YSES-1 c: Comparative Exoplanet Study

• The paper highlights that YSES-1 c is the outer, directly imaged giant planet whose atmospheric properties serve as a benchmark for comparative planetology.
• High-resolution CRIRES+ and JWST spectroscopy revealed secure detections of key molecules like H2O and CO, along with the first direct observation of silicate clouds.
• Comparative analyses with YSES 1 b and stellar abundances underscore formation pathways involving core accretion, migration, and oxygen sequestration in clouds.

YSES-1 c is the outer directly imaged giant planet in the YSES 1 system, also known as TYC 8998-760-1. The host is a 16.7 ± 1.4 Myr K3IV star in the Lower Centaurus Crux subgroup of Sco-Cen, and it hosts two wide-orbit companions discovered by the Young Suns Exoplanet Survey: YSES 1 b, with 14 ± 3 M_Jup at ~160 au, and YSES 1 c, with 6 ± 1 M_Jup at ~320 au. YSES-1 c has become a benchmark object because it combines direct-imaging accessibility, high-resolution K-band detections of atmospheric molecules, JWST spectroscopy from 0.6 to 12 μm, and a host star with a measured near-solar abundance pattern, allowing unusually explicit comparisons between planetary and stellar chemistry (Baburaj et al., 20 Oct 2025, Zhang et al., 2024, Hoch et al., 25 Jul 2025).

1. System architecture and comparative context

YSES-1 c is the outer of the two YSES planets, and its large projected separation places it in the class of very wide-orbit super-Jovian companions. The system has been treated as an “ideal laboratory” for studying the early evolution of young giant planets because both companions orbit the same young star yet exhibit distinct atmospheric and circumplanetary properties. That architecture is central to the literature on YSES-1 c: the object is rarely discussed in isolation, but rather as one half of a comparative pair whose shared natal environment makes differential atmospheric interpretation unusually informative (Hoch et al., 25 Jul 2025, Zhang et al., 2024).

The comparative setting is especially important because the two planets are similar in broad category but different in several retrieved properties. In the high-resolution CRIRES+ analysis, YSES 1 b has C/O = 0.57 ± 0.01 and $v\sin i = 5.34 \pm 0.14\,\mathrm{km\,s^{-1}}$, whereas YSES 1 c is reported with solar or subsolar C/O and a substantially larger projected rotation velocity. This contrast has been used to motivate interpretations involving different spin-axis inclinations, different braking histories, or different formation and transport pathways within the same stellar system (Zhang et al., 2024).

A recurring theme in the YSES-1 c literature is that wide separation does not by itself determine formation history. The object’s chemistry, cloud structure, and dynamical placement are all treated as coupled constraints. This suggests that YSES-1 c is best understood not simply as a wide-orbit imaged planet, but as a chemically and microphysically resolved comparative case within a multi-planet direct-imaging system.

2. Stellar abundance baseline and the chemical reference frame

The host-star abundance baseline is a defining part of the modern interpretation of YSES-1 c. The high-resolution spectroscopic survey of directly imaged companion hosts emphasizes that the era of JWST has enabled atmospheric abundance measurements of elements such as C and O to very high precision, and that planet-formation inferences therefore require corresponding host-star abundances. In that framework, YSES 1 is analyzed as a chemically important reference rather than as a mere catalog entry (Baburaj et al., 20 Oct 2025).

For YSES 1, atmospheric parameters were obtained using high-resolution GHOST spectroscopy and MCMC spectral fitting. Because the system shows Hα emission from accreting planets, the Hα order was not used for stellar-parameter determination; instead, the analysis used the Na I D region and other orders. The final stellar parameters are $T_{\rm eff} = 4709 \pm 45$ K, $\log g = 4.28 \pm 0.09$, and $[M/H] = -0.02 \pm 0.03$, i.e. essentially solar metallicity. The abundance scale is defined explicitly as

$\log{\epsilon_X} = \log{X} - \log{H} + 12.$

Using custom MARCS C/O synthetic grids and selected carbon and oxygen lines, the spectral-fit abundances for YSES 1 are $[C/H] = -0.04 \pm 0.04$, $[O/H] = +0.05 \pm 0.03$, and $\mathrm{C/O} = 0.45 \pm 0.05$. The adopted solar reference is Solar C/O = 0.55, so the stellar ratio is slightly below but statistically consistent with solar (Baburaj et al., 20 Oct 2025).

