[Grammar] he did not....partly because

[Oceanlike]

In this original sentence, can I leave out the comma (after "it")?
If not, why?

(a) He did not believe the news when he heard it, partly because the competition was fierce.

(b) He did not believe the news when he heard it partly because the competition was fierce.

 

[GoesStation]

In addition to indicating where the speaker would pause, the comma tells the reader that partly isn't modifying he heard it. You can't omit it.

 

[Rover_KE]

I'm a comma minimalist, but that one's definitely mandatory.