HD 86226 c: Short-Period Sub-Neptune

• HD 86226 c is a short-period sub-Neptune transiting a bright solar-type star, identified via joint TESS photometry and radial-velocity analysis.
• The featureless HST near-infrared transmission spectrum rules out a clear, solar-metallicity H/He atmosphere at 6.5σ, suggesting either high metal enrichment or refractory clouds.
• Its architecture, featuring an outer giant planet companion, makes HD 86226 c an important benchmark for studies of formation, volatile retention, and atmospheric evolution.

Searching arXiv for the specified papers to ground the article in current records. HD 86226 c is a short-period sub-Neptune transiting the bright, solar-type star HD 86226. It was identified in TESS photometry and characterized through joint transit and radial-velocity analyses, which established a planet with radius $2.16 \pm 0.08\,R_\oplus$, mass $7.25^{+1.19}_{-1.12}\,M_\oplus$, orbital period $3.98442 \pm 0.00018\ {\rm d}$, and equilibrium temperature $1311 \pm 28\ {\rm K}$ assuming zero Bond albedo (Teske et al., 2020). Subsequent HST transmission spectroscopy found that the planet’s near-infrared spectrum is essentially featureless, with a constant transit depth of $418 \pm 14\ {\rm ppm}$ and a spectral modulation of only $0.01^{+0.17}_{-0.01}$ scale heights for an H/He-dominated atmosphere, excluding a cloud-free solar-metallicity atmosphere at $6.5\,\sigma$ confidence (Kahle et al., 17 Jul 2025). Within the broader architecture of the HD 86226 system, the planet is also notable as the inner transiting companion to a long-period giant planet, HD 86226 b, making it relevant to studies of inner light planets in systems with outer giant planets (Delisle et al., 30 Sep 2025).

1. Discovery, confirmation, and system context

HD 86226 c was detected by the Transiting Exoplanet Survey Satellite mission around the bright ($V=7.9$) star HD 86226, also designated TOI-652 and TIC 22221375. TESS observed the star in short-cadence mode in Sector 9, and the SPOC pipeline flagged a periodic transit-like signal at about 3.98 days. An independent detrending of the TESS light curve followed by a box-fitting least squares periodogram recovered the same signal, with transit depth $\sim 360$ ppm, signal-to-noise $\sim 20$, and period near 3.98 days (Teske et al., 2020).

The validation sequence was extensive. The reported checks included TESS centroid tests, odd-even transit depth checks, ghost diagnostics, and bootstrap validation from the SPOC Data Validation Report. Ground-based follow-up photometry with LCOGT/SINISTRO ruled out neighboring eclipsing binaries, and speckle imaging with SOAR found no nearby companions within $7.25^{+1.19}_{-1.12}\,M_\oplus$0 and constrained other companions to $7.25^{+1.19}_{-1.12}\,M_\oplus$1 beyond $7.25^{+1.19}_{-1.12}\,M_\oplus$2. Archival and new radial velocities confirmed that the signal is associated with the primary star, and TOI-652.01 was therefore identified as the planet HD 86226 c (Teske et al., 2020).

The system had already been known to host a long-period giant planet, HD 86226 b. Earlier Doppler work had reported the outer companion as a very eccentric giant, but later and more comprehensive analyses revised that picture substantially. The TESS-era joint fit yielded for HD 86226 b a period of $7.25^{+1.19}_{-1.12}\,M_\oplus$3, RV semi-amplitude $7.25^{+1.19}_{-1.12}\,M_\oplus$4, eccentricity $7.25^{+1.19}_{-1.12}\,M_\oplus$5, minimum mass $7.25^{+1.19}_{-1.12}\,M_\oplus$6, and semi-major axis $7.25^{+1.19}_{-1.12}\,M_\oplus$7 (Teske et al., 2020). A later homogeneous RV re-analysis using CORALIE, HARPS, and PFS treated HD 86226 c specifically as the inner planet in a system with a known outer giant and confirmed that the short-period planet is robustly present (Delisle et al., 30 Sep 2025).

This architecture places HD 86226 c in a category of systems used to investigate whether outer giant planets correlate with or suppress the presence of inner light planets. The later survey paper identified HD 86226 as one of three systems in a 26-star Giant sample with an inner planet detection, while emphasizing that the sample size remained too small for a definitive correlation statement (Delisle et al., 30 Sep 2025). A plausible implication is that HD 86226 c is important not only as an atmospheric target but also as a benchmark for formation and system-architecture studies.

