According to Nielsen’s estimates, 29.8 million viewers tuned in to the two-hour debut of American Idol’s ninth season, its first without Paula Abdul as a judge. Although the premiere audience was down compared to the 30.4 million who watched the first episode last season. Though American Idol is still the biggest show on television, ratings have thinned the last few years. Combined with judge Simon Cowell leaving after this season, there is concern at Fox that Idol will lose even more of its viewership and <gasp> potentially come in second in the ratings.

Cowell is unlikely to say anything on the air about his move until the live episodes of Idol begin in February with new host Ellen DeGeneres.

Yet, besides landing 29.9 million viewers, American Idol ruled the night among adults ages 18-49, attracting 15.5 million in the demographic, which matches a year ago.

NBC and ABC appeared to take the biggest hit from American Idol. CBS’ NCIS, which averaged 20.1 million viewers in the 8 p.m. hour, and NCIS: Los Angeles averaged 15.5 million viewers at 9 p.m.

American Idol wasn’t the only show doing big numbers Tuesday night. NBC’s The Tonight Show spiked with viewers t tuning in to see what host Conan O’Brien would say on the air regarding his situation. Earlier this week, O’Brien issued a statement saying he was not interested in doing The Tonight Show if the network moved it to 12:05 a.m. from 11:35 p.m. to make room for Jay Leno’s return from prime time to late night.

CBS also won the 10 p.m. hour following Idol with The Good Wife (13.8 million), which outperformed The Jay Leno Show on NBC (6.2 million) and The Forgotten on ABC (4.4 million). NBC ranked third on the night, averaging 8.1 million for The Biggest Loser from 8 to 10, while ABC finished a distant fourth.