I wonder if they picked pronouns for the planets in accordance with the associated gods of Greco-Roman mythology? IIRC, the Greco-Roman sun gods were all male (e.g. https://en.wikipedia.org/wiki/Helios).
If I'm correct they'd use male pronouns for the sun, Mars, and Jupiter; and female pronouns for Earth, the Moon (Luna), and Venus.
Germanic languages, of which English was one, have this too and "sun" is Germanic in etymology. Most PIE-derived languages had this at one point but some, including English, lost it.
In German the sun is female though (and the moon male). In French the sun seems to be male and the moon female (I don't know the language though, just looked it up), so maybe these are traces of the French influence into the English language?
Old English, i.e. the language of Beowulf, is a Germanic language. With the Norman invasion, English became heavily influenced by the Romance languages. So English has roots in both both old German and old French.
One place you see this is in terms relating to food. Words relating to rustic activities like agriculture and raising animals such as "cow" tend to have German roots. While culinary words like beef and veal tend to have french roots.
It seems that sun was feminine and the moon was masculine in Old English up to about the 16th century:
"sun (n.) [...] Old English sunne was feminine (as generally in Germanic), and the fem. pronoun was used in English until 16c.; since then masc. has prevailed." [1]
"moon (n.) [...] A masculine noun in Old English." [2]
I wonder if they picked pronouns for the planets in accordance with the associated gods of Greco-Roman mythology? IIRC, the Greco-Roman sun gods were all male (e.g. https://en.wikipedia.org/wiki/Helios).
If I'm correct they'd use male pronouns for the sun, Mars, and Jupiter; and female pronouns for Earth, the Moon (Luna), and Venus.