Astronomers led by University of Chicago graduate student Brandon Park Coy report evidence of an atmosphere on the rocky super -Earth HD 3167 b, located about 154 light- years away in the constellation Pisces. It is the coolest “lava world” found so far with such evidence.

Key details

Planet properties: HD 3167 b is a rocky super- Earth that orbits its K- type host star in just one Earth Day. Its star- facing side is expected to be molten rock (hence “lava world”).

Why it’s notable: Close- in rocky planets are bombarded by stellar wind and high- energy radiation, making atmospheres hard to retain. Yet several ultra- lava worlds appear to keep them. This one is cooler than the previous examples, helping map the temperature range where atmospheres can persist.

Detection method: The team used the James Webb Space Telescope (JWST) to observe a secondary eclipse (when the planet passes behind its star). By measuring the mid- infrared light from the planet’s dayside, they found it is cooler than the theoretical maximum for a bare- rock surface. An atmosphere can redistribute heat to the night side or host reflective clouds, both of which lower the observed dayside temperature.

Possible atmosphere composition: The surface is likely silicate- rich (similar to Earth’s mantle). Earlier expectations pointed to vaporized- rock atmospheres, but data are beginning to suggest possible heavier gases such as CO₂, CO, or water. The exact makeup of HD 3167 b’s atmosphere remains uncertain and is a target for future observations.

Broader significance: Studying these extreme worlds provides a window into the early Solar System. Earth itself is thought to have gone through a magma- ocean stage shortly after formation, when planetesimal collisions left it extremely hot with a fully molten surface. Lava worlds therefore offer analogs for the conditions that shaped terrestrial planets in their first few million years.

The closer a rocky planet orbits its star, the more intense the environment becomes for any atmosphere:

Stellar wind (a stream of charged particles) is denser and stronger at smaller orbital distances. It can directly strip atmospheric gas through interactions with the planet’s magnetic field (or lack thereof) and upper atmosphere.

High- energy radiation (extreme ultraviolet and X-rays) increases dramatically closer in. This radiation heats the upper atmosphere, causing it to expand and making it easier for molecules to reach escape velocity (photoevaporation or hydrodynamic escape).

The planet also receives far more total energy, which raises surface temperatures and can drive stronger atmospheric circulation or loss processes.

For ultra- short- period (USP) planets like HD 3167 b (orbital period ~0.96 days), these effects are extreme. Theory (including the “cosmic shoreline” concept) predicts that such worlds should struggle to retain significant atmospheres over long timescales.

Yet multiple lava worlds, including HD 3167 b, the coolest one found so far with atmospheric evidence, show dayside temperatures cooler than expected for bare rock. This is interpreted as a signature of an atmosphere that either redistributes heat to the nightside or reflects incoming starlight (high albedo). Possible reasons these atmospheres persist include:

• Continuous replenishment from a molten surface (vaporized rock or outgassed volatiles)
• High mean- molecular- weight gases that are harder to strip
• Reflective clouds or hazes that reduce heating

Proximity should make atmospheres harder to keep, which is precisely why the growing sample of atmosphere- bearing lava worlds is scientifically interesting.

The result comes from a JWST program (led by Megan Weiner Mansfield) surveying about 10 ultra- hot lava worlds to search for a critical temperature transition between planets with and without atmospheres. The paper is published in The Astrophysical Journal Letters.

In short, HD 3167 b expands the sample of atmosphere- bearing rocky worlds into a slightly cooler regime and strengthens the case that many lava planets retain atmospheres, while also shedding light on the possible early history of Earth- like planets.

Evidence for an Atmosphere on the Ultra-short-period Super-Earth HD 3167 b

This paper presents JWST/MIRI Low Resolution Spectrometer (LRS) secondary- eclipse observations of the ultra- short- period (USP) lava- world super- Earth HD 3167 b. The results provide strong evidence that the planet has a thick and/or highly reflective atmosphere, making it the least irradiated USP super- Earth with such evidence to date.

