SO₂ Shoreline in Exoplanet Atmospheres

• SO₂ shoreline is an empirical detectability boundary in metallicity–temperature space that marks where the 1–100 μbar SO₂ volume mixing ratio reaches 1 ppm.
• Metallicity and temperature are the dominant controls, with increased heavy-element abundance and a transition near 600 K driving observable SO₂ levels.
• The framework aids JWST spectroscopy by providing a quantitative diagnostic to distinguish atmospheres with detectable SO₂ from those where other sulfur species prevail.

Searching arXiv for papers relevant to “SO2 shoreline” and associated sulfur-atmosphere concepts. The SO$_2$ shoreline is an empirical, model-based boundary in exoplanet parameter space that separates gas giant atmospheres expected to contain detectable sulfur dioxide from those in which SO$_2$ is likely too scarce to observe. In the formulation introduced for gas giant exoplanets, the shoreline is defined in metallicity–irradiation or metallicity–temperature space by the contour where the 1–100 $\mu$bar averaged SO$_2$ volume mixing ratio reaches $10^{-6}$, i.e. 1 ppm. It is therefore a detectability boundary rather than a physical surface in an atmosphere, and it is intended to organize sulfur chemistry, metallicity, and irradiation into a single observationally useful diagnostic for JWST-era spectroscopy (Crossfield et al., 17 Sep 2025).

1. Definition and parameterization

In its primary current usage, the SO$_2$ shoreline denotes the region in metallicity–irradiation/temperature space where SO$_2$ may be sufficiently abundant to be detectable. The nominal map spans metallicities from $0.3\times$ to $1000\times$ Solar and equilibrium temperatures from 250 to 2050 K. The shoreline itself is identified with the dashed 1 ppm contour in the main map, constructed from SO$_2$ abundances averaged over 1–100 $_2$0bar; below that contour, SO$_2$1 is “unlikely to be detectable” (Crossfield et al., 17 Sep 2025).

This definition makes the shoreline a boundary in observable parameter space rather than an atmospheric interface. The term “shoreline” is therefore metaphorical, analogous to other boundary concepts in planetary science, but here tied specifically to a practical detectability threshold. Under nominal assumptions, the central summary statement is that SO$_2$2 should be detectable for $_2$3 K and metallicities $_2$4 Solar. The mapped boundary is also described as very steep in both temperature and metallicity, so modest movement in either coordinate can shift an atmosphere from nominally detectable to nominally nondetectable (Crossfield et al., 17 Sep 2025).

A further implication of the definition is methodological: the shoreline depends on an averaging convention, a pressure range, and a practical detection threshold. It is not intended as a universal thermochemical phase boundary. This suggests that any reformulation for other observing modes, pressure levels, or planet classes would require a different contour and possibly a different geometry in parameter space.

2. Dominant controls on the shoreline

The two dominant variables shaping the gas-giant SO$_2$5 shoreline are metallicity and temperature. Metallicity is identified as the strongest overall driver: higher metallicity generally means more absolute heavy-element abundance, boosting SO$_2$6 production. Temperature controls the chemical regime, with a major transition near $_2$7 K. Below this temperature, SO$_2$8 is usually very low unless metallicity is high; for $_2$9 Solar, it is usually very low at cooler temperatures. At higher irradiation and higher temperatures, SO$\mu$0 can become detectable at much lower metallicity, down to about $\mu$1 Solar (Crossfield et al., 17 Sep 2025).

Among the single-parameter perturbations tested, the C/O ratio has the largest effect on SO$\mu$2 abundance. The models examine C/O = 0.30, 0.55, and 0.80, and increasing C/O from 0.30 to 0.80 steadily decreases SO$\mu$3, while decreasing C/O increases it. The shoreline is said to shift by roughly equal amounts at all temperatures when C/O changes, unlike most other parameters, and its shape appears to change qualitatively at C/O = 0.8. The stated interpretation is that higher C/O leaves less oxygen available to build SO$\mu$4 because more oxygen is consumed into CO/CO$\mu$5. Detecting SO$\mu$6 therefore strongly suggests C/O $\mu$7 Solar, and/or high overall metallicity (Crossfield et al., 17 Sep 2025).

The same study varies incident XUV irradiation by a factor of 30 and finds a surprisingly weak effect. For $\mu$8 K, changing XUV flux barely moves the shoreline; at cooler temperatures the effect becomes somewhat larger, but still modest. The main trend is that increasing XUV generally decreases SO$\mu$9. Vertical mixing, parameterized by $_2$0, is also weak overall for $_2$1 K, where changing $_2$2 by 4 orders of magnitude barely changes the shoreline. Below 600 K, however, lower $_2$3 dramatically reduces SO$_2$4 in the upper atmosphere, while high $_2$5 can make SO$_2$6 detectable in high-metallicity ($_2$7 Solar) planets with $_2$8 K. By contrast, internal temperature is almost negligible: testing 100 K, 300 K, and 500 K shows that SO$_2$9 is essentially insensitive to $10^{-6}$0 because the deeper layers affected by internal heat lie below the region probed by transmission (Crossfield et al., 17 Sep 2025).

