A survey of 1,000 Sun-like stars using the transit and radial velocity methods finds that the vast majority of detected exoplanets are massive and orbit very close to their stars. What is the most accurate interpretation of this finding?
AClose-in massive planets are the most common type of planet in the galaxy
BThe detection methods are biased toward large planets in short-period orbits, so the catalog reflects what is easiest to find, not what is most common
CSmall, Earth-like planets cannot form in stellar systems that already contain massive close-in planets
DThe survey was too short to detect longer-period planets, which do not exist at these distances
Both the transit and radial velocity methods are systematically biased toward detecting large planets in close orbits: large planets produce deeper transits and stronger Doppler wobbles, and short orbital periods mean more transits and faster wobbles within an observation window. This observational bias — not galactic rarity — explains the prevalence of hot Jupiters in early catalogs. As detection sensitivity improved, surveys revealed that super-Earths and sub-Neptunes are actually the most common planet types.
Question 2 Multiple Choice
A planet around a nearby star is detected by both the transit method and the radial velocity method. What unique information does combining both measurements provide that neither method alone can give?
AThe planet's surface temperature and whether it has liquid water
BThe planet's true orbital inclination and absolute distance from its star
CThe planet's bulk density, providing a first clue to whether it is rocky, icy, or gaseous
DThe planet's atmospheric composition through spectroscopic absorption features
The transit method yields the planet's radius (from the fractional brightness dip, which equals the ratio of cross-sectional areas). The radial velocity method yields the planet's minimum mass (the true mass requires knowing the orbital inclination, which the transit geometry constrains). Combining both gives mass and radius, from which bulk density = mass/volume can be calculated — distinguishing rocky, icy, or gaseous worlds. Atmospheric composition requires transmission spectroscopy, not these two methods alone.
Question 3 True / False
The radial velocity method gives only the minimum mass of a detected planet, not its true mass.
TTrue
FFalse
Answer: True
The Doppler wobble measured by radial velocity depends on the component of the star's motion along the line of sight. If the orbit is tilted away from edge-on (inclination angle i < 90°), only the projected velocity is detected — not the full orbital speed. The quantity actually measured is m·sin(i), where m is the planet's true mass. Without knowing i independently, only a lower bound is available: the planet must be at least as massive as m·sin(i). The transit method constrains inclination by requiring the orbit to be nearly edge-on, which is why combining both methods yields the true mass.
Question 4 True / False
A planet detected by the transit method can have its mass determined directly from the depth and duration of the brightness dip.
TTrue
FFalse
Answer: False
The transit light curve gives the orbital period (from the interval between transits) and the planet-to-star radius ratio (from the fractional dimming depth), but contains no information about the planet's mass. Mass requires measuring the gravitational influence of the planet on its host star — which the radial velocity method detects via the Doppler wobble. This is why combining both methods is so powerful: transit gives radius, radial velocity gives minimum mass, together they yield bulk density.
Question 5 Short Answer
Why did early exoplanet catalogs contain a disproportionate number of 'hot Jupiters,' and what does this reveal about the reliability of planet surveys as guides to the true galactic population of planets?
Think about your answer, then reveal below.
Model answer: Hot Jupiters were detected first and in large numbers because both major detection methods are biased toward exactly what hot Jupiters are: massive (large Doppler wobble, deep transit) and close-orbiting (frequent transits, fast wobble within an observation window). This does not mean hot Jupiters are common in the galaxy — it means they are the easiest to detect. As instrument sensitivity improved, surveys revealed that Earth-sized and sub-Neptune planets are far more numerous. Any planet catalog must be interpreted with attention to the selection biases of the method: absence of small, distant planets in early catalogs reflected observational limits, not physical rarity.
Observational bias is a fundamental challenge in astronomy and in science generally. A catalog reflects the sensitivity and geometry of the method, not an unbiased census of what exists. The correction of the hot-Jupiter bias as instrumentation improved is a textbook example of how understanding your instrument's limitations is as important as the measurements themselves.