Planet properties (refined in this work)

• Radius ≈ 1.60 $R_\oplus$
• Mass ≈ 4.84 $M_\oplus$
• Orbital period ≈ 0.96 days
• Equilibrium temperature $T_\mathrm{eq}$ ≈ 1786 K
• Irradiation temperature $T_\mathrm{irr}$ ∼ 2500 K (substellar temperature of a perfect blackbody)
• Bulk density lower than expected for a pure Earth- like rocky composition (compatible with either a reduced iron core or a significant atmosphere)

Key observational result

A single secondary eclipse was observed on 25 June 2025 as part of JWST GO Program 4818 (“LAVA LAMPS”).

• Measured white-light (≈5-11 µm) eclipse depth: 38 ± 11 ppm
• This is more than 5σ lower than the depth expected for a dark, maximally hot bare-rock surface (like the Moon or Mercury).

The corresponding dayside brightness temperature is significantly cooler than the theoretical maximum. This is best explained by an atmosphere that:

• Reflects a substantial fraction of incoming starlight (high Bond/effective albedo), and/or
• Efficiently redistributes heat to the nightside.

Even assuming perfect heat redistribution, a high effective albedo ($A_\mathrm{eff} \approx 0.72$, comparable to Venus) is still required. The data are inconsistent with a thin Mars- like atmosphere at ∼4σ and most consistent with a thick, highly reflective atmosphere.

The dayside emission spectrum itself is not precise enough to constrain atmospheric composition (silicate vapor vs. heavier gases such as CO₂, CO, or H₂O). Follow- up spectroscopy with JWST/NIRSpec is recommended.

Broader context

Lava worlds (rocky planets hot enough that their daysides are molten) show an apparent dichotomy: the hottest ones often have cooler- than- expected daysides (suggesting atmospheres), while cooler ones appear as bare rock. HD 3167 b helps pin down the possible transition region to roughly
2000 K ≲$T_\mathrm{irr,transition}$≲ 2500 K.

These findings challenge simple expectations from the “cosmic shoreline” (extreme irradiation should strip atmospheres) and provide an analog for the magma- ocean stage of early Earth.

The team also refined system parameters for the multi- planet HD 3167 system using the new eclipse data combined with existing photometry and radial velocities.

Journal information: Astrophysical Journal Letters

DOI: 10.3847/2041-8213/ae7f23 arXiv: 2604.11911 (open access)

Provided: University of Chicago 

Lead author: Brandon Park Coy (University of Chicago). Co- authors include researchers from UChicago, University of Maryland, Harvard & Smithsonian, Carnegie, and others.

Abstract

“Lava worlds”, Earth-sized planets hot enough (T_eq ≳ 1100 K) to melt their dayside silicate surfaces, have emerged as promising candidates for atmospheric detection and characterization. Thermal emission observations show an apparent dichotomy: The hottest lava worlds have colder daysides than the temperature of a maximally emitting bare rock, indicating the likely presence of thick and/or reflective atmospheres, while the coldest ones do not. However, where in instellation flux this potential bifurcation occurs is uncertain. We present a JWST/MIRI Low Resolution Spectrometer eclipse of the ultra-short-period (USP) lava world HD 3167 b (T_eq  = 1786 K, R = 1.6 R_⊕, P = 0.96 day) that helps bridge this gap. We measure the white-light eclipse depth to be 38 ± 11 ppm, more than 5σ lower than the expected eclipse depth of a dark, maximally hot bare rock. We use this to derive a dayside brightness temperature that is best explained by the presence of an atmosphere that cools the dayside by reflecting incoming starlight and/or efficiently redistributing heat to the planet’s nightside. An atmosphere is further compatible with the planet’s slight underdensity compared to an Earth-like composition. The corresponding dayside emission spectrum is not precise enough to constrain atmospheric composition, motivating follow-up spectroscopic observations with JWST/NIRSpec. Lastly, we use our observation and existing data to refine key planetary parameters of the HD 3167 system. HD 3167 b is currently the least irradiated USP super-Earth with evidence for an atmosphere.