4:00 PM PT -- In a totally baller move ... Jen's party is moving to a brand new private jet. The backup PJ just pulled up in front of the damaged one and their luggage is getting transferred.

The 12 women onboard have been sitting patiently waiting for their plan B to arrive, but they're gonna have to get off now to stroll to the new jet.

Jennifer Aniston's private jet bound for Mexico was forced to make an emergency landing in California ... TMZ has learned, and now she and her guests are holed up onboard.

The flight took off from LAX at 11 AM Friday heading for Cabo San Lucas, but lost a wheel or a tire during takeoff ... according to the FAA.

The jet -- which also has Courteney Cox and Molly Kimmel onboard -- made it all the way down to Mexican airspace, but the crew decided it wouldn't be safe to land in Cabo.

The flight was diverted back toward the airport in Ontario, CA ... about 60 miles east of Los Angeles. However, they still had a very full fuel tank ... which ain't ideal for emergency landings.

The plane circled for hours to burn off fuel before it was deemed safe to land ... and the captain put it down safely at 2:05 PM. Jen, Courteney, and 10 other passengers have not de-planed yet, which is why we're watching the live stream.

There's a buzz Brad Pitt is on the plane. Not to be a buzz kill, but our sources say it just ain't so. We're told all the passengers are women -- including Amanda Anka (Jason Bateman's wife) -- and it's a surprise trip for Jen's 50th birthday which they sprung on her today.

It appears everyone aboard is safe, but clearly a harrowing couple of hours for Jennifer to wrap up her birthday week.