The cosmos has grown slowly, and so too has Kyle Gann's musical probing of the aura of the planets. Six parts of his

The Planets

were played by Relache Thursday at the Franklin Institute's Fels Planetarium, a sort of satellite to the museum's current "Galileo" exhibition.

Gann launched the piece in 1994, completing "Sun," Moon," and "Venus," and Relache has been performing it, planet by planet, ever since. The "Uranus," "Mercury," and "Pluto" sections were new in this concert. The complete piece will have 10 sections, and the eight members of Relache expect to record that full cosmos this summer.

Speaking before the performance, Gann said he wasn't competing with Gustav Holst's monumental work The Planets, but was illuminating astrological connections suggested by individual planets. Listeners always have to decide if knowing the composer's thought processes is essential to understanding a piece. Would this music have sounded different had it been titled Toccata and Fun? Or danced by young people in casual clothes?

The astrological ideas suggested the moods, and Gann's witty style shaped music that is built of tingly sounds, intricate metrical joining, and compelling movement. The ensemble - flute, oboe, bassoon, viola, bass, saxophone, clarinet, percussion, and electronic keyboard - craftily blurs the distinction between dance music and something much more serious. Gann built the "Moon" section on pairs of instruments, a choice that freed bright sonorities. In "Saturn" - the apogee, as the fourth of six sections - the music moves from a dense opening to a close glowing with instrumental color and a sense of nobility. Overhead, the planetarium floated an enormous, turning Saturn as the music evolved.

The astrology behind the music said less than the music, for Gann has a sense of wit that keeps everything just beyond expectations, and always with an oblique sense of phrase. We could almost sing that theme - not quite - and be just a little wrong in predicting where the music would take us. The saxophone (Bob Butryn) lent a different cast to every ensemble, and the array of percussion suggested that celestial motion could have earthly bases as well as flights of rhythmic virtuosity. The ensemble is so at home in new gestures and conflicting inner workings that the music sounded simple, expressive, wryly descriptive, and cheerfully edgy.

The planetarium staff filled the heavens with huge planets, fleeting stars, and a sense that the composer could hear the speed of celestial time.