A map of the day-night contrast of the extrasolar planet HD 189733b

Abstract: "Hot Jupiter" extrasolar planets are expected to be tidally locked because they are close (<0.05 astronomical units, where 1 AU is the average Sun-Earth distance) to their parent stars, resulting in permanent daysides and nightsides. By observing systems where the planet and star periodically eclipse each other, several groups have been able to estimate the temperatures of the daysides of these planets. A key question is whether the atmosphere is able to transport the energy incident upon the dayside to the nightside, which will determine the temperature at different points on the planet's surface. Here we report observations of HD 189733, the closest of these eclipsing planetary systems, over half an orbital period, from which we can construct a 'map' of the distribution of temperatures. We detected the increase in brightness as the dayside of the planet rotated into view. We estimate a minimum brightness temperature of 973 +/- 33 K and a maximum brightness temperature of 1212 +/- 11 K at a wavelength of 8 microns, indicating that energy from the irradiated dayside is efficiently redistributed throughout the atmosphere, in contrast to a recent claim for another hot Jupiter. Our data indicate that the peak hemisphere-integrated brightness occurs 16$\pm$6 degrees before opposition, corresponding to a hot spot shifted east of the substellar point. The secondary eclipse (when the planet moves behind the star) occurs 120 +/- 24 s later than predicted, which may indicate a slightly eccentric orbit.

• The paper determines a temperature range from 973 ± 33 K to 1212 ± 11 K, evidencing efficient atmospheric heat redistribution.
• The paper identifies an eastward-shifted hotspot occurring 16 ± 6° before opposition, highlighting significant atmospheric wind effects.
• The paper observes a secondary eclipse delay of 120 ± 24 seconds, suggesting a marginal eccentricity contrary to expected tidal circularization.

A Map of the Day-Night Contrast of the Extrasolar Planet HD 189733b

The study, "A map of the day-night contrast of the extrasolar planet HD 189733b," presents a detailed analysis of the temperature distribution on the tidally locked hot Jupiter HD 189733b. Utilizing data from the Spitzer Space Telescope, the authors aimed to understand how efficiently the planet's atmosphere redistributes heat from its irradiated dayside to its nightside. This paper offers precise measurements and interpretation of the observed thermal emission variations as the planet orbits its host star.

Key Findings

HD 189733b was monitored over a 33.1-hour period using the 8 µm channel of the InfraRed Array Camera (IRAC) on Spitzer. The study spans slightly more than half of the planet's orbit, stretching from 2.6 hours before the transit to 1.9 hours after the secondary eclipse. Approximately 278,528 images were generated, resulting in noteworthy data on the planet’s flux variations.

1. Temperature Distribution: The brightness temperature was determined to range from a minimum of 973 ± 33 K to a maximum of 1212 ± 11 K. These measurements imply a significant redistribution of heat across the planet’s surface, counter to previous claims suggesting atmospheric inefficiency in heat distribution. Notably, the peak brightness occurs 16 ± 6 degrees before opposition, indicative of an eastward-shifted hotspot relative to the substellar point.
2. Secondary Eclipse Timing: The observed secondary eclipse occurs 120 ± 24 seconds later than anticipated, suggesting a possibly eccentric orbit. The eccentricity is constrained to e cos(ω) = 0.0010 ± 0.0002, implying a very minor deviation from a circular orbit despite expectations for rapid circularization due to tidal forces.
3. Atmosphere and Circulation Models: The study's results align with atmospheric models suggesting radiative and advection time scales are similar, promoting efficient heat distribution. This contradicted large temperature contrasts proposed by models with much shorter radiative or longer advection time scales. The existence of a notable shift in peak temperatures also underscores the role of atmospheric winds in advecting heat.

Implications

The insights gained from HD 189733b contribute substantially to our understanding of atmospheric dynamics on hot Jupiters. The efficient heat distribution suggests complex atmospheric circulation patterns possibly involving jet streams or large-scale circulation cells. For exoplanetary science, this understanding aids in refining theoretical models, especially concerning atmospheric opacity sources like H₂O and CH₄ as well as advective processes.

Future Directions

Future studies could refine these observations by utilizing longer observation periods or additional wavelengths, potentially revealing more about vertical atmospheric structures or chemical compositions. Improved models that incorporate multi-dimensional dynamics and account for various atmospheric constituents will lead to more nuanced predictions of thermal emissions and dynamics. Additionally, understanding the specific factors contributing to the observed orbital eccentricity can illuminate processes affecting tidal circularization in exoplanetary systems.

In conclusion, this research provides detailed observations of the thermal properties of HD 189733b, which serve as a valuable benchmark for atmospheric models of hot Jupiters. These findings also broader the quest for understanding exoplanetary atmospheres, particularly those with extreme irradiation conditions.