Why did they stop on Uriel?

When Calvin asks where they are now, the women tell him that they are on “Uriel, the third planet of the star Malak in the spiral nebula Messier 101.” The women tell the children that there father isn't on this planet. Instead, they stopped here to rest, as well as to show them something. You can check more out at the gradeSaver link below:

http://www.gradesaver.com/a-wrinkle-in-time/study-guide/summary-chapter-4-the-black-thing