"It could be that there are a million planets just like ours."
Translation:יכול להיות שיש מיליון כוכבי לכת בדיוק כמו שלנו.
No - כוכב is a star. Planet can either be כוכב לכת or פלנטה (planeta).
That is why I find it strange that one of the alternative answers showed planet translated simply as כוכב
I have also seen/heard people use just כוכב to mean planet... Maybe it's just one of those "yeah, it's wrong, but people say it anyway" things?
Yes, probably. Since the name in Hebrew is כוכב לכת, it does imply that a planet is a type of star (even though it's not really), so perhaps it's the source of the confusion.
No, it's totally fine. You can use כוכב for a planet, a star, or the ones in Hollywood.. If you want to be more specific you can use כוכב לכת or כוכב שבת (moving star vs stationary star = planet vs star).
But calling all the dots you see in the night sky כוכבים is just fine.
(Yeah, I know some of them are galaxies)