The Magic School Bus Lost in the Solar System by Joanna Cole, 1990.

Ms. Frizzle’s class is planning a trip to the planetarium as part of their lesson about the solar system, but of course, their magic school bus has other plans.  When they get to the planetarium and find out that it’s closed for repairs, the bus sprouts rockets and takes them on a real trip through the solar system.

This time, the class is accompanied by Arnold’s visiting cousin, Janet.  Janet is a show-off who brags constantly about everything, making up stories when she has nothing real to brag about.  She gets on the other kids’ nerves, but when they’re separated from Ms. Frizzle because she got out of the bus to fix a taillight broken in the asteroid field, Janet takes charge, using Ms. Frizzle’s lesson plan to continue the field trip, eventually figuring out how to turn the bus around to rescue her and get back to Earth.

All through the book, there are facts about the sun, the moon, and the planets in the solar system from the students’ reports.  Each time they stop at a planet, a scale shows the difference between Arnold’s weight on Earth and his weight on each planet.  The book considers Pluto to be a planet because it was written before its planet status was reconsidered.

The book is currently available online through Internet Archive.