The worst kept secret in Formula One is finally out -- Fernando Alonso is leaving Ferrari and will be replaced by Sebastian Vettel. Red Bull's four-time world champion has signed a three-year contract with the Scuderia, the oldest team in F1, from 2015. After ending his five-year stint at Ferrari, Alonso remains coy on where he will be driving next season. The double move by two of the sport's high profile world champions is the most significant in the driver market this season. But in the fickle world of F1 there are no guarantees it will work out for either of the ambitious racers. There are still seats to be filled at McLaren, Force India and Toro Rosso. With the curtain about to fall on the 2014 season at Sunday's Abu Dhabi Grand Prix, hopeful drivers have just one more chance to stake their claim for the remaining seats. Where will Alonso go ? Alonso is regarded as the best all-round driver currently racing at the elite level of motorsport, a fact many of his peers are even happily willing to acknowledge. The Spaniard -- a double world champion with Renault in 2005 and 2006 -- is regarded as the key to the F1 driver market. The 33-year-old made it clear he wanted to leave Ferrari, even though he had two years left to run on his contract with the Italian team, but he has yet to confirm where he will go next. A return to McLaren seems most likely, despite his acrimonious