An independent equivalent-width analysis yielded $[C/H] = +0.01 \pm 0.14$, $[O/H] = +0.14 \pm 0.14$, $T_{\rm eff} = 4709 \pm 45$0, $T_{\rm eff} = 4709 \pm 45$1, $T_{\rm eff} = 4709 \pm 45$2, and $T_{\rm eff} = 4709 \pm 45$3. The study states that the equivalent-width C/O agrees well with the spectral-fit value and is consistent with solar. For YSES 1 specifically, the individual host-star elemental pattern from equivalent widths is described as broadly solar: C and O are near solar, and Na, Mg, Si, Fe, Ni, etc. are all near solar, while sulfur appears somewhat elevated but with large uncertainties because the lines are weak in these late-G/K stars (Baburaj et al., 20 Oct 2025).

For YSES-1 c, this stellar baseline matters directly. The host-star study argues that directly imaged giant planets should be interpreted relative to host-star composition, so the near-solar composition of YSES 1 establishes a chemically calibrated reference against which planetary C/O, metallicity, and possible oxygen sequestration can be assessed.

3. High-resolution atmospheric detection with CRIRES+

YSES-1 c was observed with VLT/CRIRES+ on UT 2023 February 27 and 28 as part of the ESO SupJup Survey, using the K2166 setting with a 0.2″ slit at $T_{\rm eff} = 4709 \pm 45$4 over the 2.3–2.4 μm region. The slit geometry allowed YSES 1 b and c to be recorded simultaneously, while also admitting some off-axis stellar light for calibration. The total integration time was 5.3 hours. Because the companion is faint, the YSES-1 c extraction used a weighted sum along the spatial direction with a Gaussian profile fitted to the white-light PSF, followed by high-pass filtering with a 200-pixel ($T_{\rm eff} = 4709 \pm 45$52 nm) Gaussian filter to remove continuum information. The atmospheric signal for YSES-1 c therefore came essentially from line patterns rather than broadband flux (Zhang et al., 2024).

The high-resolution analysis reported secure detections of H$T_{\rm eff} = 4709 \pm 45$6O and CO. The cross-correlation S/N values are 7 for H$T_{\rm eff} = 4709 \pm 45$7O, 4 for CO, and 7.8 for the combined H$T_{\rm eff} = 4709 \pm 45$8O + CO template. In the Bayesian leave-one-out model comparison, the corresponding detection significances are 7.3$T_{\rm eff} = 4709 \pm 45$9 for H$\log g = 4.28 \pm 0.09$0O and 5.7$\log g = 4.28 \pm 0.09$1 for CO. These detections established that YSES-1 c has a directly detected molecular atmosphere even in a very high-resolution, continuum-suppressed observing mode (Zhang et al., 2024).

Composition retrievals were performed with both disequilibrium chemistry and free chemistry models. The main C/O results are $\log g = 4.28 \pm 0.09$2 in the disequilibrium model and $\log g = 4.28 \pm 0.09$3 in the free model, with the abstract and conclusion summarizing the companion as “solar or subsolar” in C/O. The authors interpret this as indicating that YSES-1 c is not carbon-rich and that its atmosphere is compatible with oxygen-rich solid accretion. Metallicities were retrieved as $\log g = 4.28 \pm 0.09$4 in the disequilibrium model and $\log g = 4.28 \pm 0.09$5 in the free model, but these were explicitly described as weak and model-dependent because of degeneracies with gravity and temperature structure (Zhang et al., 2024).

