The former Boomtown Rats singer and his partners Lord Alli, a Labour peer, and Charlie Parsons will divide pounds 15m from the television company, which owns the ITV broadcasters Carlton and Central.
It is one of Britain's largest production companies and its value is based on its contract to produce The Big Breakfast for Channel 4. That contract is thought to be worth around pounds 20m a year. It is the channel's biggest commission and only ITN's evening news programme comes close to appearing as regularly as the breakfast show.
Carlton and Planet 24 refuse to disclose the size of the deal, but City analysts believe pounds 15m reflects a realistic price for the company. However, a Planet 24 spokesman has said in the past that the asking price for the company was pounds 30m. The Big Breakfast has had a period of relative ratings success with the presenter Johnny Vaughan over the past two years, after flagging since the departure of Chris Evans in the mid-Nineties.
Geldof, who was knighted for organising Band Aid and Live Aid in the Eighties, said yesterday: "I'm as proud of Planet 24 as anything else I've done. This one company managed to shift the way television looked so nowadays our screens are awash with Planet 24 'wannabe' programming."
Dublin-born Geldof, a former music journalist who first rose to prominence with the Boomtown Rats in the post-punk wave in the late 1970s, has recently worked as a disc jockey on the London radio station Xfm, and he joked yesterday that he would now have time to follow his career in astrophysics. Last year he was paid pounds 684,000 as a director of the company. The creative brain behind the company is Charlie Parsons, who has made celebrities out of performers with youth and tabloid appeal, including Denise Van Outen, Dani Behr and Gaby Roslin. The other managing partner, Lord Alli, is a close friend of the former cabinet minister Peter Mandelson and was made a life peer last year. There has been speculation that he has wanted to sell the company to devote more time to his political career. He serves on the Foreign Office Panel 2000, which advises Robin Cook on the rebranding of Britain.
He will join Carlton as managing director of Carlton Productions. Parsons and Geldof will be connected with the company only in a consultative role.
Since its foundation in 1992 Planet 24 has managed to successfully dominate the niche market of "Yoof TV" typified by the late-night Channel 4 programme The Word. Despite some criticism of Planet 24 output, it nevertheless acted as a springboard to launch many household names.
Planet 24 employs 250 staff and makes programmes for terrestrial, satellite and cable television, as well as radio. Its terrestrial output alone is 600 hours of programming a year.
As well as The Big Breakfast, the company makes Gaytime TV for the BBC and Watercolour Challenge for Channel 4. A raft of other new programmes are also being developed but not all attempts to branch out have been successful. Plans to produce an American chat show failed after the series' ratings collapsed and Hotel Babylon for ITV received mixed reviews.
Carlton currently spends around pounds 200m a year on programmes for its own ITV companies Carlton and Central as well as for the BBC and Channel 4.
Planet 24, which employs 200 people, had profits after tax last year of pounds 1m.