---
title: 'Hot Rocks Survey: MIRI 15µm Exoplanet Atmospheres'
url: https://www.emergentmind.com/topics/hot-rocks-survey
type: topic
---

# Hot Rocks Survey: MIRI 15µm Exoplanet Atmospheres

Hot Rocks Survey is a **JWST General Observer program (GO 3730)** that uses **MIRI secondary-eclipse photometry at \(15\,\mu\mathrm{m}\)** to test whether **nine rocky exoplanets orbiting M dwarfs** retain **secondary atmospheres** or instead are consistent with **low-albedo bare rocks** [2410.11048, 2508.14210]. The survey is motivated by the observational uncertainty of terrestrial atmospheres around M dwarfs: transmission studies of rocky planets have often yielded flat or ambiguous spectra, with degeneracies among high-mean-molecular-weight atmospheres, clouds, or no atmosphere at all, and M-dwarf stellar heterogeneity can further confuse interpretation. The survey therefore emphasizes thermal emission at \(15\,\mu\mathrm{m}\), where an atmosphere should generally reduce the observed eclipse depth by redistributing heat to the nightside, and where CO\(_2\) has strong opacity [2508.14210].

## 1. Scientific rationale and survey design

The Hot Rocks Survey is framed around a specific atmospheric-evolution problem: close-in rocky planets around M dwarfs are expected to lose primordial H/He envelopes, but the survivability of heavier secondary atmospheres remains uncertain [2503.19772, 2410.11048]. M dwarfs are simultaneously attractive and difficult hosts for this test. They are small, cool, and common, so the relative planet-to-star signal is enhanced, but they also have long and active pre-main-sequence evolution, extended X-ray saturation, frequent flaring, intense XUV irradiation, and strong stellar winds, all of which can drive atmospheric escape [2410.11048].

Within that framework, the survey is not a single-target program but a comparative one. It observes targets across a range of irradiation environments in order to identify where atmospheres begin to appear and which parameters correlate with atmospheric onset [2508.14210]. One paper describes the sample as spanning equilibrium temperatures of roughly \(420\)–\(910\) K [2505.22186]. Another emphasizes that LTT 3780 b anchors the extreme hot end, while LHS 1140c anchors the cool end [2508.14210, 2505.22186].

The program’s reliance on **secondary eclipses in MIRI/F1500W** is methodologically deliberate. A deep \(15\,\mu\mathrm{m}\) eclipse is expected from a dark, inefficiently redistributing bare surface, whereas a shallower eclipse can arise from atmospheric heat transport and/or CO\(_2\) absorption in the bandpass [2508.14210, 2604.02332]. This observing strategy also made the survey a “relevant primer” for the later **Director’s Discretionary Time Rocky Worlds program** [2410.11048].

## 2. Observational architecture and inference framework

All published Hot Rocks Survey installments use **JWST/MIRI imaging in F1500W** and rely on repeated occultation measurements rather than single visits. The program extracts eclipse depths, converts them to a dayside brightness temperature or dayside flux ratio, and then compares the result to forward models for bare-rock surfaces and idealized atmospheres [2410.11048, 2503.19772, 2508.14210].

A recurring element is the conversion between eclipse depth and dayside temperature. One survey paper writes the band-integrated planet/star flux ratio as  
\[
\frac{F_p}{F_s} = \left(\frac{R_p}{R_s}\right)^2 \frac{\int \frac{\pi B_p(T_{day}, \lambda) M(\lambda)}{hc/\lambda} d\lambda}{\int \frac{F_\star(T_\star, \lambda) M(\lambda)}{hc/\lambda} d\lambda},
\]
with \(M(\lambda)\) the MIRI/F1500W throughput and \(F_\star\) an interpolated stellar spectrum [2508.14210]. Bare-rock comparisons then use a dayside temperature scaling in which \(f=2/3\) corresponds to a no-atmosphere, no-redistribution flux-weighted dayside average [2508.14210].

