A planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remains that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion in its core, and has cleared its neighboring region of planetesimals. The term planet changed from something that stimulated across the sky, to a body that orbited the Earth. When the heliocentric model gained sway in the 16th century, it became established that a planet was actually something that directly orbited the Sun. At the end of the 17th century, when the first satellites of Saturn were exposed, the terms planet and satellite were at first used interchangeably, although satellite would gradually become more common in the following century.

The energetic impacts of the smaller planetesimals will heat up the increasing planet, causing it to at least partially melt. The interior of the planet begins to differentiate by mass, mounting a denser core. Smaller terrestrial planets lose most of their atmospheres because of this accretion, but the lost gases can be replaced by out gassing from the mantle and from the succeeding impact of comets.

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