Discovery Channel and the BBC are planning a sequel to their blockbuster high-definition nature documentary series “Planet Earth.”

Jane Root, executive VP and general manager of Discovery Channel, said last week that talks are under way for a co-production of another edition of the globetrotting show, the most expensive project in Discovery’s history. Each episode of “Planet Earth” cost nearly $2 million to produce.

Discovery executives are keen to get the follow-up project launched as quickly as possible; the first 11 episodes took nearly five years to make. Though the parties have not yet agreed upon a completion date, Ms. Root said she hopes the length of production time can be shortened for the sequel.

Much of the content for the first “Planet” series was determined by producer Alastair Fothergill (“The Blue Planet”) and his veteran camera team, brainstorming wish lists of “shots that got away,” never-before-seen wildlife and unprecedented viewing angles, Ms. Root said.

The sequel will have similar ambitions, but also may take a cue from Discovery’s recently announced GreenPlanet environmentalism initiative. GreenPlanet includes a $50 million investment from Discovery in environmental programming and an eco-themed channel to be launched in 2008.

Ms. Root said the company’s decision to go forward with GreenPlanet was partly due to the success of “Planet Earth,” which in addition to strong ratings has become something of a cultural phenomenon. Critical praise has been effusive; Oprah Winfrey devoted an entire episode of her talk show to the programming.

In recent weeks, Discovery also announced a similar-sounding project, the series “10 Ways to Save the Planet,” which will premiere in 2008.

“Earth” premiered to a 3.6 average household rating, with about 15 percent of the viewership on Discovery HD Theater (about 500,000 homes), according to Nielsen Media Research. Subsequent airings have maintained at least a 3.1 household rating. Though Discovery has not yet released the series on DVD, the BBC version has been a top seller on Amazon.com.