The same analysis retrieved $\log g = 4.28 \pm 0.09$6 K, low surface gravity with $\log g = 4.28 \pm 0.09$7 in the disequilibrium model and $\log g = 4.28 \pm 0.09$8 in the free model, and a projected rotation velocity of $\log g = 4.28 \pm 0.09$9. The radial velocity is $[M/H] = -0.02 \pm 0.03$0, consistent with the primary’s $[M/H] = -0.02 \pm 0.03$1 and implying a relative offset of $[M/H] = -0.02 \pm 0.03$2. Methane was not detected; the free-chemistry upper limit is $[M/H] = -0.02 \pm 0.03$3 at 3$[M/H] = -0.02 \pm 0.03$4, and the disequilibrium analysis inferred strong CO/CH$[M/H] = -0.02 \pm 0.03$5 quenching with $[M/H] = -0.02 \pm 0.03$6 at 1$[M/H] = -0.02 \pm 0.03$7. Cloud constraints from the K band were weak: turning clouds off produced a Bayes factor of approximately 1, and the data were described as not sensitive to the main cloud deck, because the expected silicate condensation region lies below the K-band photosphere probed by CRIRES+ (Zhang et al., 2024).

4. JWST spectroscopy and the detection of silicate clouds

JWST provided a qualitatively different view of YSES-1 c. The companion was observed in low-resolution spectroscopy from 0.6 to 12 μm using NIRSpec IFU Prism and MIRI LRS, after careful PSF subtraction and spectral extraction. The resulting spectrum contains the usual near-IR and mid-IR molecular bands, including CO, H$[M/H] = -0.02 \pm 0.03$8O, CO$[M/H] = -0.02 \pm 0.03$9, and CH$\log{\epsilon_X} = \log{X} - \log{H} + 12.$0, but its most important signature is a broad mid-infrared absorption feature spanning roughly 9–12 μm, already recognizable beginning near 8.5 μm. This was interpreted as direct evidence for silicate clouds in the atmosphere of YSES-1 c and described as the first direct observation of silicate clouds in the atmosphere of the exoplanet YSES-1 c through its 9–11 μm absorption feature (Hoch et al., 25 Jul 2025).

The study used two complementary routes. A semi-empirical silicate index analysis compared the observed spectrum to cloud-free models and to brown dwarfs and young companions, and found that YSES-1 c has a very large silicate index, placing it among the strongest absorbers in the relevant regime. A second route used detailed cloud modeling with VIRGA and PICASO, together with forward models and retrievals using Exo-REM and petitRADTRANS. A central result was that cloud-free models could not reproduce the observed 9–11 μm absorption. When silicate clouds were introduced, the spectral shape could be matched (Hoch et al., 25 Jul 2025).

The preferred cloud scenarios place the silicate cloud at approximately 1 millibar, with the combination-cloud fits giving

$\log{\epsilon_X} = \log{X} - \log{H} + 12.$1

The feature requires small grains with a mean particle radius of about

$\log{\epsilon_X} = \log{X} - \log{H} + 12.$2

and the paper states that 0.1 μm is really an upper limit, because smaller grains plus higher column density can reproduce similar behavior. Larger grains such as 1 μm or 10 μm make the feature far too strong and broad and are ruled out. The best-fitting compositions are either amorphous iron-enriched pyroxene or a combination of amorphous MgSiO$\log{\epsilon_X} = \log{X} - \log{H} + 12.$3 and Mg$\log{\epsilon_X} = \log{X} - \log{H} + 12.$4SiO$\log{\epsilon_X} = \log{X} - \log{H} + 12.$5, with the latter often described as a 60–90% MgSiO$\log{\epsilon_X} = \log{X} - \log{H} + 12.$6 mixture. The authors stress degeneracies among particle size, cloud base pressure, and temperature structure, and they state that model systematics dominate the uncertainties; nonetheless, the qualitative conclusion is robust that YSES-1 c hosts high-altitude silicate clouds (Hoch et al., 25 Jul 2025).

