There's no planet in our solar system that's entirely covered by fog, in the way we think of it. However, there are some planets that have thick, hazy atmospheres that can resemble fog.

Here are a couple of examples:

* Venus: Venus has a very dense atmosphere of carbon dioxide, creating a thick, yellowish haze that traps heat and makes the planet incredibly hot.

* Titan: Titan, Saturn's largest moon, has a thick atmosphere of nitrogen and methane. This creates a hazy orange atmosphere that obscures the surface.

While these atmospheres are not fog in the traditional sense, they do create a similar effect of obscuring the surface and making it difficult to see through.