First sentence: Why this is the first line? This paragraph is the first line because it introduces the main topic. It talks about the time before Earth and other planets. This is a broad overview of the subject. It does not use any pronouns or definite articles that need prior context. Why other lines are not chosen? Paragraph 2 starts with "Those rings" which uses a pronoun that refers back to something not yet mentioned. Paragraph 3 starts with "A super-Earth" which is specific and does not introduce the topic. Paragraph 4 starts with "Astronomers" which is also specific and does not set the context. Paragraph 5 starts with "Now they" which uses a pronoun and assumes prior knowledge.

Concluding sentence: Why this is the last line? This paragraph is the last line because it wraps up the discussion. It shows the current situation of super-Earths and explains why there are fewer of them now. This gives a clear conclusion to the topic.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): There is a clear connection here. The first paragraph talks about the rings of dust, and the second paragraph explains what those rings did. This shows a logical order from the introduction of the rings to their effect on Earth.
• Pair 2 (Paragraph 2 → Paragraph 3): The second paragraph talks about how Earth did not become a super-Earth. The third paragraph then explains what a super-Earth is. This is a broader to specific strategy, moving from the effect of the rings to the definition of a super-Earth.
• Pair 3 (Paragraph 3 → Paragraph 4): The third paragraph defines what a super-Earth is, and the fourth paragraph gives information about where super-Earths used to be found. This shows a logical order from definition to discovery.
• Pair 4 (Paragraph 4 → Paragraph 5): The fourth paragraph talks about how many super-Earths used to orbit sun-like stars. The fifth paragraph explains the current situation, showing a cause and effect relationship. The invasion of planets has changed the number of super-Earths orbiting those stars.