Layered worlds
As young planets heated up, dense iron sank to the centre and lighter rock floated up — a process called differentiation. Rocky planets have cores, mantles and crusts; giant planets have deep envelopes of hydrogen and helium around dense interiors.
Atmospheres
Whether a planet keeps an atmosphere depends on its gravity, temperature and magnetic protection. Venus has a crushing carbon dioxide atmosphere and a runaway greenhouse; Mars lost most of its air and is now cold and dry; Titan, a moon, has a thicker atmosphere than Earth.
Reading surfaces
Craters count time: older surfaces have more of them. Volcanoes, river channels, ice and tectonic features record a world's activity. Rovers analyse rocks directly, while orbiters map composition from spectra.