---
title: 'YSES-1 System: Wide-Orbit Benchmark'
url: https://www.emergentmind.com/topics/yses-1-system
type: topic
---

# YSES-1 System: Wide-Orbit Benchmark

Searching arXiv for recent YSES-1 papers to ground the article in the latest literature.
The **YSES-1 system** is the planetary system centered on **TYC 8998-760-1**, a young, solar-type star originally characterized as a **K3 IV** object and identified as the first directly imaged multiplanet system around a young solar analog [2007.10991]. The system is known for its two widely separated directly imaged substellar companions, **YSES-1 b** and **YSES-1 c**, at projected separations of approximately **160 au** and **320 au**, respectively [2007.10991]. Subsequent work has substantially revised its astrophysical context: the host is now classified as a high-probability member of the **MELANGE-4** population, a distinct **\(27 \pm 3\) Myr** extended Lower Centaurus–Crux population, rather than the canonical **10–16 Myr** Lower Centaurus Crux subgroup [2212.03266]. That reassignment has altered the inferred companion masses, sharpened the system’s role as a benchmark for substellar evolution, and linked YSES-1 to a broader environment containing both directly imaged and transiting planets [2212.03266].

## 1. Discovery, identification, and stellar host

The system was established as a two-companion architecture in the Young Suns Exoplanet Survey with the detection of a second wide-orbit companion around **TYC 8998-760-1** [2007.10991]. The host star was described there as a **pre-main-sequence, solar-type star in LCC, part of Sco–Cen**, with **\(M_\star = 1.00 \pm 0.02\,M_\odot\)**, an age of **\(16.7 \pm 1.4\) Myr**, and a Gaia DR2 parallax of **\(10.54 \pm 0.03\) mas**, corresponding to **\(94.6 \pm 0.3\) pc** [2007.10991]. Later high-resolution spectroscopy gave refined stellar properties of **\(M_\star = 1.00 \pm 0.02\,M_\odot\)**, **\(R_\star = 1.01 \pm 0.02\,R_\odot\)**, **\(T_{\rm eff,\star} = 4573 \pm 10\) K**, **\([\mathrm{Fe/H}] = -0.07 \pm 0.01\)**, and **\(RV = 12.9 \pm 0.03\,\mathrm{km\,s^{-1}}\)** [2409.16660]. A later stellar SED analysis yielded closely related parameters, including **\(T_{\rm eff,*} = 4708^{+30}_{-33}\,\text{K}\)**, **\(R_* = 1.02 \pm 0.01\,R_\odot\)**, **\(A_V = 0.06^{+0.03}_{-0.04}\,\text{mag}\)**, and **\(\log(L_*/L_\odot) = -0.34 \pm 0.01\)** [2605.26805].

The nomenclature “YSES-1” derives from the Young Suns Exoplanet Survey context and denotes the first YSES system with a confirmed wide-orbit planet and the first with a directly imaged multi-companion architecture around a young solar-mass star [2007.10991]. The system’s observational importance follows directly from its geometry: the companions are widely separated from the host on the sky, which enables direct spectroscopy and astrometry with comparatively limited stellar contamination [2007.10991].

A major contextual revision came with the identification of **MELANGE-4**, a distributed nearby population in the broader Sco–Cen region [2212.03266]. In that work, **TYC 8998-760-1 (YSES-1)** was reassigned from classical LCC membership to MELANGE-4 with a **BANYAN Σ membership probability of \(99.1\%\)**, with membership probabilities in any LCC subgroup or Carina reported as effectively negligible [2212.03266]. The reassignment implies that the system is not best understood as a typical **\(\sim 16\) Myr** LCC member but rather as part of a distinct **\(27 \pm 3\) Myr** extended association whose age is anchored by lithium depletion boundary, isochronal, and rotational diagnostics [2212.03266].