Atmospheric modeling across the survey has been centered on **CO\(_2\)** and **H\(_2\)O**, typically using **HELIOS** radiative-convective calculations with cloud-free assumptions and parameterized heat redistribution [2508.14210, 2503.19772, 2604.02332]. This is not incidental: at \(15\,\mu\mathrm{m}\), CO\(_2\)-rich atmospheres are expected to produce especially strong suppression of dayside flux [2508.14210]. The survey’s interpretation is therefore not limited to “atmosphere versus no atmosphere,” but often specifically “CO\(_2\)-rich secondary atmosphere versus bare rock.”

A major technical theme is that the inferential bottleneck is not only photon noise. The published papers repeatedly show that **MIRI detector settling**, extraction choices, visit-to-visit systematics, and correlated noise modeling can materially affect the ease of interpretation, even when the eclipse depth itself is comparatively robust [2410.11048, 2505.22186, 2508.14210, 2604.02332].

## 3. Published targets and principal observational results

The first five survey installments cover **LHS 1478 b**, **TOI-1468 b**, **LHS 1140c**, **LTT 3780 b**, and **GJ 3473 b** [2410.11048, 2503.19772, 2505.22186, 2508.14210, 2604.02332]. Together they establish the survey’s current empirical basis.

| Planet | F1500W eclipse result | Main interpretation |
|---|---:|---|
| LHS 1478 b | \(138\pm53\) ppm | Possible shallow eclipse; majority of atmospheric scenarios considered are consistent, but two visits are inconsistent [2410.11048] |
| TOI-1468 b | \(311\pm31\) ppm | Mostly consistent with no atmosphere and zero Bond albedo at 1.65-\(\sigma\); pure CO\(_2\) or H\(_2\)O atmospheres above 1 bar ruled out over 3-\(\sigma\) [2503.19772] |
| LHS 1140c | \(273\pm43\) ppm | \(>5\sigma\) eclipse; highly consistent with a low-albedo bare rock [2505.22186] |
| LTT 3780 b | \(312\pm38\) ppm | Consistent with thermal emission from a bare rock surface; CO\(_2\)-based atmospheres down to 0.01 bar ruled out at \(>3\sigma\) [2508.14210] |
| GJ 3473 b | \(186\pm45\) ppm | Atmospheric and bare-rock interpretations both remain consistent; thick CO\(_2\) atmospheres excluded [2604.02332] |

The individual papers sharpen this comparison. **LHS 1478 b**, the first published target, produced a **possible shallow eclipse** at the expected time for a circular orbit, but one of the two visits was a non-detection because of significantly larger unexplained systematics [2410.11048]. That paper therefore treated the result as tentative rather than definitive.

**TOI-1468 b** produced three eclipses with individual depths of \(239\pm52\) ppm, \(341\pm53\) ppm, and \(357\pm52\) ppm, and a joint depth of \(311\pm31\) ppm. The measured thermal emission was mostly consistent with no atmosphere and zero Bond albedo at 1.65-\(\sigma\), while pure CO\(_2\) or H\(_2\)O atmospheres with a surface pressure above 1 bar were ruled out over 3-\(\sigma\) [2503.19772].

**LHS 1140c**, the coolest and least irradiated survey target, yielded a deep eclipse detected at \(>5\sigma\), with a recovered dayside brightness temperature of \(561\pm44\) K, close to the theoretical maximum of \(537\pm9\) K. The paper ruled out pure CO\(_2\) atmospheres with surface pressure \(\ge 10\) mbar and pure H\(_2\)O atmospheres with surface pressure \(\ge 1\) bar at \(>3\sigma\) [2505.22186].

**LTT 3780 b**, the survey’s highest-irradiation target at \(111\times\) Earth’s instellation, delivered a joint eclipse depth of \(312\pm38\) ppm. The inferred dayside temperature was \(1143^{+104}_{-99}\) K, corresponding to \(98\pm9\%\) of the maximum no-redistribution temperature. That paper confidently ruled out CO\(_2\)-dominated atmospheres down to 0.01 bar surface pressure to greater than \(3\sigma\) [2508.14210].

