This problem is a good example of the conflict inherent in good sentence construction: on one hand we have rules that govern our usage of language, and on the other we have the desire to express meaning within the framework of those rules. Often some sort of compromise is involved.
The basic rules governing "verbing" modifiers are as follows:
A
'comma -ing' modifier modifies the clause before it, and requires a logical connection between the two actions (usually a consequence or simultaneous actions). Some examples:
The student sat in a chair, reading a book.
I broke a window, making my dad angry.A
modifier with -ing and no comma is a simple noun modifier. For example:
Look at the birds flying overhead.
I saw a person driving badly.Check out the SC Strategy Guide for more on these.
I think "circling" modifies "most of them" rather than 'planets'.
I guess that you interpret part of the meaning of the sentence as follows:
"In the past several years, astronomers have detected more than 80 massive planets, most of them circling other stars." That's actually a pretty nonsensical sentence. I mean, all planets circle starts, and it implies that some of the planets that they found are in our solar system!
Now, as Ron explained in one of the posts above, in the problem cited we have the second type of modifier. If the sentence said this -
"In the past several years, astronomers have detected more than 80 massive planets, circling other stars." - it would have a crazy meaning, implying that the astronomers detected the planets while they (the astronomers) were circling other stars.
Really, the sentence can be broken down to this: "In the past several years, astronomers have detected more than 80 massive planets circling other stars." However, the writer wants to add a bit more information about the planets (i.e. another noun modifier) but has a dilemma. There are already two noun modifiers touching 'planets' (the adjective 'massive' and the 'circling...' modifier).
So the writer decides to squeeze in the phrase 'most of them at least as large as Jupiter' as a bit of a compromise. It would be either that or breaking the sentence up into two parts and the second option would be rather cumbersome.