An exoplanet is observed transiting its star. Astronomers measure the transit depth at multiple wavelengths and find it is deeper at 1.4 μm (a water vapor absorption band) than at adjacent wavelengths. The correct interpretation is:
AThe planet is physically larger at wavelengths where water absorbs light
BWater vapor in the planet's atmosphere absorbs starlight at 1.4 μm, making the atmosphere optically thicker so the planet appears larger at that wavelength
CThe star emits less light at 1.4 μm, making the planet's shadow more pronounced
DWater on the planet's surface reflects light at 1.4 μm back toward the star, reducing the observed transit depth
The planet's physical size doesn't change. At wavelengths where atmospheric molecules absorb strongly, the atmosphere is optically thicker — its photosphere extends to a higher altitude, blocking more starlight. This makes the transit appear deeper, as if the planet has a larger effective radius at that wavelength. The transmission spectrum is this wavelength-dependent apparent radius, encoding the atmospheric absorption profile. Option A conflates the observable effect with the physical mechanism. Option C describes stellar emission variation, not atmospheric absorption. Option D describes reflection spectroscopy, not transmission geometry.
Question 2 Multiple Choice
Compared to a hot Jupiter (large, hot, low gravity, hydrogen-rich atmosphere), a rocky Earth-sized exoplanet would produce transmission spectral features that are:
ALarger, because a rocky planet's denser atmosphere creates stronger absorption lines per unit altitude
BSimilar in size, because molecular absorption cross-sections are the same regardless of planet size or atmospheric composition
CMuch smaller, because a cold, heavy, high-gravity atmosphere has a tiny scale height, making it compact and its molecular features barely detectable
DAbsent entirely, because rocky planets cannot retain atmospheres
Scale height H = kT/(μg), where T is temperature, μ is mean molecular weight, and g is surface gravity. A small rocky planet likely has lower temperature, heavier atmospheric gases (N₂, CO₂ dominate rather than H₂), and higher surface gravity — all three factors reduce scale height and make the atmosphere compact. Hot Jupiters have puffy atmospheres (high T, low μ, low g) with easily detectable features spanning hundreds of kilometers. Rocky planet features are tiny, pushing current instruments to their limits. Option D is wrong — many rocky planets have atmospheres; detecting their spectral signatures is simply difficult.
Question 3 True / False
A featureless, flat transmission spectrum from an exoplanet could indicate either the complete absence of an atmosphere or the presence of high-altitude clouds that block molecular absorption features from below.
TTrue
FFalse
Answer: True
This ambiguity is one of the principal challenges in transmission spectroscopy. High-altitude aerosol layers create an opaque floor that blocks the view of lower atmospheric layers where molecular absorption occurs, producing a flat spectrum indistinguishable from an airless body. Distinguishing these scenarios requires additional observations: wavelength-dependent slopes from cloud scattering, thermal emission spectroscopy, or wide-wavelength observations that reveal cloud particle properties. Several early super-Earth observations returned flat spectra that were later attributed to clouds rather than absent atmospheres.
Question 4 True / False
Transmission spectroscopy directly images the exoplanet's disk during transit to map where different atmospheric molecules are distributed.
TTrue
FFalse
Answer: False
Transmission spectroscopy never directly images the planet. It measures the wavelength-dependent fraction of starlight blocked during a transit — a single integrated depth measurement at each wavelength. What is measured is the total opacity of the atmospheric limb (the thin ring of atmosphere visible at the planet's edge) at each wavelength. No current telescope can spatially resolve an exoplanet's disk during a transit; the planet is millions of times fainter than its star and angularly unresolvable. The technique is entirely indirect, inferring atmospheric composition from small differences in how much starlight is blocked at different wavelengths.
Question 5 Short Answer
Explain how molecular absorption features appear in a transmission spectrum, and why the grazing geometry of a transit amplifies the signal compared to ordinary laboratory spectroscopy.
Think about your answer, then reveal below.
Model answer: During transit, starlight passes through the atmospheric limb at a grazing angle, traversing an extremely long optical path through the gas. At wavelengths where molecules absorb (e.g., water at 1.4 μm), even trace concentrations remove detectable starlight because the path length is thousands of kilometers. The atmosphere appears optically thicker, causing a deeper transit. At transparent wavelengths, less is absorbed and the transit is shallower. Plotting transit depth versus wavelength produces the transmission spectrum, directly encoding the atmospheric molecular composition.
The path length amplification follows Beer's Law: absorbance scales with concentration × path length. In laboratory spectroscopy, path lengths are centimeters to meters. In transmission spectroscopy, the grazing geometry means the effective path through the atmosphere is orders of magnitude longer — effectively wrapping around the planet's limb. This is why H₂O, CO₂, and CH₄ can be detected despite being minor constituents, and why the technique offers a viable path toward detecting biosignature molecules in thin rocky-planet atmospheres with future large telescopes.