Nope, you don't need to bring in any outside information to answer this question--and you shouldn't!
Let's go back and make sure we're clear on what we're trying to do in a Justify question. In a Justify question, you are trying to prove the conclusion 100% by adding new information that was not previously stated in the argument. So the first thing we need to do is identify and focus on the conclusion. The conclusion of this argument is "Pluto is not a true planet." So in the answer choices we are looking for additional information that will help us prove that Pluto is not a true planet.
Answer choice (E) says that forming in orbit around the sun exclusively is a REQUIREMENT for being a true planet. The conditional diagram for answer choice (E) would look like this:
Formed in orbit around the sun exclusively
The contrapositive, then, would look like this:
Formed in orbit around the sun exclusively True planet
So if there is a celestial body that was not formed in orbit around the sun exclusively, then it is not a true planet. What do we know about Pluto based on this stimulus? It was formed in orbit around Neptune. Therefore, Pluto was not formed in orbit around the sun exclusively and so it cannot be a true planet. We've proved our conclusion 100%!
Also notice that we can prove that conclusion without any actual knowledge or understanding of astronomy. In a Justify question, we are told to assume that the answer choices are correct. If they changed answer choice (E) to say "For a celestial body to be a true planet it must have formed in orbit around the planet Mars exclusively," it would still be a correct answer. It may not be true in the real world. But if the LSAT tells us to assume that true planets have to form around Mars and Pluto formed around Neptune, then Pluto would still not be a true planet.
A key takeaway here is that it's always important to focus on conclusions and have a clear idea of what different question types are asking you to do and what you're looking for in the answer choices--it's so easy to get confused otherwise!
Hope this helps!