A tidally locked hot Jupiter orbits very close to its star. Astronomers measure the planet's infrared brightness as it orbits — a 'phase curve.' They find the hottest region of the atmosphere is shifted slightly to the east of the substellar point (the point directly facing the star). What best explains this offset?
AStellar tidal forces pull heat eastward along the orbit
BAn equatorial superrotating jet advects heat downwind of the substellar point
CThe nightside hemisphere absorbs more starlight due to the planet's reflectivity
DThe substellar point cools faster because it faces the star directly, radiating more efficiently
The equatorial superrotating jet — an eastward wind band driven by Coriolis deflection of day-to-night pressure-gradient winds — advects the thermal maximum downwind. The substellar point is still the hottest in terms of incoming stellar flux, but the atmospheric dynamics redistribute heat eastward. This hot-spot offset is a direct observational signature of atmospheric dynamics and has been measured in multiple hot Jupiter phase curves.
Question 2 Multiple Choice
A tidally locked rocky exoplanet has a very thin atmosphere. Compared to a similar planet with a thick atmosphere, what does the thin atmosphere predict about day-night temperature contrast?
ASmaller contrast — thin atmospheres radiate heat more efficiently to space
BSmaller contrast — less mass means faster Coriolis redistribution
CLarger contrast — the thin atmosphere cannot efficiently transport heat from day to night
DThe same contrast — day-night temperature difference depends only on stellar flux
Atmospheric heat redistribution depends on the mass and thermal capacity of the atmosphere. A thin atmosphere carries very little energy per unit volume, so even fast winds cannot move much heat from the hot dayside to the cold nightside. In the extreme limit, a very thin atmosphere may partly condense on the nightside. A thick atmosphere circulates far more energy, moderating the temperature contrast. This threshold between 'too thin to redistribute' and 'thick enough' is critical for habitability assessments.
Question 3 True / False
On a tidally locked hot Jupiter, the hottest atmospheric region is centered directly on the substellar point.
TTrue
FFalse
Answer: False
This is the key observational finding from hot Jupiter phase curves: the thermal maximum is shifted eastward of the substellar point, not centered on it. Coriolis forces deflect the massive day-to-night wind circulation into a superrotating equatorial jet that advects the hot region downwind. Direct measurements of the phase curve flux asymmetry confirm this eastward offset for multiple hot Jupiters.
Question 4 True / False
Phase curve measurements of exoplanets can provide evidence for or against the existence of equatorial superrotating jets.
TTrue
FFalse
Answer: True
Phase curves map how the integrated thermal emission from the planet changes as different hemispheres come into view during the orbit. If the thermal hotspot is shifted east of the substellar point, the planet's brightness peaks before secondary eclipse (when the dayside faces us), providing direct evidence for atmospheric heat redistribution by a superrotating jet. This makes phase curves one of the primary observational windows into exoplanet atmospheric dynamics.
Question 5 Short Answer
Why is atmospheric thickness a critical parameter for determining whether a tidally locked rocky planet could maintain habitable surface conditions?
Think about your answer, then reveal below.
Model answer: A thin atmosphere transports very little heat from the hot dayside to the cold nightside, leading to extreme temperature contrasts — the dayside may be scorching while the nightside freezes out. A thick atmosphere can efficiently circulate heat via winds and convection, moderating the temperature contrast across the planet. The threshold between these regimes determines whether liquid water could exist on the surface and where.
The key insight is that habitability for a tidally locked planet is not simply about receiving the right amount of stellar flux — it is about whether the atmosphere can redistribute that heat. A planet in the 'habitable zone' with a thin atmosphere might still be uninhabitable because of extreme day-night contrasts. General circulation models explore this threshold by varying atmospheric mass and composition, making thickness one of the most important parameters in tidally locked planet habitability studies.