.mcr - Mercury
.vns - Venus
.ert - Earth
.mrs - Mars
.jpt - Jupiter
.str - Saturn
.ura - Uranus
.nep - Neptune
I'd say
.mer
.vns
Earth doesn't need a domain name but if it had one anyway, .ter
.mar
.cer -ceres
.vst - vesta
.jov
.sat
.cae
.npt
.plt -pluto
.ers - eris
.hau - haumea
What do you think?