Space words explained
Glossary
Simple definitions for the terms you'll meet across this site — from orbits and gravity to gas giants and eclipses.
- Asteroid
- A rocky object, smaller than a planet, that orbits the Sun. Most asteroids are found in the asteroid belt between Mars and Jupiter. They are leftover building blocks from the formation of the solar system 4.6 billion years ago.See it in action
- Astronomical Unit (AU)
- The average distance between Earth and the Sun, about 150 million kilometers. Astronomers use AU to measure distances in the solar system: Mars is 1.5 AU from the Sun, Jupiter is 5.2 AU, and Neptune is 30 AU.See it in action
- Atmosphere
- The layer of gases surrounding a planet or moon, held in place by gravity. Earth's atmosphere is mostly nitrogen and oxygen and makes life possible. Venus has a thick, crushing atmosphere; Mars has a thin one; Mercury and the Moon have almost none.See it in action
- Comet
- A ball of ice, dust, and rock that orbits the Sun, often on a long, stretched-out path. When a comet nears the Sun, its ice turns to gas and forms a glowing coma and tails that can stretch millions of kilometers. Halley's Comet returns about every 76 years.See it in action
- Dwarf planet
- A round body that orbits the Sun but has not cleared other objects from its orbital path — the key difference from a full planet under the IAU's 2006 definition. Pluto, Ceres, and Eris are the best-known dwarf planets.See it in action
- Eclipse
- An event where one body in space blocks light from reaching another. In a solar eclipse, the Moon passes between the Sun and Earth, casting a shadow. In a lunar eclipse, Earth passes between the Sun and Moon, and the Moon turns a coppery red.See it in action
- Gas giant
- A large planet made mostly of hydrogen and helium with no solid surface. Jupiter and Saturn are the solar system's gas giants. Their "surfaces" are just cloud tops; below, gases compress into strange liquid states.See it in action
- Gravity
- The force that pulls objects with mass toward each other. Gravity keeps planets orbiting the Sun, keeps the Moon orbiting Earth, and keeps your feet on the ground. The more massive an object, the stronger its gravity: the Sun's gravity holds the entire solar system together.See it in action
- Ice giant
- A large planet made mostly of elements heavier than hydrogen and helium, such as water, methane, and ammonia "ices." Uranus and Neptune are ice giants. They are smaller and denser than gas giants.See it in action
- Orbit
- The curved path an object takes around a star, planet, or moon because of gravity. Earth orbits the Sun once every 365.25 days, and the Moon orbits Earth about every 27 days. Orbits are usually ellipses (slightly stretched circles), not perfect circles.See it in action