## 2. System architecture and directly imaged companions

The two known companions, **YSES-1 b** and **YSES-1 c**, were initially characterized as planetary-mass objects at extreme separations from the star [2007.10991]. In the discovery-era interpretation, **YSES-1 b** had a mass of **\(14 \pm 3\,M_{\rm Jup}\)** at a projected separation of about **160 au**, while **YSES-1 c** had **\(6 \pm 1\,M_{\rm Jup}\)** at about **320 au** [2007.10991]. The latter corresponds to a mass ratio of **\(q=0.57\pm0.10\%\)** with respect to the primary [2007.10991]. The same work reported that **YSES-1 c** had **\(T_{\rm eff} = 1240^{+160}_{-170}\) K**, **\(\log g = 3.5 \pm 0.5\)**, **\(R_{\rm p} = 1.1^{+0.6}_{-0.3}\,R_{\rm Jup}\)**, and **\(\log(L/L_\odot) = -4.65^{+0.05}_{-0.08}\)**, with an empirical spectral type of **L7.5** [2007.10991].

Later spectroscopy and modeling preserved the broad qualitative architecture but changed its quantitative interpretation. High-resolution CRIRES+ work described **YSES-1 b** as an **L0** companion with photometric **\(T_{\rm eff} = 1727^{+172}_{-127}\) K** and **\(R = 3.0^{+0.2}_{-0.7}\,R_{\rm Jup}\)**, while **YSES-1 c** remained an **L7.5** object with photometric **\(T_{\rm eff} = 1240^{+160}_{-170}\) K** and **\(R = 1.1^{+0.6}_{-0.3}\,R_{\rm Jup}\)** [2409.16660]. That same study explicitly noted that more recent work identifies the host as a high-probability MELANGE-4 member with age **\(\sim 27\) Myr**, and therefore discussed both **17 Myr** and **27 Myr** mass scales, giving for YSES-1 b **\(14 \pm 3\,M_{\rm Jup}\)** at **17 Myr** and **\(21.8 \pm 3\,M_{\rm Jup}\)** at **27 Myr**, and for YSES-1 c **\(6 \pm 1\,M_{\rm Jup}\)** at **17 Myr** and **\(7.2 \pm 0.7\,M_{\rm Jup}\)** at **27 Myr** [2409.16660].

The host and companions occupy a rare part of parameter space. The system was described as the **first directly imaged multi-planet system around a solar-type star** [2409.16660], and the original discovery paper emphasized that the wide separations make it an exceptional target for follow-up with facilities such as **JWST** [2007.10991]. This suggests a dual benchmark role: the system is useful both for wide-orbit architecture studies and for comparative atmospheric characterization of coeval companions with markedly different temperatures and spectra.

## 3. Association membership, age revision, and mass reclassification

The most consequential change in the system’s interpretation is the shift from the canonical **LCC age of \(16.7 \pm 1.4\) Myr** to the **MELANGE-4 age of \(27 \pm 3\) Myr** [2212.03266]. The MELANGE-4 age was derived using three independent diagnostics. The primary anchor was the **lithium depletion boundary**, bracketed by **\(5.7 < M_{K_s} < 6.1\)** and converted to an age using several model grids, all mutually consistent; the adopted value was **\(\tau_{\rm MELANGE-4} = 27 \pm 3~\mathrm{Myr}\)** [2212.03266]. Isochronal fitting gave **\(\tau_{\rm PARSEC} = 26.0 \pm 2.1~\mathrm{Myr}\)**, and the rotation sequence was qualitatively consistent with **\(20–40\) Myr** [2212.03266].

For YSES-1 specifically, the membership analysis found that the system lies near the **center of MELANGE-4 in both position and velocity**, giving it one of the more secure kinematic assignments in the sample [2212.03266]. No new lithium or rotation measurements for YSES-1 were reported there; the age inference is therefore inherited from the ensemble properties of the association rather than from fresh age diagnostics of the host itself [2212.03266].

The revised age systematically raises the inferred companion masses. Using **ATMO 2020 non-equilibrium evolutionary models**, YSES-1 b was revised from **\(14\pm3~M_{\rm Jup}\)** at **16.7 Myr** to **\(21.8 \pm 3~M_{\rm Jup}\)** at **\(27\pm3\) Myr**, while YSES-1 c moved from **\(6\pm1~M_{\rm Jup}\)** to **\(7.2 \pm 0.7~M_{\rm Jup}\)** [2212.03266]. The study noted that YSES-1 b is thereby pushed **well into the brown-dwarf regime by most definitions**, whereas YSES-1 c remains securely planetary-mass [2212.03266].