**GJ 3473 b** produced a secure average eclipse detection of \(186\pm45\) ppm from four visits, but unlike the clearer bare-rock cases, both atmospheric and bare-rock interpretations remained consistent with the data. The paper nevertheless excluded thick CO\(_2\) atmospheres, placing a 95% credible upper limit of 1.2–6.5 bar on the surface pressure [2604.02332].

Taken together, the published installments suggest that the survey is finding multiple planets on the “bare rock” side of the atmospheric-retention boundary, but with at least one ambiguous case and one tentative shallow-eclipse case that remains unresolved.

## 4. Comparative interpretation across the sample

The survey papers place strong weight on comparative structure, not isolated detections. **LTT 3780 b** is explicitly treated as a “key anchor” at the hot extreme because it is the most highly irradiated target in the 9-planet sample and therefore a stringent test of whether highly heated rocky M-dwarf planets can sustain volatile envelopes at all [2508.14210]. Its dayside temperature, at essentially the maximum expected for a synchronously rotating low-albedo airless planet, situates it clearly on the “bare rock” side of the emerging boundary [2508.14210].

At the other end of the published sample, **LHS 1140c** matters because it is the coolest and least irradiated target in the survey, yet it also appears highly consistent with a low-albedo bare rock [2505.22186]. This does not by itself define the retention boundary, but it pushes the bare-rock regime to lower irradiation than the hotter archetypes.

**TOI-1468 b** reinforces that pattern. Its eclipse is again consistent with a hot bare rock and inconsistent with substantial cloud-free CO\(_2\)- or H\(_2\)O-rich atmospheres at and above the 1 bar level, although the paper also notes that the planet appears marginally hotter than expected and therefore discusses thermal inversion, induction heating, and instrumental artifact as possible explanations [2503.19772].

The main deviation from a simple bare-rock narrative is **LHS 1478 b**, where the first observation favored a shallow eclipse and the majority of atmospheric scenarios considered were consistent with the data, spanning CO\(_2\)-rich atmospheres with surface pressures from 0.1 to 10 bar [2410.11048]. However, because the two observations did not yield consistent results, that paper explicitly stressed that more observations were needed [2410.11048].

**GJ 3473 b** is the clearest published demonstration that a secure eclipse does not automatically imply a secure atmospheric classification. Its average depth is somewhat lower than expected for a blackbody, but the overlap between reflective/weathered bare-rock surfaces and thin or moderate atmospheres leaves the atmosphere question unresolved [2604.02332]. A plausible implication is that the survey is not only mapping a physical boundary, but also identifying where **single-band \(15\,\mu\mathrm{m}\) eclipse photometry becomes intrinsically degenerate**.

## 5. Methodological advances and MIRI time-series systematics

One of the survey’s most substantial contributions is methodological. The papers show that **MIRI time-series analysis is highly sensitive to the initial settling ramp, aperture choice, extraction weighting, and pixel-level behavior**, but they also show that robust eclipse recovery is possible when these effects are handled carefully [2508.14210, 2505.22186, 2604.02332].

**Hot Rocks Survey III** introduced a “novel approach” that joint-fits individual pixel light curves with a shared eclipse model and a flexible multi-dimensional Gaussian process that models changes in the PSF over time [2505.22186]. In simulated data, the method can weight away from particular pixels that show increased systematics, allowing eclipse depths to be recovered more robustly and precisely [2505.22186]. That paper also identified a potential trend between detector settling and the **previous filter used by MIRI** [2505.22186].

