Tukuk-OS“Planet” comes from the Greek for wanderer: to naked-eye observers the five bright wanderers — Mercury, Venus, Mars, Jupiter, Saturn — slid against the fixed star pattern over weeks, while stars held their shapes. They are still the only planets you can see without help, and even then you must catch Venus near dawn or dusk, Mars when it is bright at opposition, and Mercury low in twilight. Nothing about being “visible to the eye” makes those five special in space — it is simply chemistry of the early solar system: big, bright, close.
Planets orbit the Sun inside out: Mercury whips round in 88 days, Earth in 365, Jupiter in nearly 12 years. Every outer planet therefore gets overtaken by Earth periodically — and during the pass, the outer planet appears to slow, reverse, loop, then resume. This “retrograde” loop is pure perspective, like a car you pass seeming to fall backwards against the scenery. Mars does it roughly every 26 months at opposition, which is precisely when astronomers point their telescopes at it.
The 2006 reclassification of Pluto was not a demotion of a “ninth planet we dislike” but a definition fix: the solar system turned out to hold thousands of icy bodies sharing Pluto’s zone, and anything that dominates its orbit is a planet while anything coexisting there is a dwarf. The line is about orbital real estate, and by that test Earth, Mars, Jupiter and the rest pass cleanly.