It is thought that stars emit X-ray.
Stars are thought to emit X-ray.
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Off-hand I can't think of any context where it wouldn't be preferable to use thought rather than believed in OP's construction, but that's not central to the issue.
Converting to more likely assertions, consider these results from Google Books...
1: It is believed that Mars has (water, life, whatever) (7 hits)
2: Mars is believed to have... (716 hits)
There's no difference in meaning, but idiomatically / stylistically #1 is significantly less common, and may be perceived as rather stilted / verbose.