High-Resolution Transmission Spectroscopy of the Terrestrial Exoplanet GJ 486b: Implications for JWS

Table of LinksTable of Links

• Abstract & Introduction
• Observation
• Data Reduction and Processing
• Model Spectra
• Cross-Correlation Method
• Results
• Clouds and Hazes
• Implications for JWST
• Conclusion and Acknowledgement
• Appendix & References

8. IMPLICATIONS FOR JWST

GJ 486b will be observed by JWST in both transit and eclipse. Two transits of GJ 486b are scheduled to be observed with JWST’s Near-InfraRed Spectrograph (NIRSpec), with the G395H grism in Bright Object Time Series (BOTS) mode, as part of a larger program to search for atmospheres on planets transiting M-dwarf

stars (GO 1981[12], P.I.: K. Stevenson). Two eclipses with the Mid-Infrared Instrument (MIRI) will further probe GJ 486b’s dayside atmosphere (GO 1743[13], P. I.: M. Mansfield).

Our constraints on GJ 486b’s atmospheric composition (Section 6.2) can inform these future JWST observations. Caballero et al. (2022) show that a H2/Hedominated atmosphere can be readily detected by JWST with two transits. However, we can confidently rule out to 5σ a clear H2/He-dominated atmosphere with H2O, CH4, NH3, HCN, CO2, and CO at solar abundances.

We further investigated the prospects for JWST to detect GJ 486b’s atmosphere via simulated observations.

We used PandExo (Batalha et al. 2017) to simulate two transits of GJ 486b with NIRSpec G395H. To match the scheduled observations, we used an observation duration of 5.14 hr, the R = 2700 f290lp grism, and the S1600A1 SUB2048 subarray option. We set the number of groups per integration to the ‘optimized’ default, the saturation limit to 80% of the full well capacity, and assumed no noise floor.

We considered eight cloud-free model atmospheres compatible with our high-resolution observations. We focused on H2O and CO2 as they are the main expected absorbers over the NIRSpec G395H spectral range. The models also contained background H2. Four of the models vary the H2O abundance and atmospheric mean molecular weight while CO2 is fixed at a solar abundance (VMR ,CO2 ≈ 5×10−9 ). The other four models vary the CO2 abundance and the atmospheric mean molecular weight while H2O is fixed to a solar abundance (VMR ,H2O ≈ 5×10−4 ). The models with variable H2O have log10(VMRH2O) = −7, −5, −4 and −2, and MMWs of 2, 5, 10, and 18, respectively. The models with variable CO2 have log10(VMRCO2) = −7, −5, −4 and −1, and MMWs of 2, 3, 7, and 7.5, respectively. These values of VMR and MMW were chosen because they lie just outside the region of VMR-MMW parameter space ruled out by our high-resolution observations in Fig 11. The simulated JWST observations for these models are shown in Fig. 14. By comparing the simulated observation error bars to the size of the features in the model spectra, it is apparent that the scheduled transit observations could readily distinguish all considered models with variable CO2 abundance and a solar H2O abundance (lower four panels in Fig. 14). However, these observations may not be able to distinguish our considered models with variable H2O abundance and a solar CO2 abundance (upper four panels in Fig. 14). This indicates the complementary capabilities of high-resolution ground-based and JWST observations, because the former has greater sensitivity to H2O while the latter has greater sensitivity to CO2.

We note that our atmospheric models are unable to make predictions that could inform the eclipse observations — since they have isothermal pressuretemperature profiles, which produce featureless emission spectra. Future investigations that also consider a range of possible pressure-temperature profiles may provide useful insights into GJ 486b’s emission spectrum.

ive than JWST to oxygen in a terrestrial exoplanet’s atmosphere (Snellen et al. 2013). In this regime of high signal-to-noise ratios for terrestrial exoplanets, high-resolution spectroscopy’s unique capabilities will allow detections of Doppler shifts (which can indicate atmospheric winds; Snellen et al. 2010), line broadening (which can indicate rotation; Snellen et al. 2014) and potentially molecular isotopologues (Molli`ere & Snellen 2019).

[12] https://www.stsci.edu/jwst/science-execution/ program-information.html?id=1981

[13] https://www.stsci.edu/jwst/science-execution/ program-information.html?id=1743