Combined analysis of the 12.8 and 15 microns JWST/MIRI eclipse observations of TRAPPIST-1 b

The first JWST/MIRI photometric observations of TRAPPIST-1 b allowed for the detection of the thermal emission of the planet at 15 microns, suggesting that the planet could be a bare rock with a zero albedo and no redistribution of heat. Here we present five new occultations of TRAPPIST-1 b observed with MIRI in an additional photometric band at 12.8 microns.  In this presentation we present the results from a joint fit of the 10 eclipses and derive a planet-to-star flux ratio of 452 +/- 86 ppm and 775 +/-90 ppm at 12.8 microns and 15 microns, respectively. We show how we tested a large range of models and found that the data can be well fitted by either an airless planet model with an unweathered (fresh) ultramafic surface, that could be indicative of relatively recent geological processes, or, more surprisingly, by a thick pure CO2 atmosphere with photochemical hazes that create a temperature inversion and results in the CO2 feature being seen in emission. 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+c.