The atmospheric soundtrack for the film Avatar (2009) was composed by James Horner. Horner was an accomplished composer known for his work on numerous successful films. The soundtrack features a blend of electronic music and traditional orchestral scores, making full use of a vast array of instruments. The soundtrack is as expansive and immersive as the film itself, contributing significantly to the movie's otherworldly feel. In addition to composing the film's score, James Horner also co-wrote the film's theme song, "I See You," which was performed by Leona Lewis. James Horner has composed music for multiple high-profile films including Titanic, Braveheart, and A Beautiful Mind among others. Sadly, he passed away in a plane crash in 2015, but his legacy lives on through his incredible body of work. I hope you found this information helpful. If you have further questions on this topic or others, feel free to ask!