Selecting data from one table via categoryID field and displaying in different areas whilst excluding categoryID for one area
You can use LEFT JOIN for this:

SELECT 
     articles.*, 
     art_categories.*
FROM art_categories LEFT JOIN articles
ON articles.categoryID=art_categories.categoryID
WHERE art_categories.categoryID=8 
ORDER BY publishdate DESC LIMIT 5


as advice - avoid use *, better to list all columns by name, it also allow avoid duplicates and/or arrange aliases for them