Tucker Carlson, a conservative provocateur who joined Fox News’s prime-time ranks only three months ago, has been tapped to replace Bill O’Reilly at 8 p.m. Eastern, , the network said. Mr. Carlson has become, seemingly overnight, one of the network’s most vital players, a remarkable turnaround for a pundit whose bow-tied heyday had seemed behind him. This will be Mr. Carlson’s third time-slot change in four months — an unheard-of fluidity at Fox News after years of a more-or-less static lineup. Since January, Mr. Carlson has been the replacement for three departing Fox stars: Greta Van Susteren, Ms. Kelly and, now, Mr. O’Reilly. Tucker Carlson was a senior fellow with the Cato Institute, focusing on libertarian philosophy and the American polity. Carlson is a veteran political commentator and journalist, most recently hosting his own nightly program on MSNBC, Tucker. Carlson joined MSNBC in February 2005 from CNN, where he was the youngest anchor in the history of that network. At CNN, he hosted a number of shows, including the network’s political debate program, Crossfire. During the same period, Carlson also hosted a weekly public affairs program on PBS. In 2009, Carlson launched the right-leaning news site, TheDailyCaller.com. Carlson began his journalism career at the Arkansas Democrat-Gazette newspaper in Little Rock. His first book is entitled, Politicians, Partisans and Parasites: My Adventures in Cable News. In 2006, he competed on ABC’s Dancing with the Stars. He attended St. George's School, a boarding school in Middletown, Rhode Island. After graduation, he majored in History at Trinity College in Hartford, Connecticut. He is married to Susan Carlson, née Andrews. They have four children together: Lillie, Hopie, Dorothy, and Buckley.