Question 108·Hard·Rhetorical Synthesis
While researching a topic, a student has compiled the following notes:
- The transit method detects a planet when it crosses in front of its star, producing a measurable dip in the star’s brightness.
- From the depth of the dip, astronomers can calculate the planet’s size (radius).
- The radial‐velocity method tracks tiny shifts in a star’s spectral lines caused by a planet’s gravitational pull.
- The amplitude of these shifts reveals the planet’s mass.
- Knowing a planet’s mass and size allows scientists to compute its density.
The student wants to write one sentence explaining how scientists determine an exoplanet’s density. Which choice most effectively uses relevant information from the notes to accomplish this goal?
For this type of rhetorical synthesis question, first restate the task in your own words (what exactly must the sentence do?). Then scan the notes and underline the few key relationships you must include (here: transit → size, radial-velocity → mass, mass + size → density). When you review the answer choices, quickly eliminate any that (1) leave out one of the essential pieces, (2) add specific but unnecessary details not needed to fulfill the purpose, or (3) do not directly answer the question’s “how” or “why.” Choose the option that uses the minimum necessary relevant information to satisfy the stated goal completely and clearly.
Hints
Focus on what density requires
Look at the last bullet point in the notes. What two quantities must scientists know in order to compute a planet’s density?
Match methods to quantities
Which note tells you how scientists get a planet’s size, and which note tells you how they get its mass? Keep both of these methods in mind.
Check completeness and relevance
Eliminate any answer that uses only one method or talks mainly about a specific telescope or confirming dense planets instead of explaining the general process of finding density.
Step-by-step Explanation
Clarify the task
The question asks for one sentence that explains how scientists determine an exoplanet’s density, using the relevant notes. So the right answer must:
- Describe a process (how they do it), and
- Be clearly based on the information in the bullet points.
Identify the key information in the notes
From the notes:
- The transit method and the dip in brightness give a planet’s size (radius).
- The radial-velocity method and the star’s wobble give a planet’s mass.
- Density is computed when scientists know both mass and size.
So, any good sentence must connect: transit → size, radial-velocity → mass, and mass + size → density.
Check which choices use all the necessary pieces
Go through the options:
- Some choices talk only about transits and size or only about radial-velocity and mass.
- One choice brings in extra information about specific telescopes and “dense worlds” instead of explaining the general process.
- Only one option clearly mentions both methods, correctly states what each method provides, and then explains that using these two measurements together lets scientists find density.
Select the sentence that fully and directly answers the question
The choice that does all of this is:
Combining the transit method, which yields a planet’s size, with the radial‐velocity method, which provides its mass, enables scientists to calculate the planet’s density.
It uses relevant notes, includes both methods, correctly links them to size and mass, and explicitly states that scientists use these together to calculate density.