Camera lenses are literally telescopes. The OP is using a 105mm lens which is equivalent to a small refracting telescope (4"). The diffraction limit on an aperture that size is around 1.5 arcseconds, which is enough to resolve most naked eye planets into multiple pixels. Jupiter is up to 50 arcseconds across, Mars is up to 25.

Many refracting telescopes have smaller apertures, and "good" amateur telescopes start out at not much larger (at 6" or 8" apertures). Of note, Galileo Galilei's telescopes had apertures of 15mm, 26mm, and 38mm, which he used starting in 1609, 1612, and 1620, respectively. Newton's pioneering reflecting telescope, built in 1668, had an aperture of 2 inches (50mm).

The main constraint is always going to be atmospheric seeing. Which is where digital photographic techniques and stacking comes into play. With a modern top tier astronomical observatory they would use adaptive optics to cancel out the effects of atmospheric distortion. If you don't have that option you can simply use lots and lots of individual exposures. Each exposure represents a snapshot of the atmospheric conditions at a particular moment. If you're lucky you can get a single frame or a part of a single frame where by chance the air happens to be just right to have a minimal amount of distortion and blurring. With enough frames you can use a computer to select the individual frames and portions of frames with the best seeing and digitally combine them together to improve the overall signal to noise ratio of the final image. This allows you to approximate the performance of the same optics without atmospheric distortion.