Planetary observing asks something specific of a telescope and it is not light grasp. The planets are bright; what limits the view is resolution and contrast, which means aperture, optical quality and the steadiness of the air rather than a big light bucket.
Resolution is the specification that matters
The finest detail a telescope can separate scales directly with aperture. A 76mm instrument resolves about 1.5 arcseconds on a good night, a 130mm about 0.9, a 200mm about 0.6. Jupiter's disc is around 40 arcseconds across at opposition, so the difference between those apertures is the difference between two visible cloud bands and a dozen features.
Why long focal ratios are traditional for planets
A slower instrument, f/8 and up, reaches high magnification with ordinary eyepieces and is more forgiving of cheap ones. That is a convenience rather than an optical law: a fast instrument with good eyepieces and a barlow performs identically. The tradition persists because it makes budget eyepieces work better.
The atmosphere decides more nights than the telescope does
Seeing, the steadiness of the air, sets the ceiling on most nights. A 200mm instrument on a turbulent night shows less than a 130mm one on a steady one. This is why patient observers wait for moments of stillness rather than pushing magnification, and why planetary observers care about local conditions more than deep sky observers do.
Questions people ask about telescope
What telescope do I need to see planets?
Any of them will show the main sights. For real detail on Jupiter and Saturn, 100mm and up, with good optics and a steady night mattering more than extra aperture.
Is a reflector or refractor better for planets?
Both work. A good refractor gives slightly higher contrast for its aperture; a reflector gives more aperture for the money, which usually wins. A well collimated reflector beats a mediocre refractor of the same size.
What magnification for planets?
Between 100x and 200x on most nights, limited by the air rather than the instrument. Start lower and work up until the image degrades, then come back one step.