Exoplanet Glossary

Terrestrial planet

A world made mostly of rock and metal.

Earth, Venus, Mars, and Mercury are nearby examples; rocky does not mean habitable.

Super-Earth

A planet more massive than Earth but less massive than Neptune; usage and boundaries vary.

The name does not promise an Earth-like surface, atmosphere, or climate.

Sub-Neptune / mini-Neptune

Terms often used for planets smaller than Neptune, many with substantial gaseous envelopes.

Size categories overlap; radius alone cannot establish composition.

Gas giant / hot Jupiter

Gas giants are large, gas-rich planets. A hot Jupiter is a giant orbiting very close to its star.

A short orbit can expose a giant planet to intense heating.

Host star

The star that a planet orbits.

A planet's properties depend on what we know about its star.

Stellar spectral type

A classification tied to a star's spectrum and temperature: O, B, A, F, G, K, M run from hotter to cooler.

The Sun is a G star. An M dwarf is a cool, small main-sequence star.

Orbits & units (5)

Explore catalog values →

Astronomical unit (AU)

A distance unit of about 150 million kilometres, close to Earth's average distance from the Sun.

Use AU for distances within a planetary system.

Light-year / parsec (pc)

A light-year is the distance light travels in one year. One parsec is about 3.26 light-years.

Both measure distance, not time; our catalog's system distances use parsecs.

Orbital period

The time required for one complete orbit: the planet's year.

A planet's day describes rotation and can differ greatly from its year.

Semi-major axis / eccentricity

The semi-major axis sets an orbit's size. Eccentricity describes how stretched it is; zero is circular.

On an eccentric orbit, the planet's distance from its star changes.

Earth and Sun units

R⊕ and M⊕ compare radius and mass with Earth. R☉ compares radius with the Sun; S⊕ compares incoming starlight with Earth's.

2 R⊕ means twice Earth's radius, not twice its mass.

Try Detection Methods Lab →

Detection & evidence (6)

Transit / light curve

A transit occurs when a planet crosses its star from our viewpoint. A light curve plots brightness over time.

Repeated dips can reveal the orbital period; not every dip is planetary.

Transit depth

The fractional drop in starlight, approximately (planet radius / star radius)² for a simple transit.

Use matching radius units. A smaller star makes the same planet's dip deeper.

Radial velocity (RV)

Motion toward or away from us, measured through shifts in a star's spectrum.

A planet's gravity can cause a repeating signal; orbital inclination affects the inferred mass.

Direct imaging

Separating light from a planet from its much brighter star.

Young, bright planets far from their stars are generally easier imaging targets.

Gravitational microlensing

Gravity from a foreground object magnifies a background star; a planet can alter the brightening pattern.

This relies on a chance alignment rather than repeated transits.

Astrometry

Precise measurements of positions on the sky, including a star's tiny planet-induced motion.

It measures position changes rather than the spectral shifts used in radial velocity.

Try Habitable Zone Explorer →

Habitability (5)

Habitable zone

A range of distances where a suitable atmosphere could allow liquid water on a rocky planet's surface.

Being in this zone is not evidence of life or a guarantee of human-friendly conditions.

NASA: the habitable zone ↗

Insolation / stellar flux

The stellar energy received per unit area. Our lab compares it with the sunlight arriving at Earth.

Equal incoming energy does not guarantee equal surface temperatures.

NASA: the habitable zone ↗

Equilibrium temperature

An energy-balance temperature estimate based on absorbed starlight and emitted heat.

Assumptions about reflection and heat redistribution matter; it is not a measured surface temperature.

NASA: the habitable zone ↗

Albedo / greenhouse warming

Albedo describes reflected light. Greenhouse gases absorb and emit infrared radiation, affecting surface warming.

Our HZ lab varies these separately to explore how climate assumptions change an estimate.

Tidal locking

In synchronous rotation, a planet turns once per orbit, keeping the same side toward its star.

Permanent day and night can influence climate; this alone does not settle habitability.

Reading the data (3)

Explore the dataset →

Measured vs. derived

Some catalog values come from observations; others are calculated from other properties and models.

A filled-in radius or mass is not necessarily a direct measurement. Check the parameter's reference.

Missing value

The catalog has no usable value for that property in the selected record.

Unknown mass does not mean zero mass. Filtering can exclude planets with incomplete records.

Composite catalog

A table combining chosen properties from different studies to provide more complete planet records.

Values may use different assumptions or references and may not form one consistent model.