1.   The giant planets will remain a puzzle for a long time.

2.   Because observation of Mercury presents peculiarly daunting problems, the planet remains the least understood of the four terrestrial planets, Earth, Mars, Venus and Mercury.

3.   For decades, astronomers have thought that planets remained stationary after the initial formation of the solar nebula.

4.   So the planet remained a barren rock.

5.   The planets must remain in order, of course.

6.   The three planets will remain close together Thanksgiving evening, when a new crescent moon will join them.