3. Sulfur speciation and chemical interpretation

The SO$10^{-6}$1 shoreline is not a boundary between sulfur-bearing and sulfur-free atmospheres. Rather, it separates atmospheres in which SO$10^{-6}$2 is expected to be detectable from those in which other sulfur carriers are likely to dominate the observable sulfur budget. The gas-giant models explicitly state that SO$10^{-6}$3 is never the dominant sulfur-bearing molecule. Depending on temperature and metallicity, H$10^{-6}$4S, S$10^{-6}$5, NS, SO, SH, and even S$10^{-6}$6 or atomic S are frequently as common as, or more common than, SO$10^{-6}$7 (Crossfield et al., 17 Sep 2025).

The chemical competition varies with regime. At lower metallicities and intermediate temperatures of 400–600 K, sulfur that would otherwise appear as SO$10^{-6}$8 at higher temperatures can instead be sequestered into CS$10^{-6}$9, OCS, and S$_2$0. At the lowest temperatures, sulfur increasingly resides in higher-order allotropes such as S$_2$1. The same framework notes that CS and CS$_2$2 do not carry a substantial sulfur fraction in the network in general, but cautions that missing C–S reactions may affect this conclusion. The shoreline is therefore best understood as an observable consequence of network partitioning among sulfur species rather than as a direct marker of total sulfur abundance (Crossfield et al., 17 Sep 2025).

Despite this, SO$_2$3 remains the most easily detectable sulfur-bearing species in gas giants. The reasons given are strong mid-IR opacity, especially at 7.4 $_2$4m and 8.7 $_2$5m, comparatively strong transmission features, and broader observability than H$_2$6S. H$_2$7S can be more abundant, but its broad 3.7 $_2$8m band overlaps stronger H$_2$9O/CH$_2$0/CO$_2$1 structure and part of the signature falls in a JWST NIRSpec/G395H detector gap. SO can become appreciable at higher metallicities and temperatures and may be visible from 0.8–1.2 $_2$2m, while SO and SH could be detectable in some gas giants, potentially below $_2$3m (Crossfield et al., 17 Sep 2025).

A useful contrast is provided by modeled terrestrial exoplanet atmospheres, where SO$_2$4 is chemically short-lived in essentially all modeled H$_2$5-, N$_2$6-, and CO$_2$7-dominated cases and is rapidly converted into elemental sulfur and sulfuric acid, with aerosol formation often dominating the observational outcome. In that terrestrial context, direct detection of gaseous SO$_2$8 is unlikely unless sulfur emission rates are extremely high, typically $_2$9 Earth’s volcanic sulfur flux, and aerosol-related features are more likely observables (Hu et al., 2013). This contrast suggests that the gas-giant SO$0.3\times$0 shoreline is specifically a composition-and-detectability construct for warm, irradiated, metal-enriched atmospheres, not a generic sulfur persistence criterion across all planet classes.

4. Observational role in JWST spectroscopy

SO$0.3\times$1 is observationally important because JWST can detect it in multiple bands, notably at 4.1 $0.3\times$2m and in the 7–9 $0.3\times$3m region, especially the 7.4 $0.3\times$4m and 8.7 $0.3\times$5m bands. The longer-wavelength bands are emphasized as better metallicity tracers because the 4.1 $0.3\times$6m band can saturate at lower metallicity, whereas the 7.4 $0.3\times$7m and 8.7 $0.3\times$8m bands remain sensitive over a wider metallicity range. In this sense, the shoreline is not merely a presence–absence guide; it also structures where SO$0.3\times$9 can serve as a quantitative probe of metallicity and sulfur abundance (Crossfield et al., 17 Sep 2025).

The observational interpretation is correspondingly specific. An SO$1000\times$0 detection indicates high metallicity and/or low C/O, while an SO$1000\times$1 nondetection does not automatically imply the absence of sulfur because H$1000\times$2S or other sulfur species may dominate instead. The 7–9 $1000\times$3m region is described as especially valuable because it probes metallicity better than the 4.1 $1000\times$4m band. The comparison between modeled shorelines and currently observed planets is reported as broadly consistent: HAT-P-26b, WASP-107b, and WASP-39b lie within or near the shoreline and have SO$1000\times$5 detections, while GJ 3470b sits near the edge and any discrepancy could be due to non-solar C/O or unusual metallicity. Several lower-mass planets with nondetections lie above the nominal shoreline but have broad uncertainties, so they are not yet inconsistent. The lack of robust SO$1000\times$6 detections in many massive hot Jupiters is interpreted as consistent with those planets possibly being low metallicity (Crossfield et al., 17 Sep 2025).

