Observing the Atmospheres of Known Temperate Earth-sized Planets with JWST (1708.04239v2)

Abstract: Nine transiting Earth-sized planets have recently been discovered around nearby late M dwarfs, including the TRAPPIST-1 planets and two planets discovered by the MEarth survey, GJ 1132b and LHS 1140b. These planets are the smallest known planets that may have atmospheres amenable to detection with JWST. We present model thermal emission and transmission spectra for each planet, varying composition and surface pressure of the atmosphere. We base elemental compositions on those of Earth, Titan, and Venus and calculate the molecular compositions assuming chemical equilibrium, which can strongly depend on temperature. Both thermal emission and transmission spectra are sensitive to the atmospheric composition; thermal emission spectra are sensitive to surface pressure and temperature. We predict the observability of each planet's atmosphere with JWST. GJ 1132b and TRAPPIST-1b are excellent targets for emission spectroscopy with JWST/MIRI, requiring fewer than 10 eclipse observations. Emission photometry for TRAPPIST-1c requires 5-15 eclipses; LHS 1140b and TRAPPIST-1d, TRAPPIST-1e, and TRAPPIST-1f, which could possibly have surface liquid water, may be accessible with photometry. Seven of the nine planets are strong candidates for transmission spectroscopy measurements with JWST, though the number of transits required depends strongly on the planets' actual masses. Using the measured masses, fewer than 20 transits are required for a 5 sigma detection of spectral features for GJ 1132b and six of the TRAPPIST-1 planets. Dedicated campaigns to measure the atmospheres of these nine planets will allow us, for the first time, to probe formation and evolution processes of terrestrial planetary atmospheres beyond our solar system.

• The paper demonstrates that JWST can detect key atmospheric features in Earth-sized exoplanets using emission and transmission spectroscopy with fewer than 10 events for optimal candidates.
• The methodology employs models based on Earth, Titan, and Venus compositions to simulate spectral emissions under varying surface pressures, revealing distinct atmospheric signatures.
• The study finds that accurate mass measurements are crucial, as uncertainties can significantly increase the observing time required for robust atmospheric detections.

Observing the Atmospheres of Known Temperate Earth-sized Planets with JWST

The paper "Observing the Atmospheres of Known Temperate Earth-sized Planets with JWST" by Morley et al. explores the feasibility of characterizing the atmospheres of nine Earth-sized exoplanets orbiting nearby late M dwarfs. This paper takes advantage of the capabilities of the James Webb Space Telescope (JWST), with a particular focus on planets within the TRAPPIST-1 system and those discovered by the MEarth survey, namely GJ 1132b and LHS 1140b. The primary objective is to predict the observability of atmospheric features using model thermal emission and transmission spectra as a function of varying atmospheric composition and surface pressure.

Methodology

The paper employed a comprehensive set of models to simulate the thermal emission and transmission spectra of these exoplanets. The elemental compositions of the model atmospheres are based on known planetary bodies within our solar system: Earth, Titan, and Venus. These elemental baselines allow the authors to infer chemical compositions under the assumption of chemical equilibrium, which significantly hinges on temperature. The models examined a wide array of surface pressure conditions ranging from tenuous to thick atmospheres.

For each planetary object, the authors provided predictions regarding the observability of atmospheric features using JWST. They focused on both thermal emission spectrums, where MIRI (Mid-Infrared Instrument) is crucial, and transmission spectra utilizing NIRSpec and NIRISS instruments. The analysis includes calculating the number of transit and eclipse events required for a significant detection of atmospheric components at a 5$\sigma$ confidence level.

Key Findings

1. Target Planets: The exoplanets GJ 1132b and TRAPPIST-1b emerged as excellent candidates for emission spectroscopy with JWST requiring fewer than ten eclipse observations for a significant detection. Other planets like TRAPPIST-1c require more observations, especially for comprehensive emission photometry.
2. Surface and Atmospheric Modeling: The models anticipate a considerable variance in atmospheric compositions, implying that diverse formation and evolutionary pathways likely influence these planets' atmospheres. Venus-like compositions result in atmospheres dominated by CO$_2$, whereas Earth-like environments would primarily comprise N$_2$ and O$_2$.
3. Surface Pressure Insights: For surface pressures exceeding 1 bar, the thermal emissions showed distinct flux characteristics, sharply contrasting with thin atmospheres that exhibit nearly blackbody-like emissions due to the dominance of surface radiation.
4. Transmission Spectra: The sensitivity of the transmission spectra to both atmospheric pressure and mass of the planets was pronounced. A robust detection of spectral features was facilitated for select planets assuming central TTV (Transit Timing Variation) mass values with fewer than 20 transits required. However, inaccuracies in mass measurements could significantly adjust the observational demands.
5. Implication for Future Studies: The propensity for mass loss under high stellar X-ray/UV flux conditions from M dwarfs like TRAPPIST-1 was considered, which could implicate secondary outgassing forming the observed atmospheres.

Conclusion

Morley et al. conclude that JWST presents a viable opportunity to probe the atmospheres of newly discovered terrestrial exoplanets, ushering in a meaningful extension in the breadth of terrestrial atmospheres within the landscape of exoplanet science. These observational campaigns will not only illuminate the variety of planetary atmospheres that exist but also augment our understanding of atmospheric evolution mechanisms in planetary systems beyond our own.