If you're referring to our sun:
sun → Sun

Planets in our solar system are all proper nouns and, as far as I know, their names should be capitalized. My 2 cents...

I did. Seems like you're right. Thanks, CK.

In Wikipedia and in the most (but not all) links in it the Sun is capitalized.
http://en.wikipedia.org/wiki/Sun
The capitalization seems good gor the scientific context like this.

@_undertoad

We can use "the Sun" in different context: as the name of our star (the Sun is in the centre of the Solar system), and like "the sun is shining".
In this sentence it's used as the name of the star. I guggest to search for scientific articles, not just common Google search to get the same context as in this sentence.

As you can see
https://www.google.ru/?gws_rd=s...each+the+earth
in the most cases the Sun is capitalized, though in a few articles it's not capitalized.

@Selena777
There's some interesting discussion online, but I'm not going to argue about it. Seems like a bit of a sticky point/stylistic choice.

@_undertoad
Yes, any author is free to choose which word to use: "the Sun" or "the sun". What about me I would choose "the Sun" in this context. But they both can't be considered as a mistake, cause we can find rather trustworthy sourses, using as the first, as the second one.