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 attraction between any two objects with mass, which governs the motion of planets and stars.

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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, often serving as a stellar nursery.

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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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The boundary in a protoplanetary disk beyond which volatile compounds like water and methane could condense into solid ice 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 gas and dust surrounding a central object, like a young star, from which planets can form.

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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 of rock and ice that formed in the early solar system and gradually combined 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 marked by frequent and intense impacts of asteroids on 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 leading scientific theory for the formation of the Moon, proposing a 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 of the solar system (Mercury, Venus, Earth, Mars) characterized by their solid, rocky surfaces and high density.

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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 large outer planets (Jupiter, Saturn, Uranus, Neptune), also known as gas giants, composed mainly of hydrogen, helium, and ices.

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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 spin of a planet on its axis in the direction opposite to the orbital direction of most planets and moons.

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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.