Why are the inner planets called "Terrestrial planets"?
Answers
The four innermost planets in the Solar System (Mercury, Venus, Earth, and Mars) are sometimes called the "terrestrial" planets because of their proximity to Earth ("Terra" in Latin) and their similarity as compact solid bodies with rocky surfaces.
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Terrestrial means "Earth-like" in this context.
The inner planets (Mercury, Venus, Earth and Mars) all have a stony surface, whereas the outer planets (Jupiter, Uranus and Neptune) have no real surface, but they are so-called "gas giants", that are mostly composed of more or less liquid gases.
The inner planets are also very much smaller.
Mercury has no atmosphere to speak of: most of it is cooked off by the intense radiation of the Sun. Venus has a very dense hot atmosphere. Earth has the atmosphere we all know, partly because of the magnetic field preventing most of it cooking off, and Mars has a very thin atmosphere -- mainly because it has no magnetic field, and because it's smaller than Earth, that's why it cooled faster.
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