YSES-1 b: Benchmark Substellar Companion

• The paper establishes YSES-1 b as a young, wide-orbit substellar companion orbiting TYC 8998-760-1, enabling detailed astrometric and spectroscopic studies.
• High-resolution spectroscopy confirms a solar-like C/O ratio, robust carbon isotope measurements, and slow rotation, highlighting its atmospheric characterization.
• SED re-interpretation accounting for circumplanetary disk effects shifts YSES-1 b’s inferred properties, moving it across the planet–brown-dwarf boundary.

Searching arXiv for papers on YSES-1 b to ground the article in the current literature. YSES-1 b, also written YSES 1 b and originally designated TYC 8998-760-1 b, is a directly imaged wide-orbit substellar companion to the young solar-type star YSES 1 / TYC 8998-760-1. It is the inner of two known companions in the system, with YSES 1 c at roughly 320 AU, and it has become a benchmark object for direct-imaging studies because its projected separation of about 160 AU makes both astrometric and spectroscopic characterization feasible. Across the recent literature, YSES-1 b has been treated as a young super-Jovian object near the deuterium-burning boundary, an accreting companion with a circumplanetary disk, and, in the most recent SED-based reinterpretation, a likely brown dwarf once circumplanetary-disk effects are modeled explicitly (Zhang et al., 2024, Roberts et al., 17 Sep 2025, Darcis et al., 26 May 2026).

1. Designation, host system, and observational setting

YSES-1 b belongs to the YSES-1 exoplanet system, not to the Young Supernova Experiment. That distinction matters because the supernova survey is abbreviated YSE, not YSES, and does not define a naming convention that would produce a label such as “YSES-1 b” (Jones et al., 2020).

The companion orbits the young K3IV star YSES 1 / TYC 8998-760-1. The system is described as lying at about 94.2 pc, with modeling in later work adopting $d = 94.6 \pm 0.3$ pc, and the age is variously taken as $16.7 \pm 1.4$ Myr, $17 \pm 1$ Myr, or, in one evolutionary-model reinterpretation, $27 \pm 3$ Myr (Zhang et al., 2024, Hoch et al., 25 Jul 2025, Roberts et al., 17 Sep 2025, Darcis et al., 26 May 2026). YSES-1 b is the inner of the two directly imaged companions, at a projected separation of about 160 AU or about 1.6 arcsec, while YSES-1 c lies at about 320 AU or about 3.2 arcsec (Hoch et al., 25 Jul 2025, Roberts et al., 17 Sep 2025).

The broader system is important because it is described as the first directly imaged multi-planet system around a solar-type star, and because YSES, the Young Suns Exoplanet Survey, specifically targets young, nearby, roughly solar-mass stars for direct imaging of wide companions (Zhang et al., 2024, Liu et al., 19 May 2025). This combination of youth, wide separation, and high intrinsic luminosity makes YSES-1 b a useful laboratory for atmospheric retrieval, isotopologue measurements, circumplanetary-disk diagnostics, and orbit fitting.

2. Baseline characterization and discovery-era interpretation

In the discovery-era interpretation preserved in later summaries, YSES-1 b was treated as a planetary-mass or near-planetary-mass companion. The values repeatedly cited for that earlier picture are a mass of $14 \pm 3\,M_{\rm Jup}$, a spectral type of L0, a photometric temperature of $T_{\rm eff}=1727^{+172}_{-127}$ K, and a radius of $3.0^{+0.2}_{-0.7}\,R_{\rm Jup}$ (Zhang et al., 2024, Darcis et al., 26 May 2026). One later summary also notes that an older system age would raise the mass estimate to $21.8 \pm 3\,M_{\rm Jup}$ (Zhang et al., 2024).

Those baseline parameters already made YSES-1 b unusual. It is a young, self-luminous, wide-orbit super-Jovian companion near the planet/brown-dwarf boundary, and its large inferred radius was difficult to reconcile with standard expectations for a young substellar object (Zhang et al., 2024, Darcis et al., 26 May 2026). That tension became one of the main motivations for subsequent work: if the observed SED is affected by circumplanetary extinction or dust emission, then fits that ignore circumplanetary material may systematically bias the derived atmospheric parameters.

The object is also known to be accreting. Earlier observations detected Brackett-$\gamma$ emission, and later work cites additional accretion tracers including $\mathrm{H}\alpha$, $16.7 \pm 1.4$0, Ca II H&K, and He I emission (Hoch et al., 25 Jul 2025, Darcis et al., 26 May 2026). This placed circumplanetary material at the center of the interpretation even before the most detailed mid-infrared and SED analyses were carried out.

