Astronomy
The Thermal Emission and Phase Curves of Hot Jupiters
Quick fact
Hot Jupiters, like HD 189733b, can show temperature differences of over 1,000 °C between their permanent day and night sides, yet some manage to distribute heat surprisingly efficiently. A single observation of a planet's light as it orbits—its phase curve—can reveal this temperature map.
Why this is interesting
We all know the Moon has a dark side, but imagine an entire planet where one side is always scorching and the other always freezing. How would we ever know what that looks like from light-years away?
Read the full explanation
Understanding The Thermal Emission and Phase Curves of Hot Jupiters
Hot Jupiters are giant planets that orbit their stars extremely closely, completing a year in only a few days. Because they are so close, gravity has likely locked them into a synchronous rotation, meaning one hemisphere always faces the star (the dayside) and the other always faces away (the nightside), much like the Moon always shows us the same face. The dayside is heated intensely by the star, reaching temperatures over 1,000 °C, while the nightside would naturally cool down. But the planet is not a static rock—its atmosphere can move and transport heat. When we observe such a planet from Earth, we don't see it directly as a point of light next to the star. Instead, we can capture the planet's combined light (both reflected starlight and its own thermal emission) as it orbits. As the planet moves around the star, we see different phases: more of the dayside, then more of the nightside. If we plot the total infrared brightness of the system over time, we get a phase curve—a smooth rise and fall that reflects how much hot dayside is visible. This curve tells us not just that the planet is hot, but also how the heat is distributed across its surface and how well the atmosphere moves heat from day to night.
A deeper explanation
The brightness of a hot Jupiter in the infrared is dominated by its own thermal emission, rather than reflected starlight, because it is so hot. The amount of thermal emission we receive depends on the temperature of the hemisphere facing us and its projected area. As the planet orbits, the visible hemisphere changes, and so does the observed brightness. By carefully removing the starlight, astronomers can isolate the planet's emission and construct a phase curve. The shape of this phase curve is a direct probe of the planet's temperature structure: a phase curve with a large peak when the dayside is visible and a deep minimum when the nightside faces us indicates poor heat redistribution, meaning the nightside is very cold. Conversely, a more sinusoidal phase curve with a smaller amplitude suggests that heat is efficiently transported to the nightside, keeping it relatively warm. The mechanism behind this heat transport is atmospheric circulation. Because the dayside is intensely heated, air rises and flows toward the nightside, creating strong eastward jets due to the planet's rotation. These jet streams can carry heat across the planet, effectively redistributing energy. The efficiency of this redistribution depends on the atmospheric composition, pressure, and the presence of absorbers like clouds. For example, some hot Jupiters show an infrared hotspot offset eastward from the substellar point, indicating that winds are advecting heat before it can be re-emitted to space. Phase curve observations also allow us to measure the planet's brightness temperature at different phases, which we can then compare to models of atmospheric circulation. This comparison tests our understanding of atmospheric physics under extreme conditions, and it also helps us measure the planet's albedo and the day-night temperature contrast. Ultimately, phase curves are a powerful tool because they turn a point of light into a dynamic, weather-hosting world.