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Abstract:

Measuring the atmospheric composition of hazy sub-Neptunes like GJ 1214b through transmission spectroscopy is difficult because of the degeneracy between mean molecular weight (MMW) and haziness. It has been proposed that phase-curve observations can break this degeneracy because of the relationship between MMW and phase-curve amplitude. However, photochemical hazes can strongly affect phase-curve amplitudes as well. We present a large set of general circulation model simulations of the sub-Neptune GJ 1214b that include photochemical hazes with varying atmospheric composition, haze opacity, and haze optical properties. In our simulations, photochemical hazes cause temperature changes of up to 200 K, producing thermal inversions and cooling deeper regions. This results in increased phase-curve amplitudes and adds a considerable scatter to the phase-curve amplitude–metallicity relationship. However, we find that if the haze production rate is high enough to significantly alter the phase curve, the secondary eclipse spectrum will exhibit either emission features or strongly muted absorption features. Thus, the combination of a white-light phase curve and a secondary eclipse spectrum can successfully distinguish between a hazy, lower-MMW and a clear, high-MMW scenario.

Abstract:

The full range of conditions under which rocky planets can host atmospheres remains poorly understood, especially in the regime of close-in orbits around late-type stars. One way to assess the presence of atmospheres on rocky exoplanets is to measure their dayside emission as they are eclipsed by their host stars. Here, we present Spitzer observations of the 4.5 μm secondary eclipses of the rocky super-Earth TOI-431 b, whose mass and radius indicate an Earth-like bulk composition (3.07 ± 0.35 M⊕, 1.28 ± 0.04 R⊕). Exposed to more than 2000 times the irradiation of Earth, dayside temperatures of up to 2400 K are expected if the planet is a dark bare rock without a significant atmosphere. Intriguingly, despite the strong stellar insolation, we measure a secondary-eclipse depth of only 33 ± 22 ppm, which corresponds to a dayside brightness temperature of 1520−390+360 K. This notably low eclipse depth disagrees with the dark bare-rock scenario at the 2.5σ level, and suggests either that the planet is surrounded by an atmosphere or that it is a bare rock with a highly reflective surface. In the atmosphere scenario, the low dayside emission implies the efficient redistribution of heat to the nightside, or by molecular absorption in the 4–5 μm bandpass. In the bare-rock scenario, a surface composition made of a high-albedo mineral species such as ultramafic rock can lead to reduced thermal emission consistent with low eclipse depth measurement. Follow-up spectroscopic observations with the James Webb Space Telescope hold the key to constraining the nature of the planet.

Abstract:

The sub-Neptune GJ 1214 b has an infamously flat transmission spectrum, likely due to thick aerosols in its atmosphere. A recent JWST/MIRI spectroscopic phase curve of GJ 1214 b added to this picture, suggesting a highly reflective and metal-rich atmosphere. Using a 3D general circulation model with both photochemical hazes and condensate clouds, we characterize how different aerosol types affect the atmospheric structure of GJ 1214 b and manifest in its spectroscopic phase curve. Additionally, we reanalyze the original GJ 1214 b JWST phase curve. The reanalysis shows a hotter nightside, similar dayside temperature, and a lower, but still elevated, Bond albedo (0.42 ± 0.11) in comparison to the original results. We find that a scenario with both clouds and hazes is most consistent with the JWST phase curve. Reflective clouds or hazes are needed to explain the large Bond albedo, and hazes or a supersolar metallicity help account for the several hundred kelvin day–night temperature difference measured by the phase curve.