Because the shoreline is defined using transmission-relevant abundances in the 1–100 $1000\times$7bar region, it is intrinsically tied to observing geometry and retrieval practice. A plausible implication is that the same planet could move relative to a detectability contour if clouds, haze opacity, or different pressure weighting altered the effective line-forming region. That implication is consistent with the paper’s emphasis that the shoreline is a practical detectability map rather than a chemically exhaustive classification.

5. Relation to other “shoreline” concepts

The term “shoreline” has a broader history in exoplanet science, and the SO$1000\times$8 shoreline occupies only one part of that vocabulary. In a separate rocky-exoplanet context, sulfur observables are used as a dryness diagnostic: sustained detectable SO$1000\times$9 gas or a thick H$_2$0SO$_2$1–H$_2$2O sulfate aerosol haze is proposed as a remote indicator that a planet does not host significant surface liquid water. In that usage, an “SO$_2$3 shoreline” is an analog of the liquid-water shoreline concept, defined chemically rather than geographically. The headline result is that surface liquid water reservoirs larger than about $_2$4 Earth oceans are incompatible with sustained observable SO$_2$5 and H$_2$6SO$_2$7–H$_2$8O haze under the paper’s conservative conditions (Loftus et al., 2019).

This rocky-planet use differs fundamentally from the gas-giant shoreline. The former is a sulfur-cycle threshold tied to ocean uptake and atmospheric persistence in oxidized rocky atmospheres; the latter is a metallicity–temperature detectability contour for SO$_2$9 in giant-planet atmospheres. Conflating the two would obscure the distinct controlling physics. One concerns whether sulfur can remain observable in the presence of oceans; the other concerns where sulfur dioxide is abundant enough in giant-planet transmission regions to be detectable.

A second neighboring concept is the “cosmic shoreline,” a probabilistic boundary in the three-dimensional space of planet escape velocity, bolometric flux, and host-star luminosity for atmosphere retention in general. That framework does not define an SO$_2$00 shoreline; it addresses atmosphere presence or absence rather than composition-specific detectability, and it mentions SO$_2$01 only incidentally in the discussion of an inconclusive case (Berta-Thompson et al., 2 Jul 2025). A common misconception is therefore to treat all “shorelines” in exoplanet research as equivalent. The literature instead uses the term for multiple boundary problems, of which the gas-giant SO$_2$02 shoreline is a composition-specific case.

6. Caveats, uncertainties, and extension paths

The current gas-giant SO$_2$03 shoreline is explicitly provisional. The models include no clouds or hazes, even though clouds and hazes can mask transmission features and sequester elements into condensates. Potential sulfur clouds such as ZnS, MnS, and Na$_2$04S are argued usually to remove $_2$05 of the bulk sulfur and thus probably not to drastically change SO$_2$06 abundance, but S$_2$07 hazes can form for $_2$08 K near solar metallicity and could reduce observable sulfur above the haze layer. Water condensation is also not included for the coolest models, even though it may strongly alter thermal structure and chemistry (Crossfield et al., 17 Sep 2025).

The chemical network itself is incomplete. The models lack some reactions involving C–S bonded species and other sulfur-bearing molecules, which may cause an overestimate of SO$_2$09 at cooler temperatures, where C–S species could become more important. The atmospheric structure is not fully self-consistent because the chemistry is computed on fixed temperature profiles rather than fully coupled structure-plus-chemistry solutions. The framework is one-dimensional rather than 2D or 3D, so day/night terminator differences, longitudinal transport, and eccentric-orbit time dependence are not represented. The gravity range is also limited: the calculations use a single HAT-P-26b-like gravity, and the shoreline is said to require extension to a broader range of planetary masses and radii. Lower surface gravity is specifically noted as a factor that can increase SO$_2$10 detectability by providing a physically thicker atmosphere and more shielding against photodissociation (Crossfield et al., 17 Sep 2025).

Future development is framed around missing physics and missing laboratory inputs. The paper calls for additional observational constraints on sulfur, future work on fully self-consistent atmospheric models, 2D and 3D models, a wider range of planetary masses and radii, and studies to measure and refine reaction rates and molecular opacities of sulfur-bearing species. It also points to the need to understand newly identified SO$_2$11 pathways, including the new NUV-driven pathway identified by de Gruijter et al. (2025). A major barrier to fully characterizing sulfur chemistry is the lack of usable opacity data for species including S$_2$12, S$_2$13, S$_2$14, S$_2$15, HS$_2$16, HCS, HSO, HSO$_2$17, H$_2$18SO$_2$19, CH$_2$20S, CH$_2$21SH, and S$_2$22O; H$_2$23CS has opacity data but is not in VULCAN’s network (Crossfield et al., 17 Sep 2025).

In its present form, the SO$_2$24 shoreline is therefore best regarded as a structured observational heuristic: it identifies where SO$_2$25 should be most detectable in warm-to-hot, metal-rich, low-C/O gas giants, while making explicit that sulfur chemistry remains network-dependent, cloud-sensitive, and observationally incomplete.