To me, it seems to be a new question.
Enterprise E is not asking, if there is a shield around Alderaan or not, but where exactly it is stated, that there is a shield around Alderaan.
He says, that he does not know any official sources, which are stating, that Alderaan was protected by a planetary shield.
I don't know, if there are any novels or secondary literature, which are stating such thing.
I don't have any Incredible Cross-Sections. But I would guess, that the
Star Wars: Incredible Cross-Sections does not mention a planetary shield. The overrated
Attack of the Clones: Incredible Cross-Sections shouldn't mention it either, because the presence or absence of a planetary shield around Alderaan has nothing to do with the covered movie.
- Wookieepedia, which is always very economical with references provides only one single reference for planetary shields at all [1], but claims at the same time, that planetary shields have appeared in
- Star Wars Episode IV: A New Hope (First appearance)
- LEGO Star Wars II: The Original Trilogy (Non-canonical appearance)
- Star Wars Episode VI: Return of the Jedi
- X-wing: Wedge's Gamble
- Dark Force Rising
- The Last Command
- Retreat From Coruscant
- Specter of the Past
- Vision of the Future
- Revelation
and provides as sources the following bibliography, - Dark Empire Sourcebook
- Star Wars Sourcebook
- Hideouts & Strongholds
- Coruscant and the Core Worlds
- The Essential Guide to Weapons and Technology
- The New Essential Guide to Weapons and Technology
- Aliens in the Empire
without saying, what exactly the indiviual sources are saying. To what extent these sources are dealing with the presence of a planetary shield around Alderaan is not stated.
Those, who are believers in planetary shields, are claiming, that the movie
A New Hope does show a planetary shield and that, because the movie has the highest rank in the canon, it is irrelevant, if no other sources are confirming the existence of a planetary shield around Alderaan.
But fact is, that the novelization
A New Hope does not mention a planetary shield and that the movie is - at least - not conclusive for the presence of a planetary shield
[1] .