2. Stellar and planetary properties

The host star is consistently described as a near-solar G-type star. In the atmospheric characterization study, the adopted stellar properties were $7.25^{+1.19}_{-1.12}\,M_\oplus$8 K, $7.25^{+1.19}_{-1.12}\,M_\oplus$9, $3.98442 \pm 0.00018\ {\rm d}$0, and $3.98442 \pm 0.00018\ {\rm d}$1, and the star was noted to be relatively quiet, consistent with a 25-day rotation period and no strong photometric variability in the ground-based monitoring (Kahle et al., 17 Jul 2025). Other analyses reported closely similar stellar parameters. The TESS discovery paper derived $3.98442 \pm 0.00018\ {\rm d}$2, $3.98442 \pm 0.00018\ {\rm d}$3, $3.98442 \pm 0.00018\ {\rm d}$4, $3.98442 \pm 0.00018\ {\rm d}$5, age $3.98442 \pm 0.00018\ {\rm d}$6 Gyr, and distance $3.98442 \pm 0.00018\ {\rm d}$7 pc (Teske et al., 2020). The homogeneous RV study gave spectral type G2 V, distance $3.98442 \pm 0.00018\ {\rm d}$8 pc, effective temperature $3.98442 \pm 0.00018\ {\rm d}$9 K, metallicity $1311 \pm 28\ {\rm K}$0, radius $1311 \pm 28\ {\rm K}$1, mass $1311 \pm 28\ {\rm K}$2, chromospheric activity $1311 \pm 28\ {\rm K}$3, and rotation period $1311 \pm 28\ {\rm K}$4 d (Delisle et al., 30 Sep 2025).

For HD 86226 c itself, the joint transit and RV fit reported orbital period $1311 \pm 28\ {\rm K}$5, mid-transit time $1311 \pm 28\ {\rm K}$6 $1311 \pm 28\ {\rm K}$7, scaled semi-major axis $1311 \pm 28\ {\rm K}$8, impact parameter $1311 \pm 28\ {\rm K}$9, inclination $418 \pm 14\ {\rm ppm}$0, RV semi-amplitude $418 \pm 14\ {\rm ppm}$1, eccentricity $418 \pm 14\ {\rm ppm}$2, argument of periastron $418 \pm 14\ {\rm ppm}$3, radius $418 \pm 14\ {\rm ppm}$4, mass $418 \pm 14\ {\rm ppm}$5, semi-major axis $418 \pm 14\ {\rm ppm}$6, density $418 \pm 14\ {\rm ppm}$7, equilibrium temperature $418 \pm 14\ {\rm ppm}$8, and transit duration $418 \pm 14\ {\rm ppm}$9 hr (Teske et al., 2020).

The HST atmospheric study adopted a broadband-fit planet radius of $0.01^{+0.17}_{-0.01}$0, with the mass taken as $0.01^{+0.17}_{-0.01}$1 with $0.01^{+0.17}_{-0.01}$2 uncertainty (Kahle et al., 17 Jul 2025). This suggests that modest differences in reported radius reflect differences in analysis context rather than a disagreement over the existence or basic character of the planet.

The planet’s location near both the radius gap and the hot Neptune desert was already emphasized at discovery (Teske et al., 2020). Because the orbital period is about 4 days and the equilibrium temperature is about $0.01^{+0.17}_{-0.01}$3–$0.01^{+0.17}_{-0.01}$4 K, HD 86226 c occupies a strongly irradiated regime in which envelope retention, atmospheric loss, and compositional diversity are all astrophysically consequential.

3. Observational basis for atmospheric characterization

The principal atmospheric dataset came from the Sub-neptune Planetary Atmosphere Characterization Experiment (SPACE) Program. The observations combined HST/WFC3 G141 transmission spectroscopy over $0.01^{+0.17}_{-0.01}$5–$0.01^{+0.17}_{-0.01}$6m with HST/STIS ultraviolet characterization of the host star, including reconstruction of the stellar UV/EUV spectrum and Ly$0.01^{+0.17}_{-0.01}$7 (Kahle et al., 17 Jul 2025).

Nine transits were observed with WFC3. Because the target is bright and the scan speed was high, the spectrum shifted on the detector by up to $0.01^{+0.17}_{-0.01}$8 pixels. The analysis therefore had to model a row-position-dependent systematic. Two independent reductions were performed, using PACMAN and Eureka!, and both recovered a flat spectrum. PACMAN was treated as the more reliable result because it explicitly handled the detector-position systematics better (Kahle et al., 17 Jul 2025).

