All of the planets except earth were named after Greek roman gods and goddesses. Jupiter, Saturn, Mars, Venus and Mercury were given their names thousands of years ago. The other planets were not discovered until much later after telescopes were invented. Mercury was named after a roman winged messenger. Jupiter was named after a chief roman god. Venus was named after a roman goddess of love. Mars was named after a roman god of war. Saturn was named after a former god of agriculture. The Romans knew of seven bright objects in the sky the sun the moon and five of the brightest planets. They named them after their most important gods. Sources

http://coolcosmos.ipac.caltech.edu/ask/196-How-did-the-planets-get-their-names-

https://rome.mrdonn.org/romangods/planets.html

Book - Look Inside Space Author - Rob Lloyd Jones