The inner Solar System. Computer composite image of the four planets of the inner solar system. The planets are (clockwise from upper left): Mercury, Venus, Earth and Mars. A representation of the Sun is at centre. The inner planets all have rocky surfaces, unlike the outer 'gas giants' such as Jupiter. Earth is the only planet where water is liquid at the surface (hence suitable for organic life), although some features on Mars suggest the presence of surface water in earlier times. The planetary images come from data gathered by NASA space missions.