The Science Part
Mercury, Venus, Earth and Mars are called terrestrial planets as these have 'land' to stand-on. We may also add moon to the list. Lunar and Mervury surfaces are highly cratered. These craters were formed about 4 billion years ago when the solar system was just born (about 0.3 byr ago) and taking it's present shape. At that time there were large number of small bodies flying in the solar system and naturally colliding with the other planets.
The gaseous planets like Jupiter, Saturn, Uranus and Neptune simply swallowed these debris. But on terrestrial planets these missiles left their mark in the form of a craters.
On planets like the Earth, Venus and Mars most of these makes were eroded by changing weather on these planets. Whereas Mercury and Moon, which have little atmosphere, have retained their poke marks.
Thus - if one goes exploring planets of other suns and come across a planet which has highly cratered surface then it is quite likely that the planet has little or no atmosphere.
Study of these features on the planets has helped astronomer to get insight into how the surfaces of these planets have evolved over a period of time.
The Fun Part
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