Questions: Exoplanet Characterization via Spectroscopy

Question 1

A planet's radius is measured via transit photometry and its mass via radial velocity. Its calculated bulk density is 1.1 g/cm³ (Earth's density is 5.5 g/cm³). What does this most strongly imply about the planet's composition?

• A. It is a rocky, iron-rich world similar to Earth — low density may reflect measurement uncertainty
• B. It has a substantial gaseous or volatile-rich envelope — densities below ~2 g/cm³ indicate a sub-Neptune or gas giant composition
• C. It must be a pure water world with no atmosphere, since water has a density near 1 g/cm³
• D. The measurements are inconsistent — a planet cannot have a bulk density lower than liquid water

Answer: (B)

Bulk density is the key diagnostic for planet type. A density of 1.1 g/cm³ is far below Earth's rocky 5.5 g/cm³ and indicates the planet must be largely composed of low-density material — gas, ice, or a hydrogen-helium envelope. Rocky planets (terrestrial or super-Earth) cluster above 4–5 g/cm³. A pure water world is theoretically possible but would still need to be quite different from a rocky world; the measurement itself is not inconsistent — many confirmed sub-Neptunes have densities near 1 g/cm³.

Question 2

An astronomer measures the transit depth of a planet at many different wavelengths and finds the depth is slightly larger at 1.4 μm and 2.7 μm than at other wavelengths. What is the most likely explanation?

• A. The planet's orbit is slightly eccentric, bringing it physically closer to the star at these wavelengths
• B. Water vapor in the planet's atmosphere absorbs stellar light at these wavelengths, making the apparent planetary radius larger during transit
• C. The host star emits less flux at these wavelengths, making the transit fraction appear larger
• D. Clouds in the planet's atmosphere selectively scatter these wavelengths, amplifying the transit signal

Answer: (B)

This is the signature of transmission spectroscopy. When starlight passes through a planet's atmosphere during transit, atmospheric molecules absorb at characteristic wavelengths. At absorbing wavelengths, the atmosphere is opaque higher up, making the planet appear slightly larger — the transit depth increases. Water vapor has strong absorption features near 1.4 and 2.7 μm. By measuring how transit depth varies with wavelength, astronomers construct a transmission spectrum that reveals atmospheric composition. Option C (stellar emission) would produce the opposite effect (deeper apparent transit means smaller star flux, not wavelength-dependent planet radius).

Question 3

Combining transit photometry (radius) with radial-velocity measurements (mass) for the same planet allows astronomers to calculate bulk density, which can distinguish rocky planets from gas-dominated ones.

• T. True
• F. False

Answer: True

True — this is the foundational characterization technique. Transit depth gives the planet-to-star radius ratio; knowing the star's radius yields the planet's radius. Radial velocity gives the planet's minimum mass (and true mass when the orbital inclination is known from the transit). Dividing mass by volume gives bulk density. A density near 5.5 g/cm³ implies a rocky, Earth-like composition; below 2 g/cm³ implies substantial gas or ice. This is how the exoplanet 'radius valley' was established: planets near 1.5–2 R⊕ show a bimodal density distribution.

Question 4

Transmission spectroscopy detects atmospheric molecules by measuring light emitted directly by the planet's atmosphere during transit.

• T. True
• F. False

Answer: False

False — transmission spectroscopy measures starlight that has been filtered through the planet's atmosphere as the planet crosses the stellar disk. Molecules in the atmosphere absorb at characteristic wavelengths, reducing the transmitted starlight and making the planet appear slightly larger at those wavelengths. The planet itself emits negligible light in this measurement. It is emission spectroscopy (observing the secondary eclipse, when the planet passes behind the star) that measures light from the planet directly, revealing temperature structure and dayside composition.

Question 5

The 'radius valley' near 1.5–2 Earth radii is a gap in the distribution of known exoplanet sizes. What does its existence suggest about how the exoplanet population was shaped, rather than reflecting a primordial distribution?

Model answer: If planet sizes followed a smooth primordial distribution, we would expect a roughly continuous population across all sizes. The gap suggests a physical process has removed planets from the transition zone. The leading explanation is atmospheric escape (photoevaporation or core-powered mass loss): planets that formed with modest hydrogen-helium envelopes had those envelopes stripped away by stellar irradiation, collapsing to bare rocky cores below the valley. Planets with thick enough envelopes retained them and remain as sub-Neptunes above the valley. The valley marks the threshold where initial envelope mass determined the final fate.

The radius valley is one of the most important demographic discoveries in exoplanet science. Its sharpness and the way it shifts with orbital period (planets closer to their stars lose envelopes more easily) both support the photoevaporation interpretation. It means the observed distribution of planet sizes is sculpted by atmospheric physics, not just formation — you cannot read off formation conditions directly from the current population.