Question: Why is Venus the brightest planet and hotter than mercury?

Hi nickskii,

Two very good questions!

Now, planets don’t produce their own light, so how bright a planet looks to us here on Earth depends on
(i) How much sunlight gets to the planet
and
(ii) How much sunlight the planet reflects (which in turn depends on how big the planet is and how much light its atmosphere or surface reflects)
and
(iii) How much of the reflected light gets to us (which depends on how close the planet is to Earth).

The two planets that appear brightest to us are Venus and Jupiter: both have quite reflective atmospheres; Jupiter is much much bigger, and so wins on (ii), but Venus is much closer to the Sun, and so wins on (i) and (iii). And it just so happens that Venus appears brighter than Jupiter.

To temperature now. Mercury is much closer to the Sun than Venus, so you would expect it to be hotter. But Venus has an atmosphere that keeps the planet warm by “trapping” heat in — a very strong greenhouse effect (see this picture for a more detailed explanation: https://www.msu.edu/course/isb/202/ebertmay/drivers/ipcc_greenhouse.jpg).

Mercury, on the other hand, has no atmosphere at all (the planet is so small that its gravity is too weak to stop an atmosphere being blasted away by the Sun. So it has no “insulation”, unlike Venus, which means that the side of Mercury facing the Sun is baking hot while the side facing away from the Sun is freezing cold. (So it’s actually a bit difficult to say what THE temperature of Mercury is!)