Proserpina

A candidate name for our candidate "planet nine" is Proserpine. So: who she?

The peoples of Italy had a long relationship with the Greeks, from the Bronze Age on. Greek mythology is stuffed with references to (southern) Italy and Sicily. Both peninsulae likely contributed to the Sea Peoples. The very name "Greek" comes from a Helladic name for southern Italy, subsequently adopted by other Italians for the Hellenic race generally.

The Etruscans and then the Romans looked to the Greeks, on their own peninsula, for guidance on their mid-Italian evolving civilisations. Greek gods and Greek myths were equated with Italian gods and myths (this was easiest for the IndoEuropeans in Italy, one imagines); and, if not, such mythoi were simply adopted. A good example of the latter is Heracle(we)s.

Proserpina in Latin corresponds to Persephone. But here the Doric-through-Etruscan path doesn't hold. In fact: any Greek-through-Etruscan doesn't hold. Etruscans have told us here, too, how they named this half-dead princess: "Persipnei". The Per > Pro and the Sip > Serp happened in Latin.

Given how Latin was given to accenting the second syllable, from Persipnei I should expect "Persippina" (like Agrippina); if straight from Greek Persephónē, a fair Latinate transliteration would be "Persiponna". The "per-" prefix is familiar to Latins and they wouldn't replace it without good reason.

One etymology I've seen for Proserpina is from proserpere, "to emerge". In this case, the Latins had a goddess of the springtime, or felt like they needed to have one. They heard "Persipnei" and/or "Persephone" being uttered around them. They figured "She Who Emerges" for a good-enough epithet for the actual goddess, and so they used that.