Sentence:  it's only been in recent years that astronomers and planetary scientists have been willing to think of planets, like they've long thought of stars and galaxies, as capable of orbiting one another. And of course those objects are the <WSD>satellite planets</WSD>. 
Meaning of the word marked with <WSD> tags:  A moon of planetary mass; a natural satellite which would be considered a planet or dwarf planet if it were in direct orbit of the Sun.