3. High-resolution spectroscopy and atmospheric composition

A major advance came from high-resolution K-band spectroscopy with VLT/CRIRES$16.7 \pm 1.4$1, obtained on 2023 February 27 and 28 at $16.7 \pm 1.4$2, with the extracted YSES-1 b spectrum stated to be at $16.7 \pm 1.4$3 and covering about 2.06–2.47 $16.7 \pm 1.4$4m (Zhang et al., 2024). These data showed that the atmosphere is dominated by H$16.7 \pm 1.4$5O and $16.7 \pm 1.4$6CO, and they confirmed atmospheric $16.7 \pm 1.4$7CO at a higher significance than earlier claims. The evidence-based significance quoted for the confirmation is $16.7 \pm 1.4$8, while the residual cross-correlation analysis gives S/N $16.7 \pm 1.4$9 for $17 \pm 1$0CO (Zhang et al., 2024).

The central isotopic result is $17 \pm 1$1 in the preferred GP-including run, with a baseline disequilibrium value of $17 \pm 1$2. This is consistent with the measured stellar carbon isotope ratio of $17 \pm 1$3 within $17 \pm 1$4, and the work explicitly argues against the earlier suggestion of strong $17 \pm 1$5C enrichment (Zhang et al., 2024). The retrieved atmospheric carbon-to-oxygen ratio is similarly stable across model variants, with the system summary adopting $17 \pm 1$6. The authors interpret that composition as solar-like and therefore compatible with formation by gravitational instability or by core accretion beyond the CO iceline (Zhang et al., 2024).

The CRIRES$17 \pm 1$7 analysis also constrained rotational and kinematic properties. The retrieved projected rotation velocity is $17 \pm 1$8, and the companion RV is $17 \pm 1$9 in the baseline disequilibrium run, implying a planet-star relative RV of $27 \pm 3$0 (Zhang et al., 2024). The low $27 \pm 3$1 is notable because it makes YSES-1 b one of the slowest rotators known among similarly aged super-Jovian companions. Two explanations are explicitly considered: either the spin axis is seen at low inclination, or the object has been spun down by magnetic braking from a long-lived circumplanetary disk (Zhang et al., 2024).

Not all atmospheric parameters proved equally robust. The same high-resolution study stresses that $27 \pm 3$2, $27 \pm 3$3, and the detailed temperature-pressure structure remain model-dependent, especially under different assumptions about clouds, veiling, and T-P parameterization. By contrast, the C/O ratio and carbon isotope ratio were found to be robust across the major retrieval variants explored (Zhang et al., 2024).

4. Circumplanetary disk and mid-infrared dust diagnostics

JWST spectroscopy transformed the circumplanetary interpretation. Using NIRSpec IFU Prism and MIRI LRS, a combined spectrum from 0.6–12 $27 \pm 3$4m was obtained, and the MIRI data revealed an infrared excess from 4–14 $27 \pm 3$5m together with a broad 8–11 $27 \pm 3$6m silicate emission feature (Hoch et al., 25 Jul 2025). The analysis explicitly argues that YSES-1 b’s atmosphere is too hot for silicates to condense into atmospheric clouds, so the mid-infrared silicate signature is interpreted as emission from circumplanetary dust rather than atmospheric cloud opacity (Hoch et al., 25 Jul 2025).

The favored dust model consists of small amorphous olivine $27 \pm 3$7 grains plus a separate blackbody-like dust component. The reported minimum grain size is either $27 \pm 3$8 in the main text or $27 \pm 3$9 in the Methods; the corresponding silicate temperature is $14 \pm 3\,M_{\rm Jup}$0 K in the main text or 488 K in the Methods, and the blackbody component is $14 \pm 3\,M_{\rm Jup}$1 K or 602 K (Hoch et al., 25 Jul 2025). The equilibrium distances inferred for those components are $14 \pm 3\,M_{\rm Jup}$2 for the silicate grains and $14 \pm 3\,M_{\rm Jup}$3 for the blackbody grains, while a broader summary in the same work places the dust at roughly 12–35 $14 \pm 3\,M_{\rm Jup}$4 from the companion (Hoch et al., 25 Jul 2025).

The same study interprets the 10 $14 \pm 3\,M_{\rm Jup}$5m feature as the first clear detection of silicates in a circumplanetary disk. Because pronounced silicate emission requires small grains, the result implies a population of submicron-to-micron dust rather than a disk dominated solely by large, nearly blackbody emitters (Hoch et al., 25 Jul 2025). The authors further suggest that the small, hot olivine grains may be second-generation thermally processed dust produced by collisions of larger satellite-forming bodies in the circumplanetary disk. This connects YSES-1 b to questions of moon formation and collisional processing in circumplanetary environments (Hoch et al., 25 Jul 2025).

