Interplanetary means "between the planets".
it mean inside the planet No, it means 'within the orbits of the planets' as in 'interplenetary travel' - travelling between planets. Well Google says it means "In or between planets;"interplanetary travel"
exosphere
This is the correct spelling of "space", both the interplanetary version and the "closet space" usage.
Hermann Oberth wrote it
Exosphere
There are more than two regions of space: Geospace Cislunar space Interplanetary space Interstellar space Intergalactic space
one molecule per cubic cm
outer (as in "outer space")
Actually interplanetary space does have weather, but not in the same sense as the weather on Earth e.g. it never rains in space. The Sun emits particles known as the Solar Wind and also sheds huge amounts of matter and energy in what are known as Coronal Mass Ejections.
they built it in outer space
There is no antonym for outer space.
outer (as in "outer space")