I'm a native English speaker and I agree with you that the given explanation doesn't make sense. However, these two forms are the way you will usually see these sentences. On the other hand, I've also heard people use 'the' in the second sentence and a few using 'to'. As far as I know, English speaking countries don't have any special groups of people monitoring the general population's speaking and writing, besides English teachers and I can't see where they've been extremely successful in stopping or promoting certain practices.

Talking of the Earth as a planet, it should always be "the" the same as the Sun and the Moon, the only reason why other planets don't have "the" is because those are actually names of gods.

"Example 2: Which planet is nearest Earth?" In example 2, I would say it as "Which planet is nearest the Earth?" I would use the word "the". "Earth" is a name; it is the name of the planet, and the name we give to soil too почва. When we use "the" it is like a description (a demonstrative pronoun); we are talking about a planet, and "the" earth in particular. The Earth/this Earth, not another Earth. It is like saying "I see the red car", rather than any car, it is THE red car, that particular car and no other car. I think your question has nothing particular about this planet, but about using "the" to describe one particular object/one specific object, or a general or class of objects. You could say, (but it isn't good English) "Earth goes round the sun" , but I use the word "the" to describe our sun, and no other sun. "The earth goes around the sun" with no capital E, so the word "the" shows it as this Earth, and no other Earth. This particular earth, this specific planet. In the West, in the Germanic and Latin languages, and Greek, we use the word "the" when describing a specific object. In Russian you don't use the word "the". I find it hard to explain, the idea of what is not in your language.

Mercury, Venus, and Earth are inner planets. Mercury, Venus, and Earth revolve around the sun. Earth is the only planet with life. Some people think the Earth is flat. (The Earth is visualized as our planet, not one of eight in the solar system) The Earth is mostly covered by water. (same) Water and earth make mud. (Here earth has a second meaning of soil.)