A later SED-focused analysis of YSES-1 b went further. By modeling circumplanetary-disc extinction and emission, it inferred a significantly higher intrinsic luminosity for the companion and then used **BT-Settl evolutionary tracks** to derive a mass of **\(25.7^{+4.1}_{-3.6}\,M_J\)** if the system is **\(16.7 \pm 1.4\) Myr** old, or **\(41.6^{+3.6}_{-3.4}\,M_J\)** if it is **\(27 \pm 3\) Myr** old [2605.26805]. That work concluded explicitly that, once circumplanetary disc effects are included, YSES-1 b moves into the **brown dwarf regime** [2605.26805].

A concise summary of the age-dependent companion mass scales discussed in the literature is useful.

| Companion | Earlier mass scale | Revised mass scale |
|---|---:|---:|
| YSES-1 b | \(14 \pm 3\,M_{\rm Jup}\) at \(16.7\) Myr | \(21.8 \pm 3\,M_{\rm Jup}\) at \(27\pm3\) Myr; alternatively \(25.7^{+4.1}_{-3.6}\) or \(41.6^{+3.6}_{-3.4}\,M_J\) from CPD-aware luminosity modeling |
| YSES-1 c | \(6 \pm 1\,M_{\rm Jup}\) at \(16.7\) Myr | \(7.2 \pm 0.7\,M_{\rm Jup}\) at \(27\pm3\) Myr |

This evolution in the mass estimates has produced a central ambiguity in the system’s classification. In the original YSES interpretation, both outer bodies were treated as giant planets [2007.10991]. In the revised age and CPD-aware framework, **YSES-1 c** remains a planetary-mass companion, but **YSES-1 b** is plausibly better described as a low-mass brown dwarf [2212.03266; 2605.26805]. This suggests that “YSES-1 system” now occupies a boundary case between wide-orbit planetary systems and low-mass hierarchical substellar multiples.

## 4. Orbital architecture, astrometry, and spin–orbit geometry

The original discovery paper used limited astrometric arcs and therefore treated the dynamical architecture in simplified terms, assuming **\(a_b = 160\) au** and **\(a_c = 320\) au** and exploring a grid of eccentricities under coplanar assumptions [2007.10991]. With those assumptions, the authors found that **circular orbits are stable**, while **mildly eccentric orbits for either/both components (\(e > 0.1\)) are chaotic on Gyr timescales**, implying either **in-situ formation** or a very specific past interaction involving an unseen third companion [2007.10991].

That picture has since changed for **YSES-1 b**, whose orbit was first fully constrained using **VLTI/GRAVITY astrometry**, literature SPHERE/NACO astrometry, and a **CRIRES+** relative radial velocity measurement [2509.14321]. The reported orbital posteriors were:

- **\(a = 146^{+16}_{-10}~\mathrm{AU}\)**
- **\(e = 0.44^{+0.17}_{-0.18}\)**
- **\(i = 90.6^{+1.1}_{-1.0}~\mathrm{deg}\)**
- **\(\Omega = 32.3^{+0.2}_{-0.2}~\mathrm{deg}\)**
- **\(\omega = 315^{+8}_{-11}~\mathrm{deg}\)**
- **\(M_{\rm tot} = 1.02^{+0.02}_{-0.02}~M_\odot\)**
- **\(\pi = 10.61^{+0.01}_{-0.01}~\mathrm{mas}\)** [2509.14321]

From these medians, the derived orbital period is of order **\(\sim 1700\) yr**, with **periastron \(q \approx 82\) au** and **apastron \(Q \approx 210\) au** [2509.14321]. The paper emphasized that this is the **first full orbit fit** for the system and that the eccentricity is no longer effectively prior-dominated [2509.14321].

The methodological basis for the new orbit fit is also part of the system’s significance. The study combined **four representative GRAVITY epochs**—selected from eight to minimize correlated systematics—with SPHERE/NACO imaging and the planetary relative RV **\(\Delta \mathrm{RV}_{\rm planet-star} = -1.87 \pm 0.04~\mathrm{km\,s^{-1}}\)** from CRIRES+ [2509.14321]. The orbit was fit with **orbitize!** using **ptemcee**, with fitted parameters \((a,e,i,\Omega,\omega,T_0)\), \(M_{\rm tot}\), and \(\pi\), under standard priors including a log-uniform prior on \(a\) and a uniform prior on \(e\in[0,1]\) [2509.14321].

