A Highly Reflective Atmosphere on the Lava World TOI-561b Revealed by JWST/NIRSpec Phase Curve
Abstract
Ultra-short period (USP) rocky planets are expected to be depleted of any substantial gaseous envelope due to the intense irradiation they receive from their host star, making the recent detection of an atmosphere around TOI-561~b particularly surprising. That finding was based on the planet's bulk density and dayside emission spectrum, but full-orbit phase curve observations offer a more comprehensive way to constrain the presence and characteristics of a planet's atmosphere. In this paper, we map TOI-561~b's 3-5~μm emission using JWST/NIRSpec multi-orbit spectroscopic phase curves, explicitly accounting for the curvature of the out-of-eclipse baseline and possible hotspot offsets. From a simple energy balance argument as well as comparing to general circulation models, we find the phase curve observations are best explained with high Bond albedo and moderate global heat transfer. Our 37-hour continuous observation provides a long baseline that allows us to model stellar granulation, the planetary phase curve, and dayside emission simultaneously, effectively disentangling these signals and yielding a dayside emission spectrum that is more robust against stellar variability and consistent with previous eclipse-only fits. Using general circulation model outputs, we constrain where clouds can plausibly form and test candidate compositions, finding that silicate clouds, such as SiO2 and MgSiO3, can form on the dayside near the terminator and explain the observed albedo. Our findings confirm that TOI-561~b appears to have a global reflective atmosphere, suggesting exchange of volatiles with the interior to maintain it.
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BibTeX
@misc{boucher2026highly,
title = {{A Highly Reflective Atmosphere on the Lava World TOI-561b Revealed by JWST/NIRSpec Phase Curve}},
author = {Samuel Boucher and Lisa Dang and Alex McGinty and Johanna K. Teske and Raymond Pierrehumbert and Anjali A. A. Piette and Nicole L. Wallack and Angharad Weeks and Neil T. Lewis and Tim Lichtenberg and Mykhaylo Plotnykov and Emma Postolec and Daniel Huber and Timothy R. Bedding and Harrison Nicholls and Bo Peng and Mark Hammond and Diana Valencia},
howpublished = {arXiv},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2608.21519},
eprint = {2608.21519},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.21519},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - A Highly Reflective Atmosphere on the Lava World TOI-561b Revealed by JWST/NIRSpec Phase Curve
AU - Samuel Boucher
AU - Lisa Dang
AU - Alex McGinty
AU - Johanna K. Teske
AU - Raymond Pierrehumbert
AU - Anjali A. A. Piette
AU - Nicole L. Wallack
AU - Angharad Weeks
AU - Neil T. Lewis
AU - Tim Lichtenberg
AU - Mykhaylo Plotnykov
AU - Emma Postolec
AU - Daniel Huber
AU - Timothy R. Bedding
AU - Harrison Nicholls
AU - Bo Peng
AU - Mark Hammond
AU - Diana Valencia
PY - 2026
DA - 2026/08/28
PB - arXiv
DO - 10.48550/arxiv.2608.21519
UR - https://arxiv.org/abs/2608.21519
AB - Ultra-short period (USP) rocky planets are expected to be depleted of any substantial gaseous envelope due to the intense irradiation they receive from their host star, making the recent detection of an atmosphere around TOI-561~b particularly surprising. That finding was based on the planet's bulk density and dayside emission spectrum, but full-orbit phase curve observations offer a more comprehensive way to constrain the presence and characteristics of a planet's atmosphere. In this paper, we map TOI-561~b's 3-5~μm emission using JWST/NIRSpec multi-orbit spectroscopic phase curves, explicitly accounting for the curvature of the out-of-eclipse baseline and possible hotspot offsets. From a simple energy balance argument as well as comparing to general circulation models, we find the phase curve observations are best explained with high Bond albedo and moderate global heat transfer. Our 37-hour continuous observation provides a long baseline that allows us to model stellar granulation, the planetary phase curve, and dayside emission simultaneously, effectively disentangling these signals and yielding a dayside emission spectrum that is more robust against stellar variability and consistent with previous eclipse-only fits. Using general circulation model outputs, we constrain where clouds can plausibly form and test candidate compositions, finding that silicate clouds, such as SiO_2 and MgSiO_3, can form on the dayside near the terminator and explain the observed albedo. Our findings confirm that TOI-561~b appears to have a global reflective atmosphere, suggesting exchange of volatiles with the interior to maintain it.
ER -