For example, some giant “planet” the size of our sun which has a surface like the moon, floating through space in darkness.
Tangentially, how are we sure that black holes aren’t these? Are we misinterpreting absence of light as black holes when instead they could just be large dark objects?
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It isn’t clear what you mean by “massive” since you might be shifting between “high mass” (the traditional definition for physics) and “large size” (the semantical use).
A dense massive (high mass) object will be crushed by its own gravity. The extremes of these objects would be black holes and neutron stars (if not quite as massive). There is no escaping the logic of gravity. Something that “starts” with the size of the sun but consists of dense matter like the earth could not hold itself as a planet because the gravity would very quickly lead to it collapsing.
Our sun has an average density of about 1.4 times that of liquid water on earth. Earth has an average density about 6 times that of liquid water. So an “earth” the size of the sun would be around 64 times more massive (in mass) than the sun. The gravity from such a mass at such density would very likely compress itself into a black hole. For a rocky type planet, the theoretical size limit would be (likely, ELI5) around twice the diameter of earth. For a gas type planet like Jupiter, the mass limit would be around 300 times the mass of the earth. (For reference the sun is around 300,000 times the mass of the earth)
There are “objects” that can be far more massive but much less dense and therefore occupy a large volume. They would resemble gas clouds rather than a planet. Some of these clouds will eventually form new stars and solar systems (it isn’t a static picture over time)
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