Muted Features in the JWST NIRISS Transmission Spectrum of Hot Neptune LTT 9779b

Abstract

The hot Neptune desert is one of the most sparsely populated regions of the exoplanet parameter space, and atmosphere observations of its few residents can provide insights into how such planets have managed to survive in such an inhospitable environment. Here, we present transmission observations of LTT 9779 b, the only known hot Neptune to have retained a significant H/He-dominated atmosphere, taken with JWST NIRISS/SOSS. The 0.6–2.85 μm transmission spectrum shows evidence for muted spectral features, rejecting a perfectly flat line at >5σ. We explore water- and methane-dominated atmosphere scenarios for LTT 9779 b’s terminator, and retrieval analyses reveal a continuum of potential combinations of metallicity and cloudiness. Through comparisons to previous population synthesis works and our own interior structure modeling, we are able to constrain LTT 9779 b’s atmosphere metallicity to 20–850× solar. Within this range of metallicity, our retrieval analyses prefer solutions with clouds at millibar pressures, regardless of whether the atmosphere is water or methane dominated—though cloud-free atmospheres with metallicities >500× solar cannot be entirely ruled out. By comparing self-consistent atmosphere temperature profiles with cloud condensation curves, we find that silicate clouds can readily condense in the terminator region of LTT 9779 b. Advection of these clouds onto the dayside could explain the high dayside albedo previously inferred for this planet and be part of a feedback loop aiding the survival of LTT 9779 b’s atmosphere in the hot Neptune desert.

Transmission Spectroscopy of the Habitable Zone Exoplanet LHS 1140 b with JWST/NIRISS

“…Each integration was composed of n group = 3, filling approximately 50% of the full well to minimize charge migration and nonlinearity effects (Albert et al 2023). We made use of two independent data reduction pipelines, namely SOSSISSE (Lim et al 2023) and supreme-SPOON (Feinstein et al 2023;Radica et al 2023;Benneke et al 2024;Radica et al 2024), both yielding consistent transmission spectra with a mean (median) absolute deviation of 0.72σ (0.60σ). More details of both pipelines and their comparison are provided in Appendix A.…”

“…For the systematic component, we use GP regression to model two nuisance signals also reported in other NIRISS data sets (e.g., Coulombe et al 2023;Benneke et al 2024;Radica et al 2024): a beat pattern introduced by the observatory thermal control (McElwain et al 2023), and correlated structures on the timescale of a few minutes suggestive of stellar granulation (e.g., Kallinger et al 2014;Grunblatt et al 2017;Pereira et al 2019). We use a combination of two simple harmonic oscillator (SHO) kernels in celerite to jointly model these systematic signals.…”

“…We fit for an amplitude σ beat and a quality factor Q beat for this oscillation. As in Radica et al (2024), we use a critically damped SHO term (Q fixed to 0.5 ) for the granulation-like signal and fit for an amplitude σ gra and a timescale τ gra of this stochastic variation. An extra jitter term σ jitter is added in quadrature to the diagonal of the covariance matrix to account for excess noise.…”