The quoted small-dust mass is $14 \pm 3\,M_{\rm Jup}$6 g and also $14 \pm 3\,M_{\rm Jup}$7; the same paper additionally prints $14 \pm 3\,M_{\rm Jup}$8 for the small-dust mass, but those unit conversions are not internally consistent as printed, so the safest summary is to retain the values as given rather than reconcile them (Hoch et al., 25 Jul 2025).

5. SED re-interpretation and the planet–brown-dwarf boundary

A later study revisited YSES-1 b with new MagAO-X optical photometry in $14 \pm 3\,M_{\rm Jup}$9, $T_{\rm eff}=1727^{+172}_{-127}$0, and $T_{\rm eff}=1727^{+172}_{-127}$1, combined with archival VLT/SPHERE and VLT/NACO data, and fitted the companion SED with the species framework, BT-Settl-CIFIST atmospheric models, and nested sampling via MultiNest (Darcis et al., 26 May 2026). The new MagAO-X observations detected the companion in $T_{\rm eff}=1727^{+172}_{-127}$2 and $T_{\rm eff}=1727^{+172}_{-127}$3 with S/N $T_{\rm eff}=1727^{+172}_{-127}$4 and $T_{\rm eff}=1727^{+172}_{-127}$5, but not in $T_{\rm eff}=1727^{+172}_{-127}$6 (Darcis et al., 26 May 2026).

The key methodological step was to compare a pure-photosphere model with an extended model that included a circumplanetary-disk contribution represented by dust extinction plus a single-temperature blackbody. In the no-CPD case, the fit yielded $T_{\rm eff}=1727^{+172}_{-127}$7, $T_{\rm eff}=1727^{+172}_{-127}$8, $T_{\rm eff}=1727^{+172}_{-127}$9, and $3.0^{+0.2}_{-0.7}\,R_{\rm Jup}$0. With the CPD included, the fit shifted to $3.0^{+0.2}_{-0.7}\,R_{\rm Jup}$1, $3.0^{+0.2}_{-0.7}\,R_{\rm Jup}$2, $3.0^{+0.2}_{-0.7}\,R_{\rm Jup}$3, $3.0^{+0.2}_{-0.7}\,R_{\rm Jup}$4 mag, $3.0^{+0.2}_{-0.7}\,R_{\rm Jup}$5, $3.0^{+0.2}_{-0.7}\,R_{\rm Jup}$6, $3.0^{+0.2}_{-0.7}\,R_{\rm Jup}$7, and $3.0^{+0.2}_{-0.7}\,R_{\rm Jup}$8 (Darcis et al., 26 May 2026).

This was not merely a qualitative shift. The reduced $3.0^{+0.2}_{-0.7}\,R_{\rm Jup}$9 improved from 3.75 to 1.46, and the Bayesian evidence increased from $21.8 \pm 3\,M_{\rm Jup}$0 to $21.8 \pm 3\,M_{\rm Jup}$1, corresponding to $21.8 \pm 3\,M_{\rm Jup}$2 and a strong preference for the CPD-inclusive model (Darcis et al., 26 May 2026). In physical terms, the addition of extinction and disc emission transforms YSES-1 b from a cool, inflated companion into a hotter, smaller, intrinsically more luminous object. The authors explicitly argue that this resolves the previously identified “large radius anomaly” (Darcis et al., 26 May 2026).

The classification consequence is substantial. Using the CPD-corrected luminosity and BT-Settl evolutionary tracks, the same study derives $21.8 \pm 3\,M_{\rm Jup}$3 at $21.8 \pm 3\,M_{\rm Jup}$4 Myr and $21.8 \pm 3\,M_{\rm Jup}$5 at $21.8 \pm 3\,M_{\rm Jup}$6 Myr, compared with the earlier $21.8 \pm 3\,M_{\rm Jup}$7 interpretation (Darcis et al., 26 May 2026). In that framework, YSES-1 b moves into the brown-dwarf regime.

The reinterpretation is consequential but not unconstrained. The authors are explicit that the CPD prescription is simplified, that the fit is based on photometry rather than a contiguous spectrum, and that there is a notable discrepancy with JWST/NIRSpec at wavelengths $21.8 \pm 3\,M_{\rm Jup}$8, where the JWST spectrum shows 15–45% lower flux than both the older photometry and the best-fit model (Darcis et al., 26 May 2026). A plausible implication is that the planet-versus-brown-dwarf classification remains sensitive to how circumplanetary extinction, thermal dust emission, and cross-epoch inconsistencies are modeled.