The same work derived a stellar spin inclination of **\(i_\star = 78^{+8}_{-11}~\mathrm{deg}\)** from the stellar rotation period, radius, and \(v\sin i_\star\), and compared it with the orbital inclination to obtain a **line-of-sight stellar obliquity** of **\(\Delta i \approx 12^{+11}_{-8}~\mathrm{deg}\)** [2509.14321]. The distribution peaks near zero and was described as consistent with a relatively well-aligned system [2509.14321].

A compact summary of the current orbital knowledge is therefore appropriate.

| Quantity | YSES-1 b |
|---|---:|
| Semi-major axis | \(146^{+16}_{-10}\) au |
| Eccentricity | \(0.44^{+0.17}_{-0.18}\) |
| Inclination | \(90.6^{+1.1}_{-1.0}\) deg |
| Approximate period | \(\sim 1700\) yr |
| Line-of-sight stellar obliquity | \(12^{+11}_{-8}\) deg |

No comparable orbit fit yet exists for **YSES-1 c**. Its mutual inclination with b, true 3D obliquity, and long-term dynamical relation to the outer architecture remain unconstrained [2509.14321]. This is an important limitation because the original stability analysis assumed low eccentricities for both objects [2007.10991], whereas the new solution for b has **moderate eccentricity** [2509.14321]. A plausible implication is that the older stability arguments must be revisited with updated orbital elements and eventual constraints on c.

## 5. Atmospheric composition, clouds, and circumplanetary material

YSES-1 has become a comparative atmospheric laboratory because both companions have now been studied with high-resolution spectroscopy and JWST mid-infrared observations. The CRIRES+ study measured molecular, elemental, isotopic, rotational, and radial-velocity properties for both companions [2409.16660]. For **YSES-1 b**, it confirmed **\(^{13}\mathrm{CO}\)** at higher significance, reporting a **\(^{12}\mathrm{CO}/^{13}\mathrm{CO}\)** ratio of **\(88 \pm 13\)**, consistent with the primary’s **\(66 \pm 5\)** within uncertainties [2409.16660]. The same retrievals gave **\(\mathrm{C/O} = 0.58 \pm 0.01\)** and **\([\mathrm{M/H}] = 0.04 \pm 0.05\)** in the disequilibrium+GP model [2409.16660]. For **YSES-1 c**, the study reported the first high-resolution detections of **H\(_2\)O** and **CO** in the atmosphere at **\(7.3\sigma\)** and **\(5.7\sigma\)**, respectively, with a retrieved **\(\mathrm{C/O} = 0.36 \pm 0.14\)** and **\([\mathrm{M/H}] = -0.26 \pm 0.40\)** [2409.16660].

The same work found sharply different projected spin rates: **\(v\sin i = 5.34 \pm 0.14~\mathrm{km\,s^{-1}}\)** for YSES-1 b and **\(11.3 \pm 2.1~\mathrm{km\,s^{-1}}\)** for YSES-1 c [2409.16660]. The authors suggested that this may indicate either different spin-axis inclinations or **effective magnetic braking by the long-lived circumplanetary disk around YSES-1 b** [2409.16660]. This directly connects the spectroscopic results to later JWST evidence for circumplanetary dust around b.

JWST spectroscopy transformed the understanding of both companions. In the mid-infrared study, YSES-1 c was reported to show the **first direct observations of silicate clouds in the atmosphere of the exoplanet YSES-1 c through its 9–11 micron absorption feature**, while YSES-1 b exhibited the **first circumplanetary disk silicate emission around its sibling planet** [2507.18861]. For YSES-1 c, the clouds were inferred to be composed of either **amorphous iron-enriched pyroxene** or a combination of **amorphous MgSiO\(_3\)** and **Mg\(_2\)SiO\(_4\)**, with particle sizes **\(\le 0.1~\mu\mathrm{m}\)** at **1 millibar** pressure [2507.18861]. Forward modeling and retrievals placed the companion at **\(T_{\rm eff} \approx 950–1100~\mathrm{K}\)**, **\(\log g \approx 3.0–3.7\)**, **\(R_{\rm p} \approx 1.2–1.5~R_{\rm Jup}\)**, and **\(\log(L/L_\odot)\approx -4.74\)** [2507.18861].

