Planets

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Nebula Noun

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[neb-yuh-luh]

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Nebula

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A vast interstellar cloud of dust, hydrogen, helium, and other ionized gases, often serving as a stellar nursery.

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Example: This image shows a nebula, a giant cloud of gas and dust in space where new stars and planets are born.

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Frost Line Noun

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[frawst layn]

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Frost Line

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The specific distance in a solar nebula from the central star where it is cold enough for volatile compounds to condense.

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Example: In a young solar system, the frost line is the boundary where it's cold enough for ices to form, leading to icy planetesimals.

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Planetesimals Noun

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[plan-i-tes-uh-muhlz]

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Planetesimals

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Small, solid bodies formed from dust and rock in a protoplanetary disk that accrete to form planets and moons.

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Example: This image shows a protoplanetary disk around a young star, with a zoomed-in view revealing how small particles clump together to form planetesimals.

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Accretion disk Noun

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[uh-kree-shuhn disk]

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Accretion disk

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A rotating structure of diffuse material orbiting a central body, such as a star, from which celestial objects are formed.

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Example: This image shows a flat, rotating disk of gas and dust spiraling into a central object, like a young star, due to gravity.

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Astronomical Unit (AU) Noun

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[as-truh-nom-i-kuhl yoo-nit]

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Astronomical Unit (AU)

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A unit of length, roughly the distance from Earth to the Sun, used to measure distances within the solar system.

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Example: This diagram shows that one Astronomical Unit (AU) is the approximate distance from the Sun to the Earth.

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Terrestrial Planets Noun

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[tuh-res-tree-uhl plan-its]

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Terrestrial Planets

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The four inner planets of the solar system (Mercury, Venus, Earth, Mars) characterized by their solid, rocky compositions.

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Example: This image shows the four terrestrial planets of our solar system—Mercury, Venus, Earth, and Mars—which are the inner planets closest to the Sun.

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Jovian Planets Noun

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[joh-vee-uhn plan-its]

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Jovian Planets

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The four outer planets of the solar system (Jupiter, Saturn, Uranus, Neptune) which are large and composed mostly of gas.

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Example: This diagram shows the entire solar system, placing the Jovian planets (Jupiter, Saturn, Uranus, Neptune) in their orbits far from the Sun.

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Ice Giants Noun

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[ays jahy-uhnts]

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Ice Giants

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A type of giant planet composed mainly of elements heavier than hydrogen and helium, such as water, methane, and ammonia.

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Example: This diagram shows the inside of an ice giant planet, Neptune. Its thick mantle is made of water, ammonia, and methane ices, which is its defining feature.

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Retrograde rotation Noun

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[re-truh-greyd roh-tey-shuhn]

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Retrograde rotation

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The axial spin of a celestial body in the direction opposite to that of its orbital revolution around its primary.

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Example: This image shows that Venus spins in the opposite direction (retrograde) compared to Earth, which has a normal, or prograde, rotation.

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Asteroid Noun

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[as-tuh-roid]

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Asteroid

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A small, rocky object orbiting the Sun, primarily found in the asteroid belt between the orbits of Mars and Jupiter.

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Example: This diagram shows that most asteroids in our solar system are found in the Main Asteroid Belt, a ring of rocky objects orbiting the Sun between Mars and Jupiter.