Combined analysis of the 12.8 and 15 $\mu m$ JWST/MIRI eclipse observations of TRAPPIST-1 b

TL;DR

This study analyzes JWST/MIRI eclipse data of TRAPPIST-1 b at 12.8 and 15 micrometers, exploring whether the planet is a bare rock or has a thick CO2 atmosphere, revealing two plausible scenarios.

Contribution

It combines new multi-band JWST/MIRI eclipse observations with a global analysis to constrain TRAPPIST-1 b's surface and atmospheric properties, highlighting the complexity of interpretation.

Findings

Detection of thermal emission at 15 μm and 12.8 μm.

Two main models fit the data: a bare rock or a thick CO2 atmosphere.

Challenges in distinguishing surface vs. atmospheric features from broadband data.

Abstract

The first JWST/MIRI photometric observations of TRAPPIST-1 b allowed for the detection of the thermal emission of the planet at 15 $\mu m$, suggesting that the planet could be a bare rock with a zero albedo and no redistribution of heat. These observations at 15 $\mu m$ were acquired as part of GTO time that included a twin program at 12.8 $\mu m$ in order to have a measurement in and outside the CO$_2$ absorption band. Here we present five new occultations of TRAPPIST-1 b observed with MIRI in an additional photometric band at 12.8 $\mu m$. We perform a global fit of the 10 eclipses and derive a planet-to-star flux ratio and 1-$\sigma$ error of 452 $\pm$ 86 ppm and 775 $\pm$ 90 ppm at 12.8 $\mu m$ and 15 $\mu m$, respectively. We find that two main scenarios emerge. An airless planet model with an unweathered (fresh) ultramafic surface, that could be indicative of relatively recent geological processes fits well the data. Alternatively, a thick, pure-CO2 atmosphere with photochemical hazes that create a temperature inversion and result in the CO2 feature being seen in emission also works, although with some caveats. Our results highlight the challenges in accurately determining a planet's atmospheric or surface nature solely from broadband filter measurements of its emission, but also point towards two very interesting scenarios that will be further investigated with the forthcoming phase curve of TRAPPIST-1 b.