For YSES-1 b, the same JWST work found a **mid-IR excess between 4 and 14 μm** and interpreted it as arising from a **circumplanetary dust disk** [2507.18861]. The excess includes a warm continuum approximated by a blackbody of **\(T_{\rm CPD}\sim 500~\mathrm{K}\)** and a broad **9–11 μm** silicate emission feature [2507.18861]. Modeling of the disk emission favored **sub-micron olivine dust grains**, with fitted temperatures of **\(T_{\rm sil} \approx 485–490~\mathrm{K}\)** and **\(T_{\rm bb} \approx 600~\mathrm{K}\)** and corresponding dust locations of **\(d_{\rm sil} \approx 10.5\pm1.7~R_{\rm Jup}\)** and **\(d_{\rm bb} \approx 17.6\pm3.7~R_{\rm Jup}\)** [2507.18861]. The mass in grains below 1 mm was estimated as **\(M_{\rm dust}\simeq 6.5\times 10^{15}~\mathrm{g}\)** [2507.18861].

The later optical-to-infrared SED analysis of YSES-1 b incorporated circumplanetary-disc extinction explicitly and concluded that including a CPD yields a much better fit than a pure-atmosphere model [2605.26805]. The atmosphere-only fit gave **\(T_{\rm eff} = 1784^{+17}_{-20}\,\text{K}\)**, **\(\log g = 5.23^{+0.06}_{-0.06}\)**, **\(R_p = 2.45^{+0.07}_{-0.06}\,R_J\)**, and **\(\log(L_p/L_\odot) = -3.24^{+0.01}_{-0.01}\)**, whereas the atmosphere+CPD fit yielded **\(T_{\rm eff} = 2854^{+110}_{-94}\,\text{K}\)**, **\(\log g = 4.31^{+0.42}_{-0.43}\)**, **\(R_p = 1.58^{+0.06}_{-0.07}\,R_J\)**, **\(\log(L_p/L_\odot) = -2.80^{+0.04}_{-0.04}\)**, **\(A_V = 5.17^{+0.32}_{-0.32}\,\text{mag}\)**, and a CPD blackbody component with **\(T_{\rm disk\_bb} = 339^{+90}_{-147}\,\text{K}\)** and **\(R_{\rm disk\_bb} = 41.85^{+36}_{-23}\,R_J\)** [2605.26805]. The improvement in fit quality was quantified as **\(\chi^2_\nu = 3.75 \rightarrow 1.46\)** and **\(\log Z = 448.22 \rightarrow 459.84\)** [2605.26805].

The authors of that study argued that dust extinction and blackbody radiation from a circumplanetary disc can resolve the previously noted **large-radius anomaly** by replacing a cool, inflated object with a hotter, smaller, and more luminous substellar companion partially obscured by its CPD [2605.26805]. This suggests that YSES-1 b’s physical interpretation now depends not only on age but also on whether CPD extinction is treated explicitly in SED fitting.

## 6. Formation scenarios, dynamics, and benchmark status

The system has been central to debates over wide-orbit formation pathways since its discovery. The original YSES interpretation emphasized that **mildly eccentric orbits** for the two companions would be chaotic over Gyr timescales, which was taken to support **in-situ formation** or a very specific past scattering event [2007.10991]. Formation channels considered in the broader literature include **core accretion**, **disk gravitational instability**, **cloud fragmentation**, and **scattering from smaller radii** [2007.10991].

The newer orbit fit for YSES-1 b complicates that picture. Its **moderate eccentricity** of **\(0.44^{+0.17}_{-0.18}\)** was interpreted as neither quiescently circular nor violently radial [2509.14321]. The authors concluded that the **lower eccentricity compared to some scattered brown dwarfs and the low obliquity both favor formation involving a disk**—either in situ or by early disk instability with mild dynamical shaping—over **extreme scattering or purely binary-like formation** [2509.14321]. The same paper explicitly stated that for both **YSES 1 b** and **HR 2562 B**, the lower eccentricities favor an **in situ formation scenario** over **extreme scattering or cloud fragmentation** [2509.14321].

