Yes — on the P900, the maximum shutter speed is limited by ISO. According to the camera’s manual, at ISO 200 the longest exposure is 8 seconds, at ISO 400 it’s 4 seconds, and at ISO 800 it’s 2 seconds. So this is a camera limitation, not a setting you’re missing.

For stars, very long exposures on a normal tripod also cause star trailing as the Earth rotates. To go much longer, you’d need an equatorial tracking mount that follows the stars.

Without a tracker, the practical approach is to:

• use the widest aperture available,
• raise ISO as needed,
• take many short exposures, and
• stack them later in software after aligning the stars.

Stacking improves noise performance: combining 16 frames reduces noise by about 4×, 25 frames by 5×, and 36 frames by 6×. This is often better than trying to force one long exposure.

So with the P900, treat 2 seconds at ISO 800 as normal behavior, and aim for multiple short night-sky exposures rather than a single long one.