Let's break this down.

The sentence says that the discovery of many more planets "only $\_\_\_\_$ undergirds the argument for intelligent lifeforms."
The second part explains why: the argument depends on more than just the number of planets - it also needs planets with viable atmospheres.

So the discovery of more planets alone doesn't strongly or directly support the argument - it only supports it in a limited or indirect way.

We need two words that mean "in a limited/weak way" or "not directly."
A. subtly - means in a slight, not obvious way. Could fit.
B. minimally - means to a very small extent. Fits well.
C. obliquely - means indirectly. Fits.
D. retroactively - means applying to a past period. Doesn't fit.
E. discursively - means rambling, digressive. Doesn't fit.
F. indirectly - means not directly. Fits.

We need two that are similar and both make sense.
"Minimally" and "indirectly" both fit logically, but let's check if the test wants a pair that are closer synonyms.
"Minimally" = to a small degree
"Indirectly" = not in a direct way

Both work, but "subtly" and "obliquely" are also possible. However, "subtly" is more about being hard to notice, while "minimally" is about degree. "Obliquely" is essentially the same as "indirectly."

The best pair is B. minimally and F. indirectly, but let's see if there's a closer synonym pair: "obliquely" and "indirectly" are almost identical in meaning, so that's likely the intended pair.
C. obliquely and F. indirectly are the closest synonyms and both fit the blank perfectly. "Minimally" also fits, but if we must pick two, the test usually picks the pair that are near synonyms in the given context.

Final answer:
C. obliquely and F. indirectly