astronomical units (au) Measure of distance for planetary systems. 1 au (= 150,000,000km/0.000016 light years) is the distance between the Sun and the Earth.
binary stars Two stars that orbit around their common centre of mass.
carbon-silicate cycle The geological process that regulates the quantity of carbon dioxide in the air on Earth. As carbon dioxide is a greenhouse gas and can trap heat, this acts as a thermostat for our planet.
centre of mass Balance point in a system of objects (stars, planets, moons and so on), where the pulls from their gravity cancel one another.
eccentricity Measure of how elongated a planet’s orbit is. Zero per cent eccentricity is a perfect circle.
exomoon Moon orbiting an exoplanet.
exoplanet Planet orbiting a star that is not our Sun.
gas giant planet Planet like Jupiter, Saturn, Uranus and Neptune that has a solid core engulfed in a huge atmosphere, thousands of kilometres thick.
greenhouse effect Ability of gases in a planet’s atmosphere to absorb and reflect infrared radiation, warming the planet.
Hill radius Distance around a planet (or other object), where its own gravity dominates over that from the star. Smaller objects (such as planetesimals) in this region are pulled towards the planet.
hot Jupiter Gas giant planet that orbits close to the star.
ice line Distance from a star where it becomes cold enough for ice to form in the protoplanetary disc. Also called the snow line, or frost line.
inclination Angle a planet’s orbit is tilted from the plane of the other planets in the system (or the plane perpendicular to the rotation axis of the star).
isolation mass The mass of a growing planet once it has eaten all planetesimals within its orbit.
Kepler Space Telescope NASA space observatory that detects extrasolar planets by the transit technique.
Kozai-Lidov Mechanism Ability of another body (like a binary star or another planet) to affect the eccentricity and inclination of a planet’s orbit.
light year Distance light travels in one year. It is equivalent to 63,240au, or 9,500,000,000,000km (6,000,000,000,000mi).
migration Changing of a planet’s orbit, usually in the direction of the star. Type I and II migrations refer to orbit changes due to the drag of the gas in the protoplanetary disc. Planetesimal-driven migration is from the scattering of smaller planetesimals.
planetesimals Asteroid-sized rocks a few kilometres to a few hundred kilometres in size that formed on the way to building planets.
protoplanetary disc The disc of gas and dust that circles newly forming stars, out of which planets are formed.
radial velocity technique Detection of a planet from the slight wobble in a star’s motion. This gives the orbital time for the planet (and therefore its distance from the star), and the minimum estimate of its mass.
red dwarf Star smaller and cooler than our Sun. Also called an M dwarf.
resonant orbits The orbits of neighbouring planets with periods (time for one orbit) in exact integer ratios; for example, one planet orbits twice in the time taken for the second planet to orbit once. Such orbits are very stable and difficult to break.
rogue planet A planet that does not orbit a star.
super Earth Planet with a radius of 1.25–4 Earth radii. Such planets may be rocky or have Neptune-like thick atmospheres.
Spitzer Space Telescope NASA space observatory that detects infrared heat.
temperate zone Region around a star where surface temperatures on a planet exactly like the Earth allow liquid water to exist. Also called the habitable zone, or Goldilocks zone.
terrestrial planet A planet like Mercury, Venus, Earth and Mars that is predominantly rock with a thin atmosphere.
tidal heating Heat generated by flexing a planet or moon due to the varying gravitational pull from the star or planet when in an eccentric orbit.
tidal locking Orbit of a planet or moon where one side always faces its host (star or planet).
transit technique Detection of a planet from the slight dip in a star’s light as the planet passes across its surface. This gives the orbital time for the planet and its radius.
transit timing variations (TTV) Changes in the time between planet transits due to the gravitational tugs of other planets (or potentially moons) in the same system. This gives the mass of the planets.