According to the book Galileo by John L. Heilbron, Galileo was convinced that a comet is not a planetary-like body and he has defended such a view in his book Assayer.
I have found in Internet a paper about this controversy.
“What the interpretation of the Assayer as the great manifesto of the New Science has usually passed in silence, however, is that the positions Galileo takes in the essay concerning its explicit subject matter—the nature and position of comets—are extremely conservative, not to say reactionary.”
“What he [Galileo] does make completely clear, however, is that he would not allow that the comet is a planetary-like body, residing above the moon:
A comet is not one of the wandering stars which become visible in a manner similar to that of some planet.”
‘Galileo does not have a strong position of his own as to what comets really are and his trust in the Aristotelian theory is tentative and qualified at best. Why, then, should he rely on this theory to cast doubt on the parallax technique and vehemently reject Tycho and Jesuits’ breakthrough that comets are “not sublunar but clearly celestial”? It is not the case that Galileo some deep aversion to parallax considerations, or that he was always so committed to Aristotelian meteorology or convinced in the errors of Tycho’s way. In a series of public lectures following the super nova of 1604 he took a clear Tychonic position and used the lack of parallax to argue that the new star was super lunar. And it is worth stressing again that the curious concept of frictionless motion in a medium is a small part of the price that Galileo has to pay for this rejection. Insisting that the comets should not be placed above the moon, Galileo is undermining an important mathematical empirical tool, the parallax calculation, and adopting a qualitative and analogical mode of explanation, Aristotelian in essence if not fully Aristotelian in details. Most surprising: both in declining to accept Tycho’s demonstrations for the changes in the heavens and in employing the notion of “elemental sphere” in his own account, Galileo re-embraces the sharp distinction between the sub- and super-lunar realms.’