glass rain.

A light-year is the distance light travels in one year—about 6 trillion miles (9.5 trillion km). Used to measure how far exoplanets are from Earth.

Radial velocity is a detection method: measuring a star’s tiny “wobble” from a planet’s gravity pulling it side to side.

A red dwarf is a small, cool star—most common in the galaxy. Many exoplanets orbit them, with dramatic red sunsets.

A rogue planet is a planet drifting through space without orbiting any star—free-floating and super cold.

A super earth is a rocky planet 1.5–2 times Earth’s size—bigger gravity, possibly thicker atmospheres or oceans.

Transit method is the most common detection method: spotting a planet by the tiny dip in starlight when it passes in front.

Tidal locking is when one side of a planet always faces its star (like our Moon to Earth)—eternal day on one side, eternal night on the other.

Water vapor is a key molecule scientists look for in exoplanet atmospheres—hint of possible oceans and habitability.