They remained friends throughout his ill-fated relationship with Angelina Jolie.

And eagle-eyed onlookers spotted Brad Pitt, 55, sneaking past the paparazzi as he joined the stars at ex-wife Jennifer Aniston

Actor Brad, who’s reportedly been quietly dating Charlize Theron since January, went incognito in a flat cap as he rushed inside Jennifer’s favorite West Hollywood spot, the historic Sunset Tower.

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He wasn’t alone, with stars including Katy Perry and boyfriend Orlando Bloom, chat show host Ellen DeGeneres, Gwyneth Paltrow, Barbra Streisand, Jason Bateman and Kate Hudson joining the celebrations.

Jennifer, whose birthday falls on February 11, attended the soirée in eye-catching style, stepping out in a stylish all-black jumpsuit with criss-cross detailing on the back.

With her golden locks styled in voluminous waves and her makeup applied in typically impeccable style, the star looked relaxed and happy as she was feted by her famous friends, as well as her actor dad John Aniston.

Also among the attendees was George Clooney, who arrived with his glamorous wife Amal. Jennifer has long been friends with George and visited him and Amal at their lake house in Como last summer.

During an appearance on The Ellen DeGeneres Show, the Friends star revealed how incredible it was to see George become a father.

‘It’s amazing to see him with children. That is what we talked about, that anything can happen,’ she told the host.

After several years of being celebrated as one of Hollywood’s most glamorous couples, Jennifer and Brad shocked fans when they announced that they were divorcing in 2005.

The two began dating after meeting on a blind date their respective agents set up in 1998. They quickly became the ‘it’ couple of the early 2000s, with their every move making headlines.

Two years after they met, Jennifer and Brad tied the knot during a romantic ceremony in Malibu, California. Their marriage ended on January 5, 2005. The couple announced that they had ‘formally’ separated.

It soon emerged that he had begun a relationship with Angelina Jolie, with the actress later confirming years later that they fell in love while filming Mr And Mrs Smith in 2003, while he was still married to Jennifer.

Throughout his 11-year relationship with Angelina, tabloid stories regularly emerged that Brad was still in contact with Jennifer, seeking advice from his former wife despite his new love’s objections.

And it seems that following Brangelina’s divorce, Brad may well have renewed his friendship with his ex.

On Saturday, the exes had plenty of mutual friends to celebrate with. Katy Perry and Orlando Bloom turned heads as they arrived at Jen’s party on his gray Husqvarna motorcycle.

Orlando wore jeans and a blue fleece-lined jacket, while Katy showed off her trademark quirky sense of style, Katy donned a black, pleated leather-inspired jumpsuit.

She stood-out with her eye-catching footwear, sporting off-white trainers with dozens of shimmering beads.

The California Girls hitmaker styled her platinum blonde locks into a cropped quiff and added a touch of glamour with silver, shimmering earrings. Katy topped off the look with a black handbag and a leather coat.

It was a Big Little Lies night out for Reese Witherspoon and her co-star Laura Dern as the pair were among the many celebrities who attended the star-studded bash.

Reese undoubtedly had a fantastic night at the Sunset Tower Hotel as she was spotted looking slightly bleary-eyed after leaving the party.

The Legally Blonde star, 42, was radiant as ever for the evening, donning a black mini-dress. Wrapping up in a matching faux-fur coat, the actress added height to her frame with a pair of black, strappy heels.

Letting her blonde locks hang loose down her shoulders, Reese looked in high spirits as she departed her famous friend’s soirée.

Laura, 51, cut a glamorous figure for the evening, sporting a flowing black, leather-inspired dress which she paired with a white, long-sleeved top. Styling her tresses into loose curls, she also donned black boots and carried a matching handbag.

Chat show host Ellen DeGeneres was also spotted joining the birthday shindig, looking casually cool in tailored tracksuit bottoms with a navy bomber jacket.

During the Dumplin’ star’s appearance on The Ellen Show, the host promised a special hour-long appearance for her friend on the occasion of her 50th birthday.

Iron Man and Avengers co-star Robert Downey Jr. showed up to Jennifer’s shindig looking every bit like Tony Stark.

The acclaimed actor made one of the toasts during Jennifer’s ill-fated wedding to Justin Theroux, which ended with their separation in late 2017.

Jason Bateman, her co-star in 2010’s The Switch, stepped out of a car with a goatee, a rarity for the usually clean-shaven actor.

Kate Hudson stopped by the birthday party in a shimmering black dress. She was accompanied by her beau Danny Fujikawa, with whom she recently welcomed daughter Rani Rose.

The How To Lose A Guy In 10 Days actress recently appeared with Jennifer in the romantic comedy Mother’s Day.

There was also plenty of Hollywood royalty at the Marley & Me star’s bash. Barbra Streisand and her husband James Brolin made an appearance, as did Diane Keaton, who wore her trademark finger-less gloves and wide-brimmed black hat.

A smiling David Arquette, a figure from Jennifer’s Friends days, made a special appearance.

After he married her co-star Courteney Cox, the NBC sitcom’s cast famously appended ‘-Arquette’ to all of their names to poke (loving) fun at their colleague.

Tobey Maguire’s ex-wife Jennifer Meyer, who designed the Horrible Bosses star’s wedding ring from her marriage to Justin Theroux, dropped by the party as well.