The JWST spectrum also established atmospheric context. YSES-1 c is described as roughly late-L type ($\log{\epsilon_X} = \log{X} - \log{H} + 12.$7 L7.5) and near the L/T transition, and it is much redder than typical directly imaged planets and brown dwarfs. The silicate absorption begins at longer wavelengths than in field brown dwarfs and objects such as VHS 1256 b, which the authors interpret as evidence that clouds in YSES-1 c are physically different, plausibly because the atmosphere is younger, lower gravity, higher altitude, and/or has different grain properties. The same work states that the silicate clouds are responsible for the extreme reddening of the spectrum and that up to roughly 18–20% of oxygen may be tied up in silicate condensates, reducing the gas-phase C/O to a bulk value around 0.64–0.65 (Hoch et al., 25 Jul 2025).

5. Microphysical interpretation: TwoPop cloud structure

Subsequent microphysical analysis connected the JWST silicate signature to explicit cloud-formation models. In this framework, YSES-1 c is interpreted as having a two-layer/TwoPop structure: a deeper, optically thick cloud deck shaping the broad thermal spectral energy distribution, plus a thinner and more vertically extended high-altitude silicate cloud responsible for the 9–11 μm Si–O absorption feature. Panchromatic coverage from 0.6 to 12 μm is described as crucial because shorter wavelengths constrain the thermal continuum and photospheric cloud opacity, while the 9–11 μm region constrains the upper cloud’s presence, composition, and particle size (Kiefer et al., 13 Mar 2026).

The equilibrium-condensation model Virga is formulated by balancing upward mixing against gravitational settling,

$\log{\epsilon_X} = \log{X} - \log{H} + 12.$8

with a height-dependent parameterization for $\log{\epsilon_X} = \log{X} - \log{H} + 12.$9. For YSES-1 c, a single-material, single-cloud Virga model does not reproduce the full observed Si–O feature well. The best fits require a TwoPop cloud with a base cloud of Mg$[C/H] = -0.04 \pm 0.04$0SiO$[C/H] = -0.04 \pm 0.04$1 or MgSiO$[C/H] = -0.04 \pm 0.04$2 depending on the tested case, and an extended upper cloud of MgSiO$[C/H] = -0.04 \pm 0.04$3. The best Virga TwoPop solution reported in the YSES-1 c table is the Mg$[C/H] = -0.04 \pm 0.04$4SiO$[C/H] = -0.04 \pm 0.04$5 base + MgSiO$[C/H] = -0.04 \pm 0.04$6 extended cloud case, with $[C/H] = -0.04 \pm 0.04$7 and $[C/H] = -0.04 \pm 0.04$8 (Kiefer et al., 13 Mar 2026).

The time-dependent microphysical model Nimbus includes diffusion, settling, nucleation via Modified Classical Nucleation Theory (MCNT), and growth in the diffusion- and collision-limited regime. Single-material Nimbus runs with MgSiO$[C/H] = -0.04 \pm 0.04$9 alone or Mg$[O/H] = +0.05 \pm 0.03$0SiO$[O/H] = +0.05 \pm 0.03$1 alone could not reproduce the Si–O feature satisfactorily; both reported $[O/H] = +0.05 \pm 0.03$2, with $[O/H] = +0.05 \pm 0.03$3 for base-only MgSiO$[O/H] = +0.05 \pm 0.03$4 and $[O/H] = +0.05 \pm 0.03$5 for base-only Mg$[O/H] = +0.05 \pm 0.03$6SiO$[O/H] = +0.05 \pm 0.03$7. The preferred Nimbus interpretation is again TwoPop Mg$[O/H] = +0.05 \pm 0.03$8SiO$[O/H] = +0.05 \pm 0.03$9 base + MgSiO$\mathrm{C/O} = 0.45 \pm 0.05$0 extended, with $\mathrm{C/O} = 0.45 \pm 0.05$1 and $\mathrm{C/O} = 0.45 \pm 0.05$2 (Kiefer et al., 13 Mar 2026).

