It's worth pointing out from the start that the rumour surrounding Matt Smith's role in 'Star Wars: Episode IX' came from an anonymous, unverified source.
In other words, you'll need to take this news with a Death Star-sized pinch of salt. The Weekly Planet Podcast's 'Hot Scoop or Shot of Poop' segment is claiming that 'Doctor Who' star Matt Smith's role in 'Star Wars: Episode IX' is none other than...
Emperor Palpatine. This guy.
Once again, it can't be stressed enough how unlikely this is, but that's what the claim is. The role was originally played by Shakespearean actor Ian McDiarmid, and was first introduced in 1983's 'Return of the Jedi'. Since then, Emperor Palpatine has turned up in comics, video games, the prequel trilogy and novels.
The problem with this rumour is that 'Star Wars: Episode IX' is wrapped up in secrecy. There's no way of knowing until there's official confirmation or there's on-set photos confirming it. On a surface level, Matt Smith could probably play the role quite convincingly.
He certainly has the bone structure and the Received Pronunciation voice that McDiarmid has. Not only that, Variety reported that Smith would play a "key role" when he was first announced back in August of this year.
Playing the Emperor certainly fits that description, but again, there's no way of knowing.
'Star Wars: Episode IX' arrives in Irish cinemas on December 20th, 2019.