You have to make a contribution to physics that is internationally recognized to have greatly advanced human knowledge in physical science. Minor discoveries such as the kind that scientists routinely make practically every day, do not qualify. For instance, the discovery of planets outside our solar system would probably not be enough to garner a Nobel Prize, especially since astronomy is not one of the categories in which Nobel Prizes are awarded. So as an astronomer, forget the Nobel Prize. We do have several other prestigious prizes in astronomy so you could qualify for one of those. Again, your discovery must be one that significantly enhances astronomy, and divides research in a spectacular way between 'before' and 'after' your discovery. Theories, of them selves, do not qualify unless they are substantiated by significant experimental evidence.