Meaning: between or among planets; "interplanetary travel"
"What are they?" whispered Jaska, standing fearlessly at Sarka's side. "Interplanetary cars?
I spent that whole night in one constant nightmare; in the heart of a volcano, and from the deepest depths of the earth I saw myself tossed up amongst the interplanetary spaces under the form of an eruptive rock.
And always, as they stared and literally willed the cubes which piloted and were the motive power of the aircars to speed and more speed, that marvelous display of interplanetary fireworks which had aroused the concern of Sarka the Second.