Atmospheric composition studies add another layer. For **YSES-1 b**, the near-stellar **C/O** and stellar-consistent carbon isotope ratio were interpreted as compatible with either **gravitational instability** or **core accretion beyond the CO iceline with substantial incorporation of solids** [2409.16660]. For **YSES-1 c**, the retrieved **\(\mathrm{C/O}=0.36\pm0.14\)** was described as either solar or subsolar; if genuinely subsolar, the paper suggested this could indicate **accretion of oxygen-rich solids** and perhaps formation inside the CO iceline followed by outward scattering [2409.16660]. Because the uncertainties remain large, that inference was framed cautiously [2409.16660].

The CPD-aware modeling of YSES-1 b tilts the classification and formation discussion further toward a brown-dwarf-like object. That study argued that the combination of **large separation**, **high mass**, **brown-dwarf-like nature**, and a **long-lived CPD** makes YSES-1 b more consistent with **stellar-like formation**—such as fragmentation of the protostellar core or disc gravitational instability—than with **core accretion plus migration** [2605.26805]. This interpretation is stronger than the earlier age-only revision and depends directly on the CPD-corrected luminosity scale [2605.26805].

At the same time, the system remains a benchmark regardless of which formation path proves correct. The MELANGE-4 age is based on an LDB and consistent isochrone fitting rather than on an individual-star age estimate, which makes the system particularly useful for calibrating evolutionary models in the **20–30 Myr** regime [2212.03266]. YSES-1 thus now serves as a benchmark in at least three senses: a **wide-orbit architecture benchmark**, a **young substellar atmosphere benchmark**, and a **CPD benchmark** with both accretion signatures and silicate dust emission [2507.18861; 2605.26805].

## 7. Position within the broader MELANGE-4 planetary environment

The reinterpretation of YSES-1 as a MELANGE-4 member places it in a broader common-age environment that contains both directly imaged and transiting planets [2212.03266]. The same association includes **YSES-2** and **HD 95086** as hosts of directly imaged planetary-mass companions, while **HD 109833** hosts a two-planet transiting system discovered in TESS data [2212.03266]. This mixed sample is unusual because it links **wide, massive companions** and **close-in transiting planets** at a common age of **\(27 \pm 3\) Myr** [2212.03266].

Within this framework, YSES-1 is no longer an isolated LCC outlier but part of a distributed population whose age is set by ensemble diagnostics [2212.03266]. That association-level age calibration improves the interpretability of the system’s luminosities, masses, and cooling-track positions [2212.03266]. Wood et al. noted that this makes YSES-1 a stronger benchmark for testing planetary and substellar evolutionary models than in the original YSES analysis, because the age is tied to an LDB and CMD-fit scale rather than to a single-star isochrone [2212.03266].

The existence of both **inner transiting planets** and **outer directly imaged companions** in the same 27 Myr environment also offers a broader comparative context [2212.03266]. For YSES-1 specifically, this does not imply any known inner planets, but it situates the system inside an age-calibrated laboratory spanning radii, masses, orbital separations, and atmospheric regimes [2212.03266]. A plausible implication is that the system’s long-term importance will depend not only on improved modeling of YSES-1 b and c themselves, but also on comparative analysis across the full MELANGE-4 planetary census.

In its current understanding, the YSES-1 system comprises a young solar-type host, one outer companion that is increasingly interpreted as a low-mass brown dwarf with a circumplanetary disc, and a second outer companion that remains securely planetary-mass, with both embedded in a **\(27 \pm 3\) Myr** moving-group-like population [2212.03266; 2605.26805]. That combination of revised age, direct imaging, orbit fitting, atmospheric spectroscopy, silicate cloud detection, and circumplanetary-disk emission is what makes YSES-1 one of the most technically informative wide-orbit substellar systems presently known.

Source: https://www.emergentmind.com/topics/yses-1-system