Abstract
Context.The TRAPPIST-1 planetary system is favourable for transmission spectroscopy and offers the unique opportunity to study rocky planets with possibly non-primary envelopes. We present here the transmission spectrum of the seventh planet of the TRAPPIST-1 system, TRAPPIST-1 h (RP= 0.752R⊕,Teq= 173 K) usingHubbleSpace Telescope (HST), Wide Field Camera 3 Grism 141 (WFC3/G141) data.Aims.Our purpose is to reduce the HST observations of the seventh planet of the TRAPPIST-1 system and, by testing a simple atmospheric hypothesis, to put a new constraint on the composition and the nature of the planet.Methods.First we extracted and corrected the raw data to obtain a transmission spectrum in the near-infrared (NIR) band (1.1–1.7 μm). TRAPPIST-1 is a cold M-dwarf and its activity could affect the transmission spectrum. We corrected for stellar modulations using three different stellar contamination models; while some fit the data better, they are statistically not significant and the conclusion remains unchanged concerning the presence or lack thereof of an atmosphere. Finally, using a Bayesian atmospheric retrieval code, we put new constraints on the atmosphere composition of TRAPPIST-1h.Results.According to the retrieval analysis, there is no evidence of molecular absorption in the NIR spectrum. This suggests the presence of a high cloud deck or a layer of photochemical hazes in either a primary atmosphere or a secondary atmosphere dominated by heavy species such as nitrogen. This result could even be the consequence of the lack of an atmosphere as the spectrum is better fitted using a flat line. Variations in the transit depth around 1.3 μm are likely due to remaining scattering noise and the results do not improve while changing the spectral resolution. TRAPPIST-1 h has probably lost its atmosphere or possesses a layer of clouds and hazes blocking the NIR signal. We cannot yet distinguish between a primary cloudy or a secondary clear envelope using HST/WFC3 data; however, in most cases with more than 3σconfidence, we can reject the hypothesis of a clear atmosphere dominated by hydrogen and helium. By testing the forced secondary atmospheric scenario, we find that a CO-rich atmosphere (i.e. with a volume mixing ratio of 0.2) is one of the best fits to the spectrum with a Bayes factor of 1.01, corresponding to a 2.1σdetection.
Highlights
The TRAPPIST-1 planetary system was discovered by Gillon et al (2016) and Gillon et al (2017), using the Transiting Planets and PlanetIsimals Small Telescope (Gillon et al 2011, 2013)
We present the first attempt to characterise the atmosphere of the seventh planet of the system, TRAPPIST-1 h
There is no evidence of molecular absorption in the recovered spectrum of TRAPPIST-1h from the two retrieval results
Summary
The TRAPPIST-1 planetary system was discovered by Gillon et al (2016) and Gillon et al (2017), using the Transiting Planets and PlanetIsimals Small Telescope (Gillon et al 2011, 2013). Important scientific efforts have been carried out to observe, characterise, and model the seven planets orbiting this M8-type star. This is motivated by the fact that the TRAPPIST-1 system offers the most favourable conditions to study rocky planets in the hab-. TRAPPIST-1 is close (39.14 light years), cool (2559 K), and small (0.117 R ), making it favourable for observations (Gillon et al 2017). M-type stars stay for millions of years in the pre-main sequence (PMS) phase, during which planets are exposed to strong non-thermal extreme UV (EUV) and far-UV irradiation, which is expected to lead to atmospheric hydrodynamical escape (Vidal-Madjar et al 2003; Bourrier et al 2017b) and a runaway greenhouse effect (Ramirez & Kaltenegger 2014). Atmospheric erosion might have stripped all planets in the TRAPPIST-1 system of their atmospheres
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