6. Orbit, spin-orbit architecture, and dynamical interpretation

YSES-1 b is also one of the few very wide directly imaged companions for which a full orbit fit has now been obtained. New VLTI/GRAVITY astrometry from eight epochs, together with the CRIRES$21.8 \pm 3\,M_{\rm Jup}$9 relative RV measurement, were analyzed with orbitize! using the ptemcee parallel-tempered MCMC sampler. To minimize correlated short-timescale systematics, the actual fit used four astrometric epochs, approximately one per month (Roberts et al., 17 Sep 2025).

The resulting posterior gives $\gamma$0 AU, $\gamma$1, $\gamma$2, $\gamma$3, $\gamma$4, total system mass $\gamma$5, and parallax $\gamma$6 mas (Roberts et al., 17 Sep 2025). The headline result is the moderate eccentricity. The authors emphasize that high-precision GRAVITY astrometry plus the direct relative RV overcame the usual difficulty of constraining eccentricity for such a distant companion and yielded the first full orbit fit for the system (Roberts et al., 17 Sep 2025).

The same work derives a line-of-sight stellar obliquity of $\gamma$7, with the stellar spin-axis line-of-sight inclination estimated as $\gamma$8 (Roberts et al., 17 Sep 2025). Given the orbital inclination of $\gamma$9, the system is consistent with low line-of-sight spin-orbit misalignment. The authors explicitly state that they find no evidence of significant misalignment between YSES 1 b’s orbit and the star’s spin axis (Roberts et al., 17 Sep 2025).

Dynamically, the moderate eccentricity disfavors an extremely violent scattering history, but it is not a nearly circular orbit either. The interpretation in the orbit paper is therefore deliberately nuanced: the lower eccentricity favors in situ formation over extreme scattering or cloud fragmentation, yet the posterior is not peaked near $\mathrm{H}\alpha$0, so a more complex dynamical history, potentially involving early scattering, is not ruled out (Roberts et al., 17 Sep 2025). The outer companion YSES 1 c remains a major unknown because there are not yet enough astrometric measurements to constrain its orbit. That unresolved outer architecture limits inferences about mutual inclination, long-term stability, and whether YSES 1 c helped excite the eccentricity of YSES 1 b (Roberts et al., 17 Sep 2025).

7. Scientific status and unresolved issues

YSES-1 b now occupies an unusual position in the directly imaged companion literature. On the one hand, high-resolution spectroscopy gives a coherent picture of a young, wide-orbit, accreting companion with a broadly stellar carbon isotope ratio, a stellar-like C/O ratio, and a very low projected spin rate (Zhang et al., 2024). On the other hand, JWST and optical-to-thermal-IR SED analyses indicate that circumplanetary material strongly affects the observed flux distribution, producing both an infrared excess and silicate emission and potentially biasing atmospheric and evolutionary inferences if ignored (Hoch et al., 25 Jul 2025, Darcis et al., 26 May 2026).

The classification issue is therefore not settled by a single dataset. Several works continue to treat YSES-1 b in planetary-mass or super-Jovian terms, largely following the earlier $\mathrm{H}\alpha$1 interpretation and the system’s role as a directly imaged multiplanet architecture (Zhang et al., 2024, Hoch et al., 25 Jul 2025, Roberts et al., 17 Sep 2025). By contrast, the CPD-inclusive SED modeling argues that the object is substantially hotter and more luminous than previously inferred and therefore lies firmly in the brown-dwarf regime under either plausible age assumption (Darcis et al., 26 May 2026). This suggests that YSES-1 b has become a case study in how circumplanetary extinction and thermal dust emission can shift a young companion across the nominal planet/brown-dwarf boundary.

Several open problems remain. The atmospheric structure, metallicity, and gravity are still model-dependent in the spectroscopy; the dust geometry and optical properties of the circumplanetary material are simplified in the SED analyses; the photometric and JWST spectroscopic fluxes are not fully consistent at $\mathrm{H}\alpha$2; and the orbit of YSES 1 c is still unconstrained (Zhang et al., 2024, Darcis et al., 26 May 2026, Roberts et al., 17 Sep 2025). For those reasons, YSES-1 b is best regarded not as a fully settled object class but as a technically important substellar companion whose interpretation depends sensitively on how atmospheric retrieval, disk physics, and evolutionary modeling are coupled.