D and E: By 1999, astronomers have discovered
Here, have discovered (present perfect) cannot serve to express an action completed by 1999(in the PAST).
Eliminate D and E.
A VERBing modifier serves to express an action that takes place at the same time as the MAIN VERB.
In B, the usage of orbiting seems to imply that planets were ORBITING the 17 nearby stars only IN THE PAST -- when the stars were DISCOVERED by astronomers.
Since the intended meaning is that planets are STILL orbiting the 17 nearby stars, eliminate B.
C: that there were 17 stars
Here, the usage of were seems to imply that the 17 nearby stars no longer exist.
Since the intended meaning is that the 17 nearby stars DO still exist, eliminate C.
Hence the answer will A