Is this a run-on sentence..

[rambharosey]

Scientists have discovered more than 40 asteroids, most of them as large as or larger than Saturn and circle other stars.

There are two Independent clauses connected by a comma, and hence this is a run-on sentence:

1. Scientists have discovered more than 40 asteroids
2. Most of them circle other stars.

Is my understanding correct?

[Bennevis]

Scientists have discovered more than 40 asteroids, most of them (being) as large as or larger than Saturn and circle other stars.

Since it doesn't say "most of them are as large as or larger than Saturn...", it should be OK, but "being" would really help.

[bhaisahab]

The second part of the sentence makes no sense.

[Bennevis]

The second part of the sentence makes no sense.

Yes. I didn't notice the "circle other stars" part. It is a run-on sentence. No doubt about it.