Astronomers have identified several Earth-sized exoplanets in the habitable zone _____ orbit stars remarkably similar to our Sun.

Let's begin by understanding the meaning of this sentence. We'll use our understanding of pause points and segment the sentence as shown - understanding and assimilating the meaning of each segment bit by bit!

Sentence Structure

• Astronomers have identified several Earth-sized exoplanets
• in the habitable zone [?]
• orbit stars remarkably similar to our Sun.

Understanding the Meaning

Let's start reading from the beginning:

"Astronomers have identified several Earth-sized exoplanets in the habitable zone"

So we're being told about:

• A discovery astronomers made
• They found exoplanets (planets outside our solar system)
• These are Earth-sized
• They're in the "habitable zone" (the area where conditions might support life)

Now we reach the blank. Let's look at our choices:

• zone (nothing added)
• zone, (just a comma)
• zone, and (comma plus "and")
• zone that (the word "that")

To see what works here, let's read the rest of the sentence and understand what it's saying!

The sentence continues: "orbit stars remarkably similar to our Sun"

What does this mean?

• These exoplanets orbit around stars
• Those stars are very similar to our Sun

So the complete picture is:

• Astronomers found Earth-sized exoplanets in the habitable zone
• These exoplanets orbit stars that are like our Sun

Now, what do we notice about the structure here?

The phrase "orbit stars remarkably similar to our Sun" is giving us additional information about WHICH exoplanets we're talking about:

• It's describing the exoplanets
• But "orbit" is a verb that needs a subject
• What orbits? The exoplanets do!

So we need a word that:

• Refers back to "exoplanets"
• Can act as the subject for the verb "orbit"
• Connects this descriptive information to the noun it describes

The word "that" does exactly this job. It creates a describing clause:

• "exoplanets...that orbit stars"
• "that" refers to the exoplanets
• "that" serves as the subject of "orbit"

The correct answer is D (zone that).

This creates: "several Earth-sized exoplanets in the habitable zone that orbit stars remarkably similar to our Sun" - perfectly describing which exoplanets the astronomers found.

GRAMMAR CONCEPT APPLIED

Connecting Descriptive Clauses to Nouns

When you want to add a descriptive clause (a group of words with its own verb) to give more information about a noun, you need a connecting word called a relative pronoun (like "that," "which," or "who" in grammar terms). This connecting word does two important jobs:

1. Links the descriptive information to the noun it's describing
2. Serves as the subject of the verb in the descriptive clause

The Pattern:

• Noun + that/which/who + verb + rest of description

Examples:

• "The scientist discovered a fossil that dates back millions of years"
  • "that" refers back to "fossil"
  • "that" acts as the subject of "dates"
• "Students who study regularly perform better on tests"
  • "who" refers back to "students"
  • "who" acts as the subject of "study"

In our question:

• "exoplanets in the habitable zone that orbit stars remarkably similar to our Sun"
  • "that" refers back to "exoplanets"
  • "that" acts as the subject of "orbit"
  • The clause "that orbit stars..." describes which exoplanets

Without the relative pronoun, the descriptive clause has no grammatical connection to the noun and creates a structural error.