A Thick Volatile Atmosphere on the Ultrahot Super-Earth TOI-561 b

TL;DR

This study presents the first dayside emission spectrum of the ultrahot USP planet TOI-561 b using four JWST/NIRSpec secondary eclipses, testing whether a bare rock or a volatile-rich atmosphere best explains the data. Two independent data-reduction pipelines (Eureka! and ExoTiC JEDI) and both blackbody and self-consistent atmospheric modeling (GENESIS with VapoRock magma-ocean outgassing) show that a 0-albedo bare-rock surface cannot reproduce the observed 3–5 μm emission, while volatile-rich atmospheres — including magma-ocean–driven reservoirs — can. Retrievals yield brightness temperatures around $T_b \sim 1.8\times 10^{3}$ K, well below the bare-rock limit, and the data prefer atmospheres with opacity windows at shorter wavelengths, not a thin rock-vapor layer; CO$_2$ or SiO-only compositions are disfavored. The findings imply that planetary-scale magma oceans can retain substantial volatiles over Gyr timescales, reshaping our understanding of USP formation, atmospheric evolution, and the applicability of the cosmic shoreline to exoplanets.

Abstract

Ultrashort-period (USP) exoplanets -- with $R_p \leq 2~$R$_{\oplus}$ and periods $\leq$1 day -- are expected to be stripped of volatile atmospheres by intense host star irradiation, which is corroborated by their nominal bulk densities and previous eclipse observations consistent with bare rock surfaces. However, a few USP planets appear anomalously under-dense relative to an Earth-like composition, suggesting an exotic interior structure (e.g., core-less) or a volatile-rich secondary atmosphere increasing their apparent radius. Here we present the first dayside emission spectrum of the low-density (4.3$\pm$0.4 g~cm$^{-3}$) USP planet TOI-561 b, which orbits an iron-poor, alpha-rich, $\sim$10 Gyr old thick disk star. Our 3-5 $μ$m JWST/NIRSpec observations demonstrate the dayside of TOI-561 b is inconsistent with a bare-rock surface at high statistical significance, suggesting instead a thick volatile envelope that is cooling the dayside to well below the $\sim$3000 K expected in the bare-rock or thin-atmosphere case. These results reject the popular hypothesis of complete atmospheric desiccation for highly irradiated exoplanets and support predictions that planetary-scale magma oceans can retain substantial reservoirs of volatiles, opening the geophysical study of ultrahot super-Earths through the lenses of their atmospheres.

Figures (8)

• Figure 1: Phase-folded Eureka! (top) and ExoTiC JEDI 2 (bottom) independently normalized white-light curves of secondary eclipses of TOI-561 b. We show the phase-folded eclipses with instrumental noise removed for (a,c) NRS1 (2.8627040 to 3.7143560 microns) and (b,d) NRS2 (3.8199180 to 5.0627574 microns) (see §\ref{['sec:light_curve_fitting']} for details). The black points with associated errors show the 20 minute binned Eureka! or ExoTiC JEDI 2 data and the red lines show the best-fit astrophysical model from the joint fit to the four eclipses. From the NRS1 and NRS2 Eureka! white-light curves, we measure eclipse depths of 25$\pm$4 ppm and 46$\pm$5 ppm for Eureka!, and 53$^{+6}_{-4}$ ppm and 60$^{+7}_{-8}$ ppm for ExoTiC JEDI 2.
• Figure 2: The JWST/NIRSpec emission spectrum of TOI-561 b is inconsistent with a zero-albedo bare-rock surface. The black symbols and error bars show the Eureka! (circles), ExoTiC JEDI 1 (squares), and ExoTiC JEDI 2 (triangles) reductions/fits. There is no offset applied between the NRS1 and NRS2 detectors (gap around 3.75 $\mu$m). The gray dashed line shows the expected emission spectrum for a bare rocky surface assuming zero Bond albedo. The colored lines show model spectra simulated using a 1D self-consistent atmospheric model and different chemical compositions (see the legend). We assume no day-night heat redistribution in the rock vapor case and efficient day-night heat redistribution in the cases without rock vapor. Volatile-rich atmospheric compositions are able to reproduce the observed brightness temperature. The points plotted here are in Table \ref{['tab:eclipse_depths']}.
• Figure 3: TOI-561 b's dayside brightness temperature presents the strongest evidence out of all rocky planets for the presence of an atmosphere. As measured from the $\sim$3-5 $\mu$m G395H emission spectrum, the brightness temperature of TOI-561 b (red star symbols, one for each data reduction/fit) is well below the irradiation temperature expected assuming a zero-albedo, zero-heat-redistribution planet (dashed gray line; see the text). The observed trend in the dayside brightness temperature of the current sample of observed rocky planets suggests that USPs with highly irradiated daysides ($T_{\rm{irr}}>$2500 K) have atmospheres (that may be rock vapor) evaporated from their molten daysides, although note that wavelength ranges are different depending on the observing mode (e.g., MIRI/LRS covers $\sim$6.5-11.5 $\mu$m). The data for this plot are provided in Table \ref{['tab:brightness_temps_lit']} in Appendix \ref{['sec:Appendix_additional_figures']}.
• Figure 4: The "cosmic shoreline", which divides solar system planets into those with and without atmospheres, is not universal across exoplanetary systems. The empirical relation between the estimated cumulative XUV irradiation and escape velocity, $I_{\rm{XUV}} \propto$v$_{\rm{esc}}^{4}$ (solid purple line), is in line with predictions of XUV stellar radiation driving atmospheric escape in planets 2017ApJ...843..122Z; see also Chatterjee2024 and Ji2025 for more analytical predictions. The arbitrary envelope around the cosmic shoreline represents uncertainty based on factors like host star type, atmospheric composition, and initial volatile content. The cumulative XUV irradiation for each planet is estimated based on the simple scaling relation (Eq. 27) in 2017ApJ...843..122Z. Planets marked in colors have measured dayside brightness temperatures from Spitzer or JWST that are consistent with (red) or less than (blue) the maximum dayside brightness temperature (see Figure \ref{['fig:brightness_temp']} and Table \ref{['tab:brightness_temps_lit']} in Appendix \ref{['sec:Appendix_additional_figures']}). The planets are divided into those above (circles) and below (triangles) the expected transition in planet density from rocky to not (just) rocky ($\sim$0.8 in $\rho$/$\rho_e$, the ratio between the density of the planet and the density for the same mass planet given an Earth-like composition; Plotnykov2024). Given the high irradiation and low escape velocity of TOI-561 b (the light blue symbol outlined in dark blue), our JWST/NIRSpec evidence for a thick atmosphere is in strong conflict with the empirical "cosmic shoreline" hypothesis.
• Figure A.1: White-light curves of secondary eclipses of TOI-561 b (a-d). The four individual consecutive secondary eclipse white-light curves with best fit models from the joint fits to the Eureka! eclipses. In each panel, NRS1 (2.8627040 to 3.7143560 microns) is in blue and NRS2 (3.8199180 to 5.0627574 microns) is in red. The unbinned points are shown in the background and the two minute binned points as shown with their errors. The best fit models are shown in black.