Well-known or well known?

A student asked whether the two words well and known are always hyphenated when they appear next to each other.

He was probably thinking about a sentence such as (1) below:

(1) The well-known author visited our school.

No, they are not always hyphenated.

When the Adverb + Participle pair precedes a noun as in (1), we hyphenate the two words to make them into a single adjective modifying the noun.

When the Adverb + Participle sequence follows a noun, we do not need to hyphenate:

(2) The author is well known.

However, with adverbs ending in -ly, no hyphenation is needed whether the Adverb + Participle sequence precedes or follows the noun:

(3) He is a highly paid employee.
(4) That employee is highly paid.