**Hot Rocks Survey V** pushed the detector analysis further by identifying **three distinct settling behaviors across the PSF**: low-illumination edge pixels with little or no settling, highly illuminated core pixels with strong exponential-like ramps, and an intermediate annulus with exponential behavior of opposite sign to the core [2604.02332]. Because different apertures weight these regions differently, the apparent light-curve ramp changes with aperture size [2604.02332]. The fourth GJ 3473 b visit, which had different illumination history because the target had previously been observed in F560W, was highlighted as an especially instructive case [2604.02332].

**Hot Rocks Survey IV** on LTT 3780 b is notable for its exhaustive reduction-dependence analysis. The authors performed **two independent reductions**, explored linear, exponential, and Gaussian-process detrending models, varied the number of integrations trimmed from the start of the visit from 100 to 600, and used **Bayesian model averaging** over the best-supported models to incorporate systematic-model uncertainty [2508.14210]. They emphasized that nearly all tested fits agreed within \(\sim 1\sigma\), and almost all within \(\sim 2\sigma\), with discrepant fits strongly disfavored by Bayesian evidence [2508.14210].

**Hot Rocks Survey I** provides the contrasting cautionary example: a plausible eclipse candidate can remain scientifically inconclusive when one visit is degraded by a systematic of the same order as the astrophysical signal [2410.11048]. The combined lesson is that survey-level conclusions depend not only on nominal sensitivity, but also on visit repeatability, illumination history, pixel-level modeling, and multi-pipeline cross-checking.

## 6. Scientific significance, limits of single-band photometry, and future directions

The published Hot Rocks Survey papers collectively strengthen the case that **many highly irradiated rocky planets around M dwarfs appear airless or nearly so in thermal emission** [2503.19772, 2505.22186, 2508.14210]. LTT 3780 b is explicitly described as a hot-boundary anchor on the “bare rock” side of the retention threshold [2508.14210]. TOI-1468 b and LHS 1140c extend that picture to other parts of the survey parameter space [2503.19772, 2505.22186].

At the same time, the survey has exposed the limits of what **single-band F1500W eclipse photometry** can accomplish. **GJ 3473 b** shows that even a confident eclipse detection can remain degenerate between atmospheric and airless interpretations, because reflective, textured, or weathered surfaces can overlap the same \(15\,\mu\mathrm{m}\) depth range as thin or moderate atmospheres [2604.02332]. **LHS 1478 b** shows that shallow-eclipse interpretations can be undermined by visit-to-visit inconsistency [2410.11048]. This suggests that the survey is not merely producing yes-or-no atmospheric classifications; it is also empirically delimiting the regimes in which single-band thermal photometry is decisive and the regimes in which it is not.

The survey papers therefore converge on a common follow-up strategy. **LTT 3780 b** is described as an excellent target for follow-up JWST observations because its short period makes repeated eclipse or phase-curve observations efficient, and simulated **NIRSpec/G395H** and **MIRI/LRS** follow-up could distinguish some surface compositions and probe nightside emission directly [2508.14210]. **GJ 3473 b** is identified as requiring future **spectroscopic or phase-curve observations** to determine whether it hosts a substantial atmosphere [2604.02332]. **TOI-1468 b** likewise identifies a full phase curve as the most direct route to determining whether an atmosphere is present [2503.19772].

A broader implication is that the Hot Rocks Survey has become both a scientific survey and a calibration program for JWST rocky-world eclipse work. It has already shown that \(15\,\mu\mathrm{m}\) eclipse photometry can decisively rule out wide classes of CO\(_2\)-rich and H\(_2\)O-rich atmospheres on some targets, but it has also shown that the same observable cannot always uniquely distinguish a bare rock from a more weakly absorbing or more compositionally complex atmosphere [2505.22186, 2508.14210, 2604.02332]. In that sense, the survey’s lasting significance lies in both outcomes: it is mapping the atmospheric-retention boundary around M dwarfs, and it is defining the methodological boundary of what MIRI/F1500W eclipse photometry alone can establish.

Source: https://www.emergentmind.com/topics/hot-rocks-survey