Qalculate! has been my go-to calculator on my laptop for years, very happy to have it on my phone now too! And it definitely knows planet radii, try `planet("earth"; "radius")`. Specifically, it knows a bunch of attributes about atoms (most importantly to me, their atomic mass) and planets (including niche things like mean surface temperature). You can see all the data here: https://qalculate.github.io/manual/qalculate-definitions-fun...