It is not known, since we don't know all the planets in the galaxy, but in our solar system Mercury is the hottest planet, since it's closest to the sun. The hottest planet in Milky Way would be some planet closest to the biggest and hottest star there is. that is wrong, venus, due to it's hot surface and acid gases and things like that, make it the hottest planet.
Mercury has no atmosphere, and Venus has a large CO2 high pressure atmosphere, hence Venus is hotter than Mercury!
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