The planet-specific microphysical conclusions are unusually sharp. The upper cloud must contain very small particles to preserve the silicate feature, and the broader study concludes that all four planets analyzed require cluster-sized silicate particles at high altitudes, with characteristic sizes near

$\mathrm{C/O} = 0.45 \pm 0.05$3

For YSES-1 c, the preferred TwoPop Nimbus fit gives $\mathrm{C/O} = 0.45 \pm 0.05$4 for the base cloud and $\mathrm{C/O} = 0.45 \pm 0.05$5 for the upper MgSiO$\mathrm{C/O} = 0.45 \pm 0.05$6 cloud. The inferred upper-cloud value lies near the threshold identified as necessary for producing high-altitude small particles, $\mathrm{C/O} = 0.45 \pm 0.05$7, whereas the base-cloud sticking coefficient is described as broadly consistent with laboratory expectations,

$\mathrm{C/O} = 0.45 \pm 0.05$8

The authors therefore argue that YSES-1 c cannot be explained by a simple compact equilibrium cloud alone, but instead requires an optically thick base plus a tenuous, extended, nanograin-rich upper silicate layer (Kiefer et al., 13 Mar 2026).

6. Formation interpretations and current compositional tensions

The formation interpretation of YSES-1 c is driven by comparisons among stellar abundances, planetary atmospheric retrievals, and cloud-corrected chemistry. In the host-star abundance study, the key comparison is between YSES 1 at $\mathrm{C/O} = 0.45 \pm 0.05$9 and YSES-1 c at $[C/H] = +0.01 \pm 0.14$0 from prior work. Because the companion is then super-stellar in C/O, the study states that YSES-1 c’s chemistry, together with the super-solar metallicity reported in Hoch et al. (2025), favors formation by core accretion followed by migration to its present wide separation. More generally, that work includes YSES-1 c among the directly imaged companions whose super-stellar C/O and super-solar metallicity “strongly indicate a planet-like formation” (Baburaj et al., 20 Oct 2025).

A different atmospheric picture emerges from the CRIRES+ study. There, YSES-1 c is characterized as solar or subsolar in C/O, with $[C/H] = +0.01 \pm 0.14$1 emphasized in the abstract and conclusion, and this is interpreted as consistent with oxygen-rich solid accretion. In that scenario, the planet may have formed closer in, possibly inside the CO iceline or near the water iceline, accreting oxygen-rich solids or water-rich material before later dynamical scattering or outward migration to its current orbit at roughly 320 au. The same paper stresses that the S/N is low enough that the C/O result is best viewed as solar-to-subsolar, not as a definitive measurement (Zhang et al., 2024).

JWST-based cloud analysis adds a further layer. The silicate-cloud study reports that cloud condensation can sequester a substantial oxygen fraction, and it gives a bulk C/O around 0.64–0.65 after cloud correction. This does not eliminate ambiguity, but it means that compositional interpretation depends on whether one is discussing gas-phase retrievals, cloud-corrected bulk abundances, or comparisons to the stellar baseline. The paper also notes that the atmosphere is young, low gravity, and dominated by unusually high-altitude silicate cloud opacity, all of which complicate direct comparison to field brown dwarfs and to narrower-band retrieval frameworks (Hoch et al., 25 Jul 2025).

Taken together, the literature does not yield a single uncontroversial chemical narrative for YSES-1 c. One line of analysis emphasizes super-stellar planetary C/O relative to a near-solar host and therefore core accretion with migration; another emphasizes solar or subsolar C/O and therefore oxygen-rich solid accretion with subsequent outward relocation; the cloud-microphysics work shows that high-altitude silicates and oxygen sequestration are not secondary details but part of the abundance problem itself. This suggests that YSES-1 c is important precisely because it exposes the coupling among retrieval wavelength coverage, cloud physics, bulk-versus-gas-phase chemistry, and formation inference. In that sense, YSES-1 c has become a benchmark not only for young giant-planet atmospheres, but also for the limits of current comparative planetology across direct imaging, high-resolution spectroscopy, and host-star abundance analysis.