system
Given the following post, respond as if you're commenting on Reddit:
human
What makes Venus is the most shiniest planet of the solar system?

Is it the chemical composition of Venus atmosphere or its weather phenomena what generates the shiny optics phenomena? What is the physics process of this phenomenon?
gpt
I am going to assume you mean brightest rather than most reflective (but it is both)?

The brightness of a planet is determined by a few factors, and one of them is that is the most reflective (shiniest). Astronomers measure reflectivity with something called albedo, the ratio of reflected to incident light. Venus has an albedo (0.75) that is more than double the next most reflective planet - the outer planets are all around 0.3 and Mars and Mercury are around 0.15. Venus' high albedo is due to the clouds being light coloured as opposed to dark rocky surfaces of Mercury and Mars or the darker clouds of the outer planets.

The distance between Venus and the Sun is also important, the closer a planet is to the Sun the more incident radiation there is so there is obviously also more reflected radiation. Venus is almost as close to the Sun as Mercury. This follows what we call an inverse square law, a planet 3 times further away will have 9 times less incident radiation, the outer planets therefore have very low incident radiation compared to the inner planets.

The next major factor is the distance between the Earth and Venus, while Mercury is on average the closest planet to Earth, when Venus is on our side of the Sun it has the closest approach to the Earth, this will coincide with the times where Venus is exceptionally bright in the sky.  This is also an inverse square law, so if a planet is 3 times further away from us, it will be 9 times less bright.

The last factor is the size of the planet, a planet that has three times the radius will have 9 times the reflective area. The outer planets are much much larger than Venus so despite being much much further from the Sun, much much further from the Earth, and having lower albedos we can still see Saturn and Jupiter brightly throughout much of the year. Venus' relatively small size is more than made up with though by its distance to us (but only for some of its orbit, it is quite faint on the far side really).

So basically a confluence of the distance to the Sun, its size and its reflectivity and, most importantly, its distance to us all make it appear bright to us.