Ashton Kutcher

In addition to famous twins like Mary-Kate and Ashley Olsen — who celebrate their 27th birthday today — Benji and Joel Madden, and the Mowry sisters, there are a number of celebrities that you may not have known are part of a pair. Ashton Kutcher, Scarlett Johansson, Gisele Bündchen, and even Vin Diesel have twin siblings — and while some of their other halves keep a low profile, others have a place in show business as well. Click through now to see 15 stars that you didn't know had a double!

Princess Beatrice and her boyfriend, Dave Clark, hopped on a yacht off the coast of Antibes, France, with a group of friends on Friday. The lavish getaway may be turning into a tradition for the couple since they took a similar getaway together this time last year. After boarding the large vessel, Dave and Beatrice wasted no time getting into vacation mode. Dave stripped off his shirt to ride a Jet Ski and jump on a trampoline in the water, and he even showed off his skills by diving off the side of the boat. He later met up with Beatrice, who wore a black bikini, for a dip in the hot tub.

The fun continued on Saturday, when Dave and Beatrice met up with Ashton Kutcher and Mila Kunis for a visit to Saint Tropez. The group took a small boat to the shore, where they stopped at Club 55. On Sunday, they got together again for a trip to dry land after Ashton went shirtless on the deck of the yacht. Dave, Beatrice, Ashton, and Mila have already spent time together overseas since they went on a double date in London last month. Ashton and Mila are stationed in the UK while Mila films Jupiter Ascending with Channing Tatum, but they got away from England for the weekend.

Today's actors have only made it to space on screen, but some stars could actually travel out of this world as early as this year. Justin Bieber is the latest star to book a seat on a Virgin Galactic suborbital flight, joining a string of A-listers set to board the company's spaceships. As the first commercial spaceline, Virgin Galactic plans to operate several flights aboard spaceships that carry six passengers and two pilots. A seat will cost you more than $200,000. After passengers spend a few days preparing with their crew, they'll bast off from a spaceport in New Mexico for a short flight that includes several minutes of weightlessness.

As of now, the company is still testing, but Virgin Group owner Sir Richard Branson hopes to launch the first full space flight by the end of the year, making space tourism a probable reality. Besides Bieber, see which stars are planning trips.

Liam Hemsworth joined Twitter yesterday despite not being so fond of the site back in October 2009, when he convinced his now-fiancée, Miley Cyrus, to deactivate her account. Both stars have active accounts as of Wednesday, and Liam's first official tweet read, "Glad to be on board. Thanks for following." While he's only following 13 people, including Miley, Liam has already racked up over 70,000 followers in less than 48 hours. To celebrate his induction to the social media site, we've rounded up a list of celebrities who have a history of changing their minds about the microblogging site. From Megan Fox's confusion with Twitter's purpose to Ashton Kutcher's online silence following a controversial tweet, celebrities go back and forth with 140 characters.

Click through now to see all the Twitter quitters and find out why — at the end of the day — they can't seem to stay away!

Ashton Kutcher and Mila Kunis held hands and covered up in hooded sweatshirts during a rainy outing in Hampstead, North London, today. The couple grabbed lunch at an Italian restaurant before stopping for dessert at a crepe stand and devouring their sweets during the walk home.

Ashton has joined Mila across the pond while she films Jupiter Ascending with Channing Tatum, who also has the company of his pregnant wife, Jenna Dewan, in the UK. Mila and Ashton have welcomed the London scene and cuisine rather quickly and had a double date with Princess Beatrice and her boyfriend, Dave Clark, on Saturday. The foursome reportedly indulged in deep-fried bananas and tequila shots at two different locations, including London hot spot Duck & Waffle.

This week in the world of social media, celebrities stayed busy sharing their cute candids from the MTV Movie Awards, Coachella, and more! Miranda Kerr and Bar Refaeli both soaked up the sun in their native countries, while Lea Michele showed off her yoga moves during an LA hike. Ashton Kutcher showed his support for the city of Boston by snapping a photo in a Red Sox cap, and his former That '70s Show costar Wilmer Valderrama spoke at the White House to champion immigration reform.

Click through to see the rest of this week's cutest celebrity candids, and be sure to follow POPSUGAR on Instagram, Facebook, and Twitter!

Mila Kunis and her Jupiter Ascending costar Channing Tatum had a night out in London yesterday with their significant others. She brought along boyfriend Ashton Kutcher, while Channing had the company of his pregnant wife, Jenna Dewan. The foursome were reportedly able to attend the premiere of the English National Opera's Sunken Garden at the Barbican Center. The evening of opera was a bit of a change from the sci-fi material Mila and Channing have been busy with. Work on Jupiter Ascending has both Channing and Mila hunkering down in the English capital for a good period of time. Mila's apparently staying in the city itself, while Channing and Jenna are enjoying the last weeks of her pregnancy at Sienna Miller's house outside of London in the Cotswolds.

Ashton Kutcher joined Mila Kunis in London last night. The couple shared a dinner date at Scott's restaurant after attending Mila's Gemfields jewelry event together. Mila is the brand ambassador for Gemfields and she celebrated her recent appointment at a party for the brand in LA last month.

Mila's moving on to her jewelry duties following a press tour for her latest film, Oz the Great and Powerful, which took her overseas as well. She debuted the film in Moscow with costars James Franco and Michelle Williams before hitting the yellow brick road in an Alexander McQueen gown for the UK premiere at the end of February.