The most common and most likely answer to this crossword clue is the 6 letter word JOVIAN
I've seen this crossword clue in The Guardian
J
O
V
I
A
N
Buddy explains!

The straight part
We know that "from another planet" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case from another planet = jovian.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does JOVIAN mean?
JOVIAN adjective- Pertaining to the planet Jupiter.
- Pertaining to the Roman god Jove or Jupiter (the counterpart of the Greek god Zeus); Jove-like; befitting Jupiter.