The systematics modeling is an important part of the interpretation because the reported atmospheric result is not the detection of a spectral feature but the robust recovery of a nearly constant transit depth in the presence of instrumental structure. PACMAN modeled orbit-long systematics with

$0.01^{+0.17}_{-0.01}$9

where $6.5\,\sigma$0 is time since orbit start. Eureka!’s ramp model used

$6.5\,\sigma$1

These expressions are instrumental rather than atmospheric, but they delimit the reduction framework under which the transmission spectrum was extracted (Kahle et al., 17 Jul 2025).

The ultraviolet stellar characterization also matters physically. The paper notes that the star is relatively quiet and that the STIS data were used to reconstruct the stellar UV/EUV spectrum. Within the paper’s logic, this constrains the host-star environment in which the planet’s atmosphere is irradiated. The combination of near-infrared transmission spectroscopy and ultraviolet stellar characterization is therefore not merely observationally convenient; it is part of the broader attempt to place the atmosphere in the context of host-star forcing (Kahle et al., 17 Jul 2025).

4. Featureless transmission spectrum and atmospheric retrievals

The main empirical atmospheric result is that HD 86226 c has a featureless near-infrared transmission spectrum. For the PACMAN spectrum, a constant-depth fit gives

$6.5\,\sigma$2

and the data are consistent with a constant transit depth at only $6.5\,\sigma$3 from flat. The broadband WFC3 depth is

$6.5\,\sigma$4

and the spectroscopic bins mostly scatter around $6.5\,\sigma$5–450 ppm with no compelling molecular structure (Kahle et al., 17 Jul 2025).

The spectral modulation is reported as

$6.5\,\sigma$6

for an H/He-dominated atmosphere. In the paper’s interpretation, this is extraordinarily small and is far flatter than expected for a clear hydrogen-rich atmosphere (Kahle et al., 17 Jul 2025). That conclusion is strengthened by retrieval calculations using petitRADTRANS, which show that a cloud-free solar-metallicity H/He atmosphere is ruled out at

$6.5\,\sigma$7

The paper notes an important caveat. If the planet mass is allowed to float freely, the exclusion of the cloud-free solar-metallicity H/He case weakens to $6.5\,\sigma$8, but that solution requires

$6.5\,\sigma$9

which conflicts with the dynamical mass estimate (Kahle et al., 17 Jul 2025). The robust conclusion is therefore not that every H/He-rich interpretation is impossible, but that the specific case of a clear, solar-metallicity H/He atmosphere is not viable under the observed mass constraint.

The retrieval setup assumed an isothermal atmosphere at $V=7.9$0 K, hydrostatic equilibrium, a reference radius fixed by the broadband transit depth, a free reference pressure $V=7.9$1, and optional gray clouds at $V=7.9$2. The gray-cloud retrieval has a near one-to-one degeneracy between the cloud-top pressure $V=7.9$3 and the reference pressure $V=7.9$4, since both shift the effective transit radius (Kahle et al., 17 Jul 2025). This clarifies why the data strongly disfavor some classes of models while leaving a residual degeneracy between high mean molecular weight and cloud opacity.

5. Atmospheric interpretations: metal enrichment and refractory clouds

The atmospheric analysis explored two principal explanatory classes. The first is a cloud-free but metal-rich atmosphere. Using scaled-solar equilibrium chemistry and no gray cloud deck, the flat spectrum can be matched if the atmosphere has

$V=7.9$5

with a $V=7.9$6 lower limit

$V=7.9$7

This corresponds to a minimum mean molecular weight of about $V=7.9$8 u (Kahle et al., 17 Jul 2025).

The paper defines metallicity as

$V=7.9$9

where $\sim 360$0 is the abundance of elements heavier than He and $\sim 360$1 is hydrogen (Kahle et al., 17 Jul 2025). In this framework, the flatness of the spectrum is explained by the reduction of atmospheric scale height through a substantially increased mean molecular weight.

The second class of explanation is cloud opacity. The featureless spectrum can be reproduced by clouds of MnS, MgSiO$\sim 360$2, or Fe. These species were selected because their condensation curves intersect the relevant $\sim 360$3-$\sim 360$4 regime of HD 86226 c (Kahle et al., 17 Jul 2025). The analysis used radiative-convective temperature-pressure profiles and compared them with vapor-pressure curves for candidate condensates. The condensation levels of MnS, MgSiO$\sim 360$5, and Fe cross the observable atmospheric region, roughly

$\sim 360$6

while Na$\sim 360$7S, KCl, and ZnS do not (Kahle et al., 17 Jul 2025).

This distinction is central because HD 86226 c is hot enough that methane-based haze formation is suppressed. The paper explicitly states that the equilibrium temperature of about $\sim 360$8 K suppresses CH$\sim 360$9-based haze formation and favors CO as the dominant carbon reservoir (Kahle et al., 17 Jul 2025). One might therefore have expected a clearer atmosphere than in cooler sub-Neptunes, yet the observed spectrum remains nearly flat. The authors conclude that the planet likely falls into one of two categories: a very metal-rich atmosphere with large mean molecular weight, or an atmosphere obscured by high-altitude refractory clouds more typical of hot-Jupiter-like condensation physics than of cooler sub-Neptunes (Kahle et al., 17 Jul 2025).

A common misconception would be to interpret the flat spectrum as proof that the atmosphere is absent. The papers do not support that conclusion. The discovery paper states that the density is low enough to imply a small volatile envelope (Teske et al., 2020), while the atmospheric paper shows that the spectrum is compatible with either strong metal enrichment or specific cloud species rather than with an atmosphere-free rocky body (Kahle et al., 17 Jul 2025).

6. Interior structure, population context, and future tests

From mass and radius alone, HD 86226 c is not consistent with a purely rocky interpretation. The discovery paper states that the planet lies slightly above the silicate-only mass-radius curve and interprets the density as low enough to imply at least a small volatile envelope (Teske et al., 2020). The interior model adopted a layered structure with iron core, silicate mantle, water layer, and H/He envelope, with the structure equations solved using a generalized Bayesian inference approach. A notable modeling detail is that the transit radius was defined by the chord optical depth condition

$\sim 20$0

The inferred interior composition is highly degenerate. Under priors on stellar Fe/Si and Mg/Si, the Bayesian interior modeling gave core mass fraction

$\sim 20$1

mantle mass fraction

$\sim 20$2

water mass fraction

$\sim 20$3

and atmospheric mass fraction

$\sim 20$4

corresponding to an H/He mass of about $\sim 20$5 and a thickness of about $\sim 20$6 (Teske et al., 2020). These values show that the presence of volatiles is favored, while the relative contributions of rock, iron, and water are not tightly determined.

In population context, HD 86226 c is especially noteworthy because the atmospheric study concludes that it does not follow the aerosol trend of sub-Neptunes found by previous studies (Kahle et al., 17 Jul 2025). At $\sim 20$7 K, a clearer atmosphere might have been expected than on cooler sub-Neptunes, but the observed transmission spectrum remains nearly featureless. This suggests that temperature alone is not a sufficient predictor of observable spectral amplitude in the sub-Neptune regime.

The system is also used in the literature on planetary architecture. The homogeneous RV survey describes HD 86226 c as one of only three inner planet detections in a 26-star Giant sample, with all three detected inner planets having periods below 6 days (Delisle et al., 30 Sep 2025). The authors state that their relatively low number of detections seems to contradict previous studies that found a strong outer giant planet–inner light planet correlation, although they do not yet conclude a definitive correlation or anticorrelation because the sample is still small and completeness corrections are not yet fully done. HD 86226 c therefore functions as an informative but non-dispositive data point in that debate.

The atmospheric paper identifies specific future observations that could break the present degeneracy. JWST/NIRSpec G395H can look for CO$\sim 20$8 above $\sim 20$9m, and a strong CO$7.25^{+1.19}_{-1.12}\,M_\oplus$00 feature would support the metal-rich scenario, whereas a weak or absent feature would point more toward cloud opacity or an even more unusual composition. MIRI could test cloud composition because MgSiO$7.25^{+1.19}_{-1.12}\,M_\oplus$01 has a prominent feature around $7.25^{+1.19}_{-1.12}\,M_\oplus$02–$7.25^{+1.19}_{-1.12}\,M_\oplus$03m, while Fe and MnS are less distinctive in that region (Kahle et al., 17 Jul 2025). The paper also notes that optical or visible observations and phase curves could probe Rayleigh slopes, albedo, and cloud coverage.

Taken together, the available measurements define HD 86226 c as a hot, short-period sub-Neptune around a quiet G-type star, with a flat WFC3 transmission spectrum, a constant transit depth of $7.25^{+1.19}_{-1.12}\,M_\oplus$04 ppm, a spectral amplitude of only $7.25^{+1.19}_{-1.12}\,M_\oplus$05 scale heights, and a $7.25^{+1.19}_{-1.12}\,M_\oplus$06 exclusion of a cloud-free solar-metallicity H/He atmosphere (Kahle et al., 17 Jul 2025). Its significance lies in the conjunction of precise bulk properties, anomalously muted transmission features, and a system architecture that links inner sub-Neptune evolution to the presence of an outer giant companion.