Planets

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Gravity Noun

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[grav-i-tee]

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Gravity

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A fundamental force of nature that attracts any two objects with mass, holding planets in orbit around the Sun.

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Example: This diagram shows gravity as a force, represented by yellow arrows, that pulls everything towards the center of a massive object like the planet Earth.

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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 from which stars and solar systems are formed.

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Example: A nebula is a giant cloud of gas and dust in space. This image shows a planetary nebula, formed when a star sheds its outer layers.

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Frost line Noun

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[frost layn]

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Frost line

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A boundary in the early solar system beyond which it was cold enough for ices to condense into solid particles.

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Example: This diagram shows a young solar system's frost line. Inside the line, it's hot, so only rocks and metals condense. Outside, it's cold, so ices, rocks, and metals all condense.

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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 disk of dust and gas that formed around a young star, from which planetesimals and eventually planets formed.

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Example: Material pulled from a giant star by gravity forms a flat, rotating disk, called an accretion disk, around a smaller companion object.

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Planetesimals Noun

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[plan-i-tes-uh-muhls]

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Planetesimals

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Small, solid bodies formed from condensed material in the early solar system that eventually collided and merged to form planets.

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Example: This image shows a planetesimal, a large rocky body, forming in the early solar system by collecting smaller rocks and dust.

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Heavy Bombardment Period Noun

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[hev-ee bom-bard-ment peer-ee-uhd]

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Heavy Bombardment Period

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An early era in the solar system's history characterized by numerous impacts of leftover planetesimals on newly-formed planets and moons.

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Example: This image shows a young planet, like early Earth, being repeatedly struck by asteroids and comets during a period known as the Late Heavy Bombardment.

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Giant Impact Theory Noun

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[jahy-uhnt im-pakt thee-uh-ree]

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Giant Impact Theory

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The theory explaining the Moon's formation as the result of a massive collision between early Earth and a Mars-sized body.

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Example: This diagram shows the Giant Impact Theory, where a Mars-sized object named Theia collided with early Earth, creating a disk of debris that eventually formed the Moon.

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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 inner planets (Mercury, Venus, Earth, Mars) characterized by solid, rocky surfaces, high density, and relatively small size.

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Example: This image shows the four terrestrial planets—Mercury, Venus, Earth, and Mars—which are the inner planets closest to the Sun and have solid, rocky surfaces.

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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 outer planets (Jupiter, Saturn, Uranus, Neptune), also known as gas giants, characterized by large size and gaseous composition.

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Example: This diagram shows the four Jovian planets (Jupiter, Saturn, Uranus, Neptune), which are the large, gas-rich outer planets of our solar system.

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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 rotation of a planet on its axis in the opposite direction to the orbit of most other solar system bodies.

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Example: This diagram shows that while most planets like Earth rotate in one direction (prograde), Venus rotates in the opposite direction, which is called retrograde rotation.