A Thick Atmosphere on the Ultra-Hot Super-Earth TOI-561 b ✛
Context
Ultra-short-period (USP) planets with radii up to two Earth radii and orbital periods under a day were thought to lack atmospheres because of intense stellar irradiation. Most of them have densities consistent with bare rocks, but TOI-561 b stands out with an anomalously low density of 4.3 g/cm³, hinting at an unusual interior or a volatile envelope. The planet has already been the subject of debate: some studies suggested complete atmospheric evaporation, while others proposed volatile retention in a magma ocean. Observations of neighboring USPs, such as 55 Cancri e, gave contradictory results due to eclipse variability. This work provides the first spectrum of the dayside emission of TOI-561 b in the 3–5 µm range to settle the question.
Methods
Four consecutive secondary eclipses of TOI-561 b were observed with the NIRSpec spectrometer on JWST in May 2024. The data were processed with two independent pipelines, Eureka! and ExoTiC JEDI, to control for systematic errors. Eclipse depths were extracted from the light curves in seven spectral bins and compared with theoretical atmospheric models. Interpretation used Bayesian retrievals with a blackbody model and self-consistent 1D radiative-convective equilibrium models from GENESIS, coupled with the magma ocean degassing code VapoRock.
Results
The dayside brightness temperature of TOI-561 b was 1800–2150 K depending on the processing method, about 1000 K cooler than the 2950 K expected for a bare rock. This rules out both an airless surface and a thin rock-vapor atmosphere. Models with volatile compositions, either 100% H₂O or 0.1% H₂O + 99.9% O₂, reproduce the observations well, whereas a CO₂ composition fits worse because of the absorption band at 4.5 µm. The planet's low density (0.68 times Earth's at the same mass) further supports the presence of a volatile envelope.
Implications
This work shows for the first time that an ultra-short-period planet under extreme irradiation can retain a thick volatile atmosphere. Unlike previous studies that either assumed bare rocks or gave ambiguous results for 55 Cancri e, here we have a statistically significant discrepancy from the airless model. This challenges the universality of the empirical 'cosmic shoreline' that separates planets with and without atmospheres. The result supports the idea that magma oceans act as volatile reservoirs, limiting atmospheric loss to space.
Future development
The topic could advance through observations of other ultra-short-period planets around stars of different compositions, to test how iron content affects volatile retention. More accurate models of atmospheric escape for gases heavier than helium will be needed. Studying full phase curves to assess heat transport to the nightside and the role of clouds is also promising.
Impact
The results will affect models of the formation and evolution of rocky exoplanets, as well as strategies for detecting atmospheres around small planets.
Next steps
The next step will be to analyze the full phase curve of TOI-561 b to check variability and heat transport, and to compare with similar planets around stars of different chemical compositions.
Key open problems
The discovery relates to the unresolved problem of atmospheric retention on strongly irradiated rocky planets and the role of magma oceans as volatile reservoirs. It also raises questions about atmospheric escape mechanisms for high-molecular-weight gases, which are still poorly understood.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This work presents the first observations of the dayside of TOI-561 b with JWST/NIRSpec, showing that its brightness temperature is significantly lower than expected for a bare rock, which is interpreted as evidence of a thick volatile-rich atmosphere sustained by a magma ocean.
Where its strength is
The strength of this work lies in the direct spectroscopic evidence of an atmosphere on an ultra-short-period planet, obtained through two independent reduction pipelines, yielding a statistically significant discrepancy from atmosphere-free models.
Where it could go next
- It might be interesting to compare the obtained constraints on atmospheric composition with results for other hot rocky planets, for example, with the study of a lava world where the interaction between magma ocean and atmosphere is also considered.based on: A Lava World with a Cool Face arXiv:2604.11911
- Closely related is the work on the thermal memory of a planet around a dead star, which discusses the evolution of the thermal state of rocky planets; perhaps comparing their approaches could help refine the long-term stability of TOI-561 b's atmosphere.based on: The Heat Memory of a Planet Orbiting a Dead Star arXiv:2607.01316
- The archive contains a study of hydrogen atmospheres on scorching planets sustained by volcanic degassing; their degassing model could potentially be applied to estimate the contribution of volcanism to TOI-561 b's atmosphere.based on: Hydrogen Atmospheres on Scorching Planets: The Volcano Secret arXiv:2510.07378
Worth trying
- Try applying the magma ocean degassing model from the study on hydrogen atmospheres to estimate the equilibrium atmospheric composition of TOI-561 b.
- Compare predictions for thermal evolution with calculations for planets around white dwarfs to test how quickly the interior cools and how this affects the atmosphere.
- Use data on lava worlds to calibrate emission models for magma-ocean-covered surfaces in the context of TOI-561 b.
Why this may matter
This discovery could serve as a starting point for the systematic study of atmospheres on ultra-short-period super-Earths, helping to refine the boundaries of the 'cosmic shoreline' and to understand the role of magma oceans in retaining volatiles.
Nearby works in our archive
- A Lava World with a Cool Facesimilarity 0.60
- A Warm World Around a Quiet Starsimilarity 0.60
- Unexpected Atmosphere Composition of a Warm Mini-Neptunesimilarity 0.57
- A Planet Where Day Melts Hydrogen and Night Hides Cloudssimilarity 0.57
- What a Giant Planet Was Baked Fromsimilarity 0.55
- The Heat Memory of a Planet Orbiting a Dead Starsimilarity 0.55
- Hydrogen Atmospheres on Scorching Planets: The Volcano Secretsimilarity 0.55
The archive is sparse around this work: the closest review is only 0.60, against a usual 0.64. It looks like few have drilled nearby.
TOI-561 b orbits a star that is about twice as old as the Sun, roughly 10 billion years old. Yet the planet is so close to its star that its year lasts only 10.7 hours, and the dayside is hot enough to melt many minerals.
Key numbers
- Dayside brightness temperature: 1800–2150 K
- Expected bare-rock temperature: 2950 K
- Planet density: 4.3 g/cm³
- Orbital period: 0.4466 days
- Parent star age: ~10 billion years