A good unit to measure*the distance between planets would be

Astronomical Unit

Explanation:

For distances within our solar system, or other solar systems, the common unit is the 'Astronomical Unit' (A.U.) 2. For most everything else, stars, galaxies etc..., the distance unit is the parsec (pc).

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Astronomical Unit

Explanation:

For distances within our solar system, or other solar systems, the common unit is the 'Astronomical Unit' (A.U.) 2. For most everything else, stars, galaxies etc..., the distance unit is the parsec (pc).