Fridge Brilliance: Remina seems to plow through most planets, devouring them at a rapid pace. It made a beeline for Earth upon spotting it, and actually takes time to savor our planet before eating it. It's very possible that planets capable of sustaining life, with all the lovely water, vegetation, and... well, meat, taste much better than others.
Fridge Horror: Think about the sheer scale of Remina compared to Earth. Now think of Remina compared to a human. Now think about your own size compared to that of a single-celled organism. Now think about all those organisms on the food you eat every day. Do you have any understanding of what goes on with your food, on a cellular level? Every bite you take destroys billions of tiny single-celled lives, but you don't give it even a cursory thought... Just like Remina. As far as the monster was concerned, Earth was just especially tasty food. It may not have even realized that the humans were there.
Though from the fate of the astronauts, it seems that its aware of the people on the "food" its eating.
Many other planets inhabited by possible alien civilizations will suffer the same fate as Earth.
Fridge Logic: Was it ever explained what the hell was up with the cultist's tongue when he licked Remina's face?
Suffice to say, I'm pretty sure gravity doesn't work very well with planets as much as it did in the story. Maybe this was just a case of Art Major Physics?
Sci-Fi Writers Have No Sense of Scale: How does a Jupiter-sized organism form and not crush itself from its own gravity? There's a reason solid planets don't form that large—they would destroy themselves from their own gravity. Further, how does the Hellstar travel faster than light, let alone do so without killing itself? How does it withstand the vacuum of space, especially it's gigantic, sensitive eye, or its tongue? Lastly, how does it eat a planet like Earth or Jupiter without killing itself from the extreme heat, pressures, gasses and solids, radioactive material, and everything else from what it eats?
All those questions were asked inside the story by the scientists who discovered and observed it. Nobody knows.
They did say it was from another universe, maybe it operates by different physics. Or...y'know...