Haven't used the P900 myself but tested my Nikon D800e with 500mm and 1.4x, 2.0x, 3.0x extenders my old panasonic FZ20 superzoom, FZ1000 and a Sony 60+x superzoom and none come close to my cheap celestron refractor with a webcam.
I use the FZ1000 for most of my astrophotography these days, its ok for some deep space objects on the vixen polarie, and the moon. Planetary though superzooms are useless. They are just not up to the job. The planets will be little more than dots like stars. You can capture the International Space Station though.
