Harriet,
Planets are great fun, and of interest to many children.
I’m not sure that these activities need to be specifically “inquiry-based” since that has a particular meaning that references the Framework for Inquiry, which builds on the children’s understanding and asks them to extend that understanding by making testable predictions and investigating.
With the planets as a theme this could be structured research as the children notice patterns from their facts and then try to come up with explanations for those patterns.
They might see that the rocky planets are near the sun and the gas planets are far away. They might note that the small planets are near the Sun and the big planets are far away. They might think – planets near the Sun are hotter and planets further away are colder – and check that out.
Once they see that the hottest planet is Venus (2nd from the Sun) they might take the next step and look at why Venus is hotter.