Extra space when converting number with to_char

Hello,

I've encountered an unexpected behavior when converting a number to char. I
need to match an INTEGER with a VARCHAR(9) that has leading zeros. When I
tried to match them using to_char(num, '000000000') I didn't get anything. I
later found out that to_char adds a leading space. This is demonstrated with
the query below :

SELECT
string,
length(string),
'"' || string || '"' AS showThemToMe
FROM
(SELECT to_char(42, '000000000') AS string) AS example;

string | length | showthemtome
------------+--------+--------------
000000042 | 10 | " 000000042"

Is there a reason why to_char adds a leading space? Is this a bug? I can
easily fix this with trim(leading ' ' from to_char(num, '000000000')), but,
being of a curious nature, I'd like ton know why I need to do that.

--
Samuel Gilbert