Sentence:  Part of the time saved was spent on a preliminary estimate for a long-distance plan to free bound oxygen in the sands of Mars to make the <WSD>planet</WSD> more friendly to future human generations . 
Meaning of the word marked with <WSD> tags:  (astronomy) any of the nine large celestial bodies in the solar system that revolve around the sun and shine by reflected light; Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto in order of their proximity to the sun; viewed from the constellation Hercules, all the planets